Disable deployment record for release workflow

Set deployment: false on the publish environment so the release
job accesses environment protection rules and secrets without
creating a deployment record.

.github/CODEOWNERS

@@ -1 +1,4 @@
 * @actions/actions-vscode-reviewers
+
+# Owners maintaining https://github.com/actions/runner-images
+/languageservice/src/value-providers/default.ts @actions/runner-images-writers @actions/actions-vscode-reviewers

.github/workflows/buildtest.yml

@@ -14,7 +14,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [18.x, 20.x, 22.x]
+        node-version: [20.x, 22.x, 24.x]
 
     steps:
     - uses: actions/checkout@v4
@@ -37,10 +37,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v4
-    - name: Use Node.js 22.x
+    - name: Use Node.js 24.x
       uses: actions/setup-node@v4
       with:
-        node-version: 22.x
+        node-version: 24.x
         cache: 'npm'
         registry-url: 'https://npm.pkg.github.com'
     - run: npm ci

.github/workflows/publish.yml

@@ -29,7 +29,7 @@ jobs:
 
       - uses: actions/setup-node@v4
         with:
-          node-version: "16"
+          node-version: 24.x
 
       - name: Bump version and push
         run: |

.github/workflows/release.yml

@@ -50,7 +50,9 @@ jobs:
             return true;
 
   release:
-    environment: publish
+    environment:
+      name: publish
+      deployment: false
 
     needs: check-version-change
     if: ${{ needs.check-version-change.outputs.changed == 'true' }}
@@ -59,7 +61,7 @@ jobs:
 
     permissions:
       contents: write
-      packages: write
+      id-token: write
 
     env:
       PKG_VERSION: "" # will be set in the workflow
@@ -69,9 +71,8 @@ jobs:
 
       - uses: actions/setup-node@v4
         with:
-          node-version: 22.x
+          node-version: 24.x
           cache: "npm"
-          scope: '@actions'
 
       - name: Parse version from lerna.json
         run: |
@@ -97,13 +98,6 @@ jobs:
             core.summary.addLink(`Release v${{ env.PKG_VERSION }}`, release.data.html_url);
             await core.summary.write();
 
-      - name: setup authentication
-        run: echo "//registry.npmjs.org/:_authToken=${NPM_TOKEN}" >> .npmrc
-        env:
-          NPM_TOKEN: ${{ secrets.NPM_TOKEN }}
-
       - name: Publish packages
         run: |
-          lerna publish ${{ env.PKG_VERSION }} --yes --no-git-reset --no-git-tag-version
-        env:
-          NPM_TOKEN: ${{ secrets.NPM_TOKEN }}
+          npx lerna publish ${{ env.PKG_VERSION }} --yes --no-git-reset --no-git-tag-version

.prettierignore

@@ -3,4 +3,5 @@ dist
 *.md
 *.js
 *.json
-*.d.ts
+*.d.ts
+/.nx/workspace-data

docs/esm-migration-plan.md

@@ -4,6 +4,27 @@
 
 This document outlines the plan to migrate from TypeScript's deprecated `"moduleResolution": "node"` (node10) to `"moduleResolution": "node16"` or `"nodenext"`. This change is necessary because the published ESM packages have extensionless imports that don't work correctly in modern ESM environments.
 
+## TL;DR - Remaining Work
+
+- [x] expressions - Migrated ✅
+- [x] workflow-parser - Migrated ✅
+- [x] languageservice - Migrated ✅
+- [x] languageserver - Add `.js` extensions to imports ✅
+- [ ] languageserver - Update `tsconfig.build.json` to `moduleResolution: "node16"` (blocked by vscode-languageserver)
+- [ ] languageserver - Upgrade `vscode-languageserver` to stable v10+ when released
+
+**Blocker:** `vscode-languageserver@8.0.2` lacks ESM exports. Stable v10 with `exports` field needed.
+
+### ⚠️ Important: `skipLibCheck: true` Required
+
+All migrated packages use `skipLibCheck: true` in their `tsconfig.build.json`. This works around a TS2386 "Overload signatures must all be optional or required" error in `@types/node/module.d.ts`.
+
+**Why can't we just fix the error?** The error is in `@types/node`, a third-party package maintained by DefinitelyTyped. We can't modify `node_modules`, and upstream fixes take time.
+
+**Is `skipLibCheck` safe?** Yes. It only skips type checking of `.d.ts` files (declaration files from dependencies). Our own `.ts` source files are still fully type-checked. This is a common and recommended workaround for issues in third-party type definitions.
+
+---
+
 ## Issues Fixed
 
 This migration will resolve the following issues:
@@ -199,14 +220,13 @@ src/connection.ts(1,43): error TS2307: Cannot find module 'vscode-languageserver
 
 With `moduleResolution: "node16"`, TypeScript follows Node.js ESM resolution rules which require explicit `exports` for subpath imports like `vscode-languageserver/browser` and `vscode-languageserver/node`.
 
-**Status:** Verified December 2025. Version 9.0.1 is available but ESM export support is not confirmed.
+**Status:** Partial - `.js` extensions added, waiting for stable `vscode-languageserver` release with ESM exports to complete migration.
 
-**Current Decision:** The languageserver package is **deferred** from this migration until the upstream `vscode-languageserver` package adds proper ESM exports. It will continue using the old `moduleResolution: "node"` configuration.
+**Completed:** All relative imports in languageserver source files have been updated to use `.js` extensions. This is compatible with the current `moduleResolution: "node"` and will enable a seamless migration once a stable vscode-languageserver version with ESM exports is available.
 
 **Options to resolve:**
-- Wait for vscode-languageserver to add ESM exports
-- Try upgrading to vscode-languageserver v9.x to see if exports were added
-- Use a bundler to work around the module resolution
+- Wait for stable vscode-languageserver v10+ with ESM exports
+- Use pre-release `vscode-languageserver@10.0.0-next.16` (has proper exports but is unstable)
 - Fork or patch the dependency
 
 ---
@@ -218,7 +238,7 @@ With `moduleResolution: "node16"`, TypeScript follows Node.js ESM resolution rul
 | expressions | 1068 | ✅ Migrated |
 | workflow-parser | 292 | ✅ Migrated |
 | languageservice | 452 | ✅ Migrated |
-| languageserver | 6 files | ⏸️ Deferred (vscode-languageserver lacks ESM exports) |
+| languageserver | 31 | 🔶 Partial (`.js` extensions added, awaiting stable vscode-languageserver) |
 
 ---
 

expressions/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@actions/expressions",
-  "version": "0.3.32",
+  "version": "0.3.47",
   "license": "MIT",
   "type": "module",
   "source": "./src/index.ts",
@@ -44,7 +44,7 @@
     "watch": "tsc --build tsconfig.build.json --watch"
   },
   "engines": {
-    "node": ">= 18"
+    "node": ">= 20"
   },
   "files": [
     "dist/**/*"

expressions/src/completion.ts

@@ -2,6 +2,7 @@ import {DescriptionPair} from "./completion/descriptionDictionary.js";
 import {Dictionary, isDictionary} from "./data/dictionary.js";
 import {ExpressionData} from "./data/expressiondata.js";
 import {Evaluator} from "./evaluator.js";
+import {FeatureFlags} from "./features.js";
 import {wellKnownFunctions} from "./funcs.js";
 import {FunctionDefinition, FunctionInfo} from "./funcs/info.js";
 import {Lexer, Token, TokenType} from "./lexer.js";
@@ -26,13 +27,15 @@ export type CompletionItem = {
  * @param context Context available for the expression
  * @param extensionFunctions List of functions available
  * @param functions Optional map of functions to use during evaluation
+ * @param featureFlags Optional feature flags to control which features are enabled
  * @returns Array of completion items
  */
 export function complete(
   input: string,
   context: Dictionary,
   extensionFunctions: FunctionInfo[],
-  functions?: Map<string, FunctionDefinition>
+  functions?: Map<string, FunctionDefinition>,
+  featureFlags?: FeatureFlags
 ): CompletionItem[] {
   // Lex
   const lexer = new Lexer(input);
@@ -63,7 +66,7 @@ export function complete(
     const result = contextKeys(context);
 
     // Merge with functions
-    result.push(...functionItems(extensionFunctions));
+    result.push(...functionItems(extensionFunctions, featureFlags));
 
     return result;
   }
@@ -88,10 +91,15 @@ export function complete(
   return contextKeys(result);
 }
 
-function functionItems(extensionFunctions: FunctionInfo[]): CompletionItem[] {
+function functionItems(extensionFunctions: FunctionInfo[], featureFlags?: FeatureFlags): CompletionItem[] {
   const result: CompletionItem[] = [];
+  const flags = featureFlags ?? new FeatureFlags();
 
   for (const fdef of [...Object.values(wellKnownFunctions), ...extensionFunctions]) {
+    // Filter out case function if feature is disabled
+    if (fdef.name === "case" && !flags.isEnabled("allowCaseFunction")) {
+      continue;
+    }
     result.push({
       label: fdef.name,
       description: fdef.description,

expressions/src/errors.ts

@@ -12,6 +12,7 @@ export enum ErrorType {
   ErrorExceededMaxLength,
   ErrorTooFewParameters,
   ErrorTooManyParameters,
+  ErrorEvenParameters,
   ErrorUnrecognizedContext,
   ErrorUnrecognizedFunction
 }
@@ -42,6 +43,8 @@ function errorDescription(typ: ErrorType): string {
       return "Too few parameters supplied";
     case ErrorType.ErrorTooManyParameters:
       return "Too many parameters supplied";
+    case ErrorType.ErrorEvenParameters:
+      return "Even number of parameters supplied, requires an odd number of parameters";
     case ErrorType.ErrorUnrecognizedContext:
       return "Unrecognized named-value";
     case ErrorType.ErrorUnrecognizedFunction:

expressions/src/features.test.ts

@@ -0,0 +1,63 @@
+import {FeatureFlags} from "./features.js";
+
+describe("FeatureFlags", () => {
+  describe("isEnabled", () => {
+    it("returns false by default when no options provided", () => {
+      const flags = new FeatureFlags();
+      expect(flags.isEnabled("missingInputsQuickfix")).toBe(false);
+    });
+
+    it("returns false by default when empty options provided", () => {
+      const flags = new FeatureFlags({});
+      expect(flags.isEnabled("missingInputsQuickfix")).toBe(false);
+    });
+
+    it("returns true when feature is explicitly enabled", () => {
+      const flags = new FeatureFlags({missingInputsQuickfix: true});
+      expect(flags.isEnabled("missingInputsQuickfix")).toBe(true);
+    });
+
+    it("returns false when feature is explicitly disabled", () => {
+      const flags = new FeatureFlags({missingInputsQuickfix: false});
+      expect(flags.isEnabled("missingInputsQuickfix")).toBe(false);
+    });
+
+    it("returns true when all is enabled", () => {
+      const flags = new FeatureFlags({all: true});
+      expect(flags.isEnabled("missingInputsQuickfix")).toBe(true);
+    });
+
+    it("explicit feature flag takes precedence over all:true", () => {
+      const flags = new FeatureFlags({all: true, missingInputsQuickfix: false});
+      expect(flags.isEnabled("missingInputsQuickfix")).toBe(false);
+    });
+
+    it("explicit feature flag takes precedence over all:false", () => {
+      const flags = new FeatureFlags({all: false, missingInputsQuickfix: true});
+      expect(flags.isEnabled("missingInputsQuickfix")).toBe(true);
+    });
+  });
+
+  describe("getEnabledFeatures", () => {
+    it("returns empty array when no features enabled", () => {
+      const flags = new FeatureFlags();
+      expect(flags.getEnabledFeatures()).toEqual([]);
+    });
+
+    it("returns enabled features", () => {
+      const flags = new FeatureFlags({missingInputsQuickfix: true});
+      expect(flags.getEnabledFeatures()).toEqual(["missingInputsQuickfix"]);
+    });
+
+    it("returns all features when all is enabled", () => {
+      const flags = new FeatureFlags({all: true});
+      expect(flags.getEnabledFeatures()).toEqual([
+        "missingInputsQuickfix",
+        "blockScalarChompingWarning",
+        "allowCaseFunction",
+        "allowCronTimezone",
+        "allowCopilotRequestsPermission"
+      ]);
+    });
+  });
+});

expressions/src/features.ts

@@ -0,0 +1,97 @@
+/**
+ * Experimental feature flags.
+ *
+ * Individual feature flags take precedence over `all`.
+ * Example: { all: true, missingInputsQuickfix: false } enables all
+ * experimental features EXCEPT missingInputsQuickfix.
+ *
+ * When a feature graduates to stable, its flag becomes a no-op
+ * (the feature will be enabled regardless of the configuration value).
+ */
+export interface ExperimentalFeatures {
+  /**
+   * Enable all experimental features.
+   * Individual feature flags take precedence over this setting.
+   * @default false
+   */
+  all?: boolean;
+
+  /**
+   * Enable quickfix code action for missing required action inputs.
+   * @default false
+   */
+  missingInputsQuickfix?: boolean;
+
+  /**
+   * Warn when block scalars (| or >) use implicit clip chomping,
+   * which adds a trailing newline that may be unintentional.
+   * @default false
+   */
+  blockScalarChompingWarning?: boolean;
+
+  /**
+   * Enable the case() function in expressions.
+   * @default false
+   */
+  allowCaseFunction?: boolean;
+
+  /**
+   * Enable the timezone input in cron schedule mappings.
+   * @default false
+   */
+  allowCronTimezone?: boolean;
+
+  /**
+   * Enable the copilot-requests permission in workflow permissions.
+   * @default false
+   */
+  allowCopilotRequestsPermission?: boolean;
+}
+
+/**
+ * Keys of ExperimentalFeatures that represent actual features (excludes 'all')
+ */
+export type ExperimentalFeatureKey = Exclude<keyof ExperimentalFeatures, "all">;
+
+/**
+ * All known experimental feature keys.
+ * This list must be kept in sync with the ExperimentalFeatures interface.
+ */
+const allFeatureKeys: ExperimentalFeatureKey[] = [
+  "missingInputsQuickfix",
+  "blockScalarChompingWarning",
+  "allowCaseFunction",
+  "allowCronTimezone",
+  "allowCopilotRequestsPermission"
+];
+
+export class FeatureFlags {
+  private readonly features: ExperimentalFeatures;
+
+  constructor(features?: ExperimentalFeatures) {
+    this.features = features ?? {};
+  }
+
+  /**
+   * Check if an experimental feature is enabled.
+   *
+   * Resolution order:
+   * 1. Explicit feature flag (if set)
+   * 2. `all` flag (if set)
+   * 3. false (default)
+   */
+  isEnabled(feature: ExperimentalFeatureKey): boolean {
+    const explicit = this.features[feature];
+    if (explicit !== undefined) {
+      return explicit;
+    }
+    return this.features.all ?? false;
+  }
+
+  /**
+   * Returns list of all enabled experimental features.
+   */
+  getEnabledFeatures(): ExperimentalFeatureKey[] {
+    return allFeatureKeys.filter(key => this.isEnabled(key));
+  }
+}

expressions/src/funcs.ts

@@ -1,4 +1,5 @@
 import {ErrorType, ExpressionError} from "./errors.js";
+import {caseFunc} from "./funcs/case.js";
 import {contains} from "./funcs/contains.js";
 import {endswith} from "./funcs/endswith.js";
 import {format} from "./funcs/format.js";
@@ -16,6 +17,7 @@ export type ParseContext = {
 };
 
 export const wellKnownFunctions: {[name: string]: FunctionDefinition} = {
+  case: caseFunc,
   contains: contains,
   endswith: endswith,
   format: format,
@@ -53,4 +55,9 @@ export function validateFunction(context: ParseContext, identifier: Token, argCo
   if (argCount > f.maxArgs) {
     throw new ExpressionError(ErrorType.ErrorTooManyParameters, identifier);
   }
+
+  // case function requires an odd number of arguments
+  if (name === "case" && argCount % 2 === 0) {
+    throw new ExpressionError(ErrorType.ErrorEvenParameters, identifier);
+  }
 }

expressions/src/funcs/case.ts

@@ -0,0 +1,29 @@
+import {ExpressionData, Kind} from "../data/index.js";
+import {FunctionDefinition} from "./info.js";
+
+export const caseFunc: FunctionDefinition = {
+  name: "case",
+  description:
+    "`case( pred1, val1, pred2, val2, ..., default )`\n\nEvaluates predicates in order and returns the value corresponding to the first predicate that evaluates to `true`. If no predicate matches, it returns the last argument as the default value.",
+  minArgs: 3,
+  maxArgs: Number.MAX_SAFE_INTEGER,
+  call: (...args: ExpressionData[]): ExpressionData => {
+    // Evaluate predicate-result pairs
+    for (let i = 0; i < args.length - 1; i += 2) {
+      const predicate = args[i];
+
+      // Predicate must be a boolean
+      if (predicate.kind !== Kind.Boolean) {
+        throw new Error("case predicate must evaluate to a boolean value");
+      }
+
+      // If predicate is true, return the corresponding result
+      if (predicate.value) {
+        return args[i + 1];
+      }
+    }
+
+    // No predicate matched, return default (last argument)
+    return args[args.length - 1];
+  }
+};

expressions/src/index.ts

@@ -4,6 +4,7 @@ export {DescriptionDictionary, DescriptionPair, isDescriptionDictionary} from ".
 export * as data from "./data/index.js";
 export {ExpressionError, ExpressionEvaluationError} from "./errors.js";
 export {Evaluator} from "./evaluator.js";
+export {ExperimentalFeatureKey, ExperimentalFeatures, FeatureFlags} from "./features.js";
 export {wellKnownFunctions} from "./funcs.js";
 export {Lexer, Result} from "./lexer.js";
 export {Parser} from "./parser.js";

expressions/testdata/case.json

@@ -0,0 +1,157 @@
+{
+  "case": [
+    {
+      "expr": "case(true, 'first', 'default')",
+      "result": { "kind": "String", "value": "first" }
+    },
+    {
+      "expr": "case(false, 'first', 'default')",
+      "result": { "kind": "String", "value": "default" }
+    },
+    {
+      "expr": "case(true, 'first', false, 'second', 'default')",
+      "result": { "kind": "String", "value": "first" }
+    },
+    {
+      "expr": "case(false, 'first', true, 'second', 'default')",
+      "result": { "kind": "String", "value": "second" }
+    },
+    {
+      "expr": "case(false, 'first', false, 'second', 'default')",
+      "result": { "kind": "String", "value": "default" }
+    },
+    {
+      "expr": "case(1 == 1, 'equal', 'not equal')",
+      "result": { "kind": "String", "value": "equal" }
+    },
+    {
+      "expr": "case(1 == 2, 'equal', 'not equal')",
+      "result": { "kind": "String", "value": "not equal" }
+    },
+    {
+      "expr": "case(github.ref == 'refs/heads/main', 'main', github.event_name == 'pull_request', 'pr', 'other')",
+      "contexts": {
+        "github": {
+          "ref": "refs/heads/main",
+          "event_name": "push"
+        }
+      },
+      "result": { "kind": "String", "value": "main" }
+    },
+    {
+      "expr": "case(github.ref == 'refs/heads/main', 'main', github.event_name == 'pull_request', 'pr', 'other')",
+      "contexts": {
+        "github": {
+          "ref": "refs/heads/develop",
+          "event_name": "pull_request"
+        }
+      },
+      "result": { "kind": "String", "value": "pr" }
+    },
+    {
+      "expr": "case(github.ref == 'refs/heads/main', 'main', github.event_name == 'pull_request', 'pr', 'other')",
+      "contexts": {
+        "github": {
+          "ref": "refs/heads/develop",
+          "event_name": "push"
+        }
+      },
+      "result": { "kind": "String", "value": "other" }
+    },
+    {
+      "expr": "case(true, 123, 456)",
+      "result": { "kind": "Number", "value": 123 }
+    },
+    {
+      "expr": "case(false, 123, 456)",
+      "result": { "kind": "Number", "value": 456 }
+    },
+    {
+      "expr": "case(github.event == 'pull_request', 0, 1)",
+      "contexts": {
+        "github": {
+          "event": "pull_request"
+        }
+      },
+      "result": { "kind": "Number", "value": 0 }
+    },
+    {
+      "expr": "case(false, 0, 1)",
+      "result": { "kind": "Number", "value": 1 }
+    },
+    {
+      "expr": "case(true, false, true)",
+      "result": { "kind": "Boolean", "value": false }
+    },
+    {
+      "expr": "case(false, false, true)",
+      "result": { "kind": "Boolean", "value": true }
+    },
+    {
+      "expr": "case(true, '', 'default')",
+      "result": { "kind": "String", "value": "" }
+    },
+    {
+      "expr": "case(false, 'first', '')",
+      "result": { "kind": "String", "value": "" }
+    },
+    {
+      "expr": "case(true, fromJSON('[1,2,3]'), 'default')",
+      "result": { "kind": "Array", "value": [1, 2, 3] }
+    },
+    {
+      "expr": "case(true, fromJSON('{\"key\":\"value\"}'), 'default')",
+      "result": { "kind": "Object", "value": { "key": "value" } }
+    },
+    {
+      "expr": "case(false, 'first', false, 'second', false, 'third', false, 'fourth', 'default')",
+      "result": { "kind": "String", "value": "default" }
+    },
+    {
+      "expr": "case(false, 'first', false, 'second', true, 'third', false, 'fourth', 'default')",
+      "result": { "kind": "String", "value": "third" }
+    },
+    {
+      "expr": "case('not a boolean', 'first', 'default')",
+      "err": {
+        "kind": "evaluation",
+        "value": "case predicate must evaluate to a boolean value"
+      }
+    },
+    {
+      "expr": "case(1, 'first', 'default')",
+      "err": {
+        "kind": "evaluation",
+        "value": "case predicate must evaluate to a boolean value"
+      }
+    },
+    {
+      "expr": "case(null, 'first', 'default')",
+      "err": {
+        "kind": "evaluation",
+        "value": "case predicate must evaluate to a boolean value"
+      }
+    },
+    {
+      "expr": "case(fromJSON('[]'), 'first', 'default')",
+      "err": {
+        "kind": "evaluation",
+        "value": "case predicate must evaluate to a boolean value"
+      }
+    },
+    {
+      "expr": "case(fromJSON('{}'), 'first', 'default')",
+      "err": {
+        "kind": "evaluation",
+        "value": "case predicate must evaluate to a boolean value"
+      }
+    },
+    {
+      "expr": "case(true, 'first', false, 'second')",
+      "err": {
+        "kind": "parsing",
+        "value": "Even number of parameters supplied, requires an odd number of parameters: 'case'. Located at position 1 within expression: case(true, 'first', false, 'second')"
+      }
+    }
+  ]
+}

languageserver/README.md

@@ -84,6 +84,11 @@ export interface InitializationOptions {
    * Desired log level
    */
   logLevel?: LogLevel;
+
+  /**
+   * Experimental features that are opt-in
+   */
+  experimentalFeatures?: ExperimentalFeatures;
 }
 ```
 
@@ -100,6 +105,33 @@ const clientOptions: LanguageClientOptions = {
 const client = new LanguageClient("actions-language", "GitHub Actions Language Server", serverOptions, clientOptions);
 ```
 
+### Experimental Features
+
+The language server supports opt-in experimental features via the `experimentalFeatures` initialization option. These features may change or be removed in between releases.
+
+```typescript
+initializationOptions: {
+  experimentalFeatures: {
+    // Enable all experimental features
+    all: true,
+
+    // Or enable specific features
+    missingInputsQuickfix: true,
+  }
+}
+```
+
+**Available experimental features:**
+
+| Feature | Description |
+|---------|-------------|
+| `missingInputsQuickfix` | Code action to add missing required inputs for actions |
+| `blockScalarChompingWarning` | Warn when block scalars (`\|` or `>`) use implicit clip chomping, which adds a trailing newline that may be unintentional |
+
+Individual feature flags take precedence over `all`. For example, `{ all: true, missingInputsQuickfix: false }` enables all experimental features except `missingInputsQuickfix`.
+
+When a feature graduates to stable, its flag becomes a no-op and the feature will be enabled regardless of the configuration value.
+
 ### Standalone CLI
 
 After installing globally, you can run the language server directly:

languageserver/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@actions/languageserver",
-  "version": "0.3.32",
+  "version": "0.3.47",
   "description": "Language server for GitHub Actions",
   "license": "MIT",
   "type": "module",
@@ -48,8 +48,8 @@
     "actions-languageserver": "./bin/actions-languageserver"
   },
   "dependencies": {
-    "@actions/languageservice": "^0.3.32",
-    "@actions/workflow-parser": "^0.3.32",
+    "@actions/languageservice": "^0.3.47",
+    "@actions/workflow-parser": "^0.3.47",
     "@octokit/rest": "^21.1.1",
     "@octokit/types": "^9.0.0",
     "vscode-languageserver": "^8.0.2",
@@ -57,7 +57,7 @@
     "yaml": "^2.1.3"
   },
   "engines": {
-    "node": ">= 18"
+    "node": ">= 20"
   },
   "files": [
     "dist/**/*",
@@ -73,9 +73,10 @@
     "eslint-plugin-prettier": "^4.2.1",
     "fetch-mock": "^9.11.0",
     "jest": "^29.0.3",
+    "node-fetch": "^2.6.7",
     "prettier": "^2.8.3",
     "rimraf": "^3.0.2",
     "ts-jest": "^29.0.3",
-    "typescript": "^4.8.4"
+    "typescript": "^5.8.3"
   }
 }

languageserver/src/connection.ts

@@ -1,8 +1,18 @@
-import {documentLinks, getInlayHints, hover, validate, ValidationConfig} from "@actions/languageservice";
+import {
+  documentLinks,
+  getCodeActions,
+  getInlayHints,
+  hover,
+  validate,
+  ValidationConfig
+} from "@actions/languageservice";
 import {registerLogger, setLogLevel} from "@actions/languageservice/log";
 import {clearCache, clearCacheEntry} from "@actions/languageservice/utils/workflow-cache";
 import {Octokit} from "@octokit/rest";
 import {
+  CodeAction,
+  CodeActionKind,
+  CodeActionParams,
   CompletionItem,
   Connection,
   DocumentLink,
@@ -20,18 +30,19 @@ import {
   TextDocumentSyncKind
 } from "vscode-languageserver";
 import {TextDocument} from "vscode-languageserver-textdocument";
-import {getClient} from "./client";
-import {Commands} from "./commands";
-import {contextProviders} from "./context-providers";
-import {descriptionProvider} from "./description-provider";
-import {getFileProvider} from "./file-provider";
-import {InitializationOptions, RepositoryContext} from "./initializationOptions";
-import {onCompletion} from "./on-completion";
-import {ReadFileRequest, Requests} from "./request";
-import {getActionsMetadataProvider} from "./utils/action-metadata";
-import {TTLCache} from "./utils/cache";
-import {timeOperation} from "./utils/timer";
-import {valueProviders} from "./value-providers";
+import {getClient} from "./client.js";
+import {Commands} from "./commands.js";
+import {contextProviders} from "./context-providers.js";
+import {descriptionProvider} from "./description-provider.js";
+import {FeatureFlags} from "@actions/expressions";
+import {getFileProvider} from "./file-provider.js";
+import {InitializationOptions, RepositoryContext} from "./initializationOptions.js";
+import {onCompletion} from "./on-completion.js";
+import {ReadFileRequest, Requests} from "./request.js";
+import {getActionsMetadataProvider} from "./utils/action-metadata.js";
+import {TTLCache} from "./utils/cache.js";
+import {timeOperation} from "./utils/timer.js";
+import {valueProviders} from "./value-providers.js";
 
 export function initConnection(connection: Connection) {
   const documents: TextDocuments<TextDocument> = new TextDocuments(TextDocument);
@@ -41,6 +52,7 @@ export function initConnection(connection: Connection) {
   const cache = new TTLCache();
 
   let hasWorkspaceFolderCapability = false;
+  let featureFlags = new FeatureFlags();
 
   // Register remote console logger with language service
   registerLogger(connection.console);
@@ -64,6 +76,8 @@ export function initConnection(connection: Connection) {
       setLogLevel(options.logLevel);
     }
 
+    featureFlags = new FeatureFlags(options.experimentalFeatures);
+
     const result: InitializeResult = {
       capabilities: {
         textDocumentSync: TextDocumentSyncKind.Full,
@@ -75,7 +89,10 @@ export function initConnection(connection: Connection) {
         documentLinkProvider: {
           resolveProvider: false
         },
-        inlayHintProvider: true
+        inlayHintProvider: true,
+        codeActionProvider: {
+          codeActionKinds: [CodeActionKind.QuickFix]
+        }
       }
     };
 
@@ -91,6 +108,11 @@ export function initConnection(connection: Connection) {
   });
 
   connection.onInitialized(() => {
+    const enabledFeatures = featureFlags.getEnabledFeatures();
+    if (enabledFeatures.length > 0) {
+      connection.console.info(`Experimental features enabled: ${enabledFeatures.join(", ")}`);
+    }
+
     if (hasWorkspaceFolderCapability) {
       connection.workspace.onDidChangeWorkspaceFolders(() => {
         clearCache();
@@ -114,7 +136,8 @@ export function initConnection(connection: Connection) {
       actionsMetadataProvider: getActionsMetadataProvider(client, cache),
       fileProvider: getFileProvider(client, cache, repoContext?.workspaceUri, async path => {
         return await connection.sendRequest(Requests.ReadFile, {path} satisfies ReadFileRequest);
-      })
+      }),
+      featureFlags
     };
 
     const result = await validate(textDocument, config);
@@ -131,7 +154,8 @@ export function initConnection(connection: Connection) {
           getDocument(documents, textDocument),
           client,
           repos.find(repo => textDocument.uri.startsWith(repo.workspaceUri)),
-          cache
+          cache,
+          featureFlags
         )
     );
   });
@@ -167,6 +191,17 @@ export function initConnection(connection: Connection) {
     });
   });
 
+  connection.onCodeAction((params: CodeActionParams): CodeAction[] => {
+    const document = getDocument(documents, params.textDocument);
+    return getCodeActions({
+      uri: params.textDocument.uri,
+      documentContent: document.getText(),
+      diagnostics: params.context.diagnostics,
+      only: params.context.only,
+      featureFlags
+    });
+  });
+
   // Make the text document manager listen on the connection
   // for open, change and close text document events
   documents.listen(connection);

languageserver/src/context-providers.test.ts

@@ -1,9 +1,9 @@
 import {data, DescriptionDictionary} from "@actions/expressions";
 import {WorkflowContext} from "@actions/languageservice/context/workflow-context";
 import {Mode} from "@actions/languageservice/context-providers/default";
-import {contextProviders} from "./context-providers";
-import {RepositoryContext} from "./initializationOptions";
-import {TTLCache} from "./utils/cache";
+import {contextProviders} from "./context-providers.js";
+import {RepositoryContext} from "./initializationOptions.js";
+import {TTLCache} from "./utils/cache.js";
 
 describe("contextProviders", () => {
   const mockCache = new TTLCache();

languageserver/src/context-providers.ts

@@ -3,11 +3,11 @@ import {ContextProviderConfig} from "@actions/languageservice";
 import {Mode} from "@actions/languageservice/context-providers/default";
 import {WorkflowContext} from "@actions/languageservice/context/workflow-context";
 import {Octokit} from "@octokit/rest";
-import {getSecrets} from "./context-providers/secrets";
-import {getStepsContext} from "./context-providers/steps";
-import {getVariables} from "./context-providers/variables";
-import {RepositoryContext} from "./initializationOptions";
-import {TTLCache} from "./utils/cache";
+import {getSecrets} from "./context-providers/secrets.js";
+import {getStepsContext} from "./context-providers/steps.js";
+import {getVariables} from "./context-providers/variables.js";
+import {RepositoryContext} from "./initializationOptions.js";
+import {TTLCache} from "./utils/cache.js";
 
 export function contextProviders(
   client: Octokit | undefined,

languageserver/src/context-providers/action-outputs.ts

@@ -1,7 +1,7 @@
 import {ActionOutputs, ActionReference} from "@actions/languageservice/action";
 import {Octokit} from "@octokit/rest";
-import {fetchActionMetadata} from "../utils/action-metadata";
-import {TTLCache} from "../utils/cache";
+import {fetchActionMetadata} from "../utils/action-metadata.js";
+import {TTLCache} from "../utils/cache.js";
 
 export async function getActionOutputs(
   octokit: Octokit,

languageserver/src/context-providers/secrets.ts

@@ -6,10 +6,10 @@ import {warn} from "@actions/languageservice/log";
 import {isMapping, isString} from "@actions/workflow-parser";
 import {Octokit} from "@octokit/rest";
 
-import {RepositoryContext} from "../initializationOptions";
-import {TTLCache} from "../utils/cache";
-import {errorStatus} from "../utils/error";
-import {getRepoPermission} from "../utils/repo-permission";
+import {RepositoryContext} from "../initializationOptions.js";
+import {TTLCache} from "../utils/cache.js";
+import {errorStatus} from "../utils/error.js";
+import {getRepoPermission} from "../utils/repo-permission.js";
 
 export async function getSecrets(
   workflowContext: WorkflowContext,

languageserver/src/context-providers/steps.test.ts

@@ -3,9 +3,9 @@ import {getStepsContext as getDefaultStepsContext} from "@actions/languageservic
 import {Octokit} from "@octokit/rest";
 import fetchMock from "fetch-mock";
 
-import {createWorkflowContext} from "../test-utils/workflow-context";
-import {TTLCache} from "../utils/cache";
-import {getStepsContext} from "./steps";
+import {createWorkflowContext} from "../test-utils/workflow-context.js";
+import {TTLCache} from "../utils/cache.js";
+import {getStepsContext} from "./steps.js";
 
 const workflow = `
 name: Caching Primes

languageserver/src/context-providers/steps.ts

@@ -3,8 +3,8 @@ import {parseActionReference} from "@actions/languageservice/action";
 import {WorkflowContext} from "@actions/languageservice/context/workflow-context";
 import {isActionStep} from "@actions/workflow-parser/model/type-guards";
 import {Octokit} from "@octokit/rest";
-import {TTLCache} from "../utils/cache";
-import {getActionOutputs} from "./action-outputs";
+import {TTLCache} from "../utils/cache.js";
+import {getActionOutputs} from "./action-outputs.js";
 
 export async function getStepsContext(
   octokit: Octokit,

languageserver/src/context-providers/variables.ts

@@ -7,10 +7,10 @@ import {isMapping, isString} from "@actions/workflow-parser";
 import {Octokit} from "@octokit/rest";
 import {RequestError} from "@octokit/request-error";
 
-import {RepositoryContext} from "../initializationOptions";
-import {TTLCache} from "../utils/cache";
-import {errorStatus} from "../utils/error";
-import {getRepoPermission} from "../utils/repo-permission";
+import {RepositoryContext} from "../initializationOptions.js";
+import {TTLCache} from "../utils/cache.js";
+import {errorStatus} from "../utils/error.js";
+import {getRepoPermission} from "../utils/repo-permission.js";
 
 export async function getVariables(
   workflowContext: WorkflowContext,

languageserver/src/description-provider.ts

@@ -1,8 +1,8 @@
 import {DescriptionProvider} from "@actions/languageservice/hover";
 import {Octokit} from "@octokit/rest";
-import {getActionDescription} from "./description-providers/action-description";
-import {getActionInputDescription} from "./description-providers/action-input";
-import {TTLCache} from "./utils/cache";
+import {getActionDescription} from "./description-providers/action-description.js";
+import {getActionInputDescription} from "./description-providers/action-input.js";
+import {TTLCache} from "./utils/cache.js";
 
 export function descriptionProvider(client: Octokit | undefined, cache: TTLCache): DescriptionProvider {
   const getDescription: DescriptionProvider["getDescription"] = async (context, token, path) => {

languageserver/src/description-providers/action-description.test.ts

@@ -1,9 +1,9 @@
 import {Octokit} from "@octokit/rest";
 import fetchMock from "fetch-mock";
-import {createWorkflowContext} from "../test-utils/workflow-context";
-import {TTLCache} from "../utils/cache";
-import {getActionDescription} from "./action-description";
-import {actionsCheckoutMetadata} from "../test-utils/action-metadata";
+import {createWorkflowContext} from "../test-utils/workflow-context.js";
+import {TTLCache} from "../utils/cache.js";
+import {getActionDescription} from "./action-description.js";
+import {actionsCheckoutMetadata} from "../test-utils/action-metadata.js";
 
 const workflow = `
 name: Hello World

languageserver/src/description-providers/action-description.ts

@@ -2,8 +2,8 @@ import {actionUrl, parseActionReference} from "@actions/languageservice/action";
 import {isActionStep} from "@actions/workflow-parser/model/type-guards";
 import {Step} from "@actions/workflow-parser/model/workflow-template";
 import {Octokit} from "@octokit/rest";
-import {fetchActionMetadata} from "../utils/action-metadata";
-import {TTLCache} from "../utils/cache";
+import {fetchActionMetadata} from "../utils/action-metadata.js";
+import {TTLCache} from "../utils/cache.js";
 
 export async function getActionDescription(client: Octokit, cache: TTLCache, step: Step): Promise<string | undefined> {
   if (!isActionStep(step)) {

languageserver/src/description-providers/action-input.test.ts

@@ -2,10 +2,10 @@ import {StringToken} from "@actions/workflow-parser/templates/tokens/string-toke
 import {Octokit} from "@octokit/rest";
 import fetchMock from "fetch-mock";
 
-import {actionsCheckoutMetadata} from "../test-utils/action-metadata";
-import {createWorkflowContext} from "../test-utils/workflow-context";
-import {TTLCache} from "../utils/cache";
-import {getActionInputDescription} from "./action-input";
+import {actionsCheckoutMetadata} from "../test-utils/action-metadata.js";
+import {createWorkflowContext} from "../test-utils/workflow-context.js";
+import {TTLCache} from "../utils/cache.js";
+import {getActionInputDescription} from "./action-input.js";
 
 const workflow = `
 name: Hello World

languageserver/src/description-providers/action-input.ts

@@ -4,8 +4,8 @@ import {isActionStep} from "@actions/workflow-parser/model/type-guards";
 import {Step} from "@actions/workflow-parser/model/workflow-template";
 import {TemplateToken} from "@actions/workflow-parser/templates/tokens/template-token";
 import {Octokit} from "@octokit/rest";
-import {fetchActionMetadata} from "../utils/action-metadata";
-import {TTLCache} from "../utils/cache";
+import {fetchActionMetadata} from "../utils/action-metadata.js";
+import {TTLCache} from "../utils/cache.js";
 
 export async function getActionInputDescription(
   client: Octokit,

languageserver/src/file-provider.ts

@@ -2,7 +2,7 @@ import {File} from "@actions/workflow-parser/workflows/file";
 import {FileProvider} from "@actions/workflow-parser/workflows/file-provider";
 import {fileIdentifier} from "@actions/workflow-parser/workflows/file-reference";
 import {Octokit} from "@octokit/rest";
-import {TTLCache} from "./utils/cache";
+import {TTLCache} from "./utils/cache.js";
 import * as vscodeURI from "vscode-uri";
 
 export function getFileProvider(

languageserver/src/index.ts

@@ -6,7 +6,7 @@ import {
 } from "vscode-languageserver/browser";
 import {createConnection as createNodeConnection} from "vscode-languageserver/node";
 
-import {initConnection} from "./connection";
+import {initConnection} from "./connection.js";
 
 /** Helper function determining whether we are executing with node runtime */
 function isNode(): boolean {

languageserver/src/initializationOptions.ts

@@ -1,3 +1,4 @@
+import {ExperimentalFeatures} from "@actions/expressions";
 import {LogLevel} from "@actions/languageservice/log";
 export {LogLevel} from "@actions/languageservice/log";
 
@@ -28,6 +29,12 @@ export interface InitializationOptions {
    * If a GitHub Enterprise Server should be used, the URL of the API endpoint, eg "https://ghe.my-company.com/api/v3"
    */
   gitHubApiUrl?: string;
+
+  /**
+   * Experimental features that are opt-in.
+   * Features listed here may change or be removed without notice.
+   */
+  experimentalFeatures?: ExperimentalFeatures;
 }
 
 export interface RepositoryContext {

languageserver/src/on-completion.ts

@@ -1,13 +1,14 @@
 import {complete} from "@actions/languageservice/complete";
+import type {FeatureFlags} from "@actions/expressions";
 import {Octokit} from "@octokit/rest";
 import {CompletionItem, Connection, Position} from "vscode-languageserver";
 import {TextDocument} from "vscode-languageserver-textdocument";
-import {contextProviders} from "./context-providers";
-import {getFileProvider} from "./file-provider";
-import {RepositoryContext} from "./initializationOptions";
-import {Requests} from "./request";
-import {TTLCache} from "./utils/cache";
-import {valueProviders} from "./value-providers";
+import {contextProviders} from "./context-providers.js";
+import {getFileProvider} from "./file-provider.js";
+import {RepositoryContext} from "./initializationOptions.js";
+import {Requests} from "./request.js";
+import {TTLCache} from "./utils/cache.js";
+import {valueProviders} from "./value-providers.js";
 
 export async function onCompletion(
   connection: Connection,
@@ -15,11 +16,13 @@ export async function onCompletion(
   document: TextDocument,
   client: Octokit | undefined,
   repoContext: RepositoryContext | undefined,
-  cache: TTLCache
+  cache: TTLCache,
+  featureFlags?: FeatureFlags
 ): Promise<CompletionItem[]> {
   return await complete(document, position, {
     valueProviderConfig: repoContext && valueProviders(client, repoContext, cache),
     contextProviderConfig: repoContext && contextProviders(client, repoContext, cache),
+    featureFlags,
     fileProvider: getFileProvider(client, cache, repoContext?.workspaceUri, async path => {
       return await connection.sendRequest(Requests.ReadFile, {path});
     })

languageserver/src/utils/action-metadata.test.ts

@@ -1,7 +1,7 @@
 import {Octokit} from "@octokit/rest";
 import fetchMock from "fetch-mock";
-import {fetchActionMetadata} from "./action-metadata";
-import {TTLCache} from "./cache";
+import {fetchActionMetadata} from "./action-metadata.js";
+import {TTLCache} from "./cache.js";
 
 // A simplified version of the action.yml file from actions/checkout
 const actionMetadataContent = `

languageserver/src/utils/action-metadata.ts

@@ -3,8 +3,8 @@ import {ActionsMetadataProvider} from "@actions/languageservice";
 import {error} from "@actions/languageservice/log";
 import {Octokit, RestEndpointMethodTypes} from "@octokit/rest";
 import {parse} from "yaml";
-import {TTLCache} from "./cache";
-import {errorMessage, errorStatus} from "./error";
+import {TTLCache} from "./cache.js";
+import {errorMessage, errorStatus} from "./error.js";
 
 export function getActionsMetadataProvider(
   client: Octokit | undefined,

languageserver/src/utils/repo-permission.ts

@@ -1,9 +1,9 @@
 import {error} from "@actions/languageservice/log";
 import {Octokit} from "@octokit/rest";
-import {RepositoryContext} from "../initializationOptions";
-import {TTLCache} from "./cache";
-import {errorStatus} from "./error";
-import {getUsername} from "./username";
+import {RepositoryContext} from "../initializationOptions.js";
+import {TTLCache} from "./cache.js";
+import {errorStatus} from "./error.js";
+import {getUsername} from "./username.js";
 
 export type RepoPermission = "admin" | "write" | "read" | "none";
 

languageserver/src/utils/username.ts

@@ -1,5 +1,5 @@
 import {Octokit} from "@octokit/rest";
-import {TTLCache} from "./cache";
+import {TTLCache} from "./cache.js";
 
 export async function getUsername(octokit: Octokit, cache: TTLCache): Promise<string> {
   return await cache.get(`/username`, undefined, () => fetchUsername(octokit));

languageserver/src/value-providers.ts

@@ -2,11 +2,11 @@ import {ValueProviderConfig} from "@actions/languageservice";
 import {WorkflowContext} from "@actions/languageservice/context/workflow-context";
 import {ValueProviderKind} from "@actions/languageservice/value-providers/config";
 import {Octokit} from "@octokit/rest";
-import {RepositoryContext} from "./initializationOptions";
-import {TTLCache} from "./utils/cache";
-import {getActionInputValues} from "./value-providers/action-inputs";
-import {getEnvironments} from "./value-providers/job-environment";
-import {getRunnerLabels} from "./value-providers/runs-on";
+import {RepositoryContext} from "./initializationOptions.js";
+import {TTLCache} from "./utils/cache.js";
+import {getActionInputValues} from "./value-providers/action-inputs.js";
+import {getEnvironments} from "./value-providers/job-environment.js";
+import {getRunnerLabels} from "./value-providers/runs-on.js";
 
 export function valueProviders(
   client: Octokit | undefined,

languageserver/src/value-providers/action-inputs.ts

@@ -3,8 +3,8 @@ import {WorkflowContext} from "@actions/languageservice/context/workflow-context
 import {Value} from "@actions/languageservice/value-providers/config";
 import {isActionStep} from "@actions/workflow-parser/model/type-guards";
 import {Octokit} from "@octokit/rest";
-import {fetchActionMetadata} from "../utils/action-metadata";
-import {TTLCache} from "../utils/cache";
+import {fetchActionMetadata} from "../utils/action-metadata.js";
+import {TTLCache} from "../utils/cache.js";
 
 export async function getActionInputs(
   client: Octokit,

languageserver/src/value-providers/job-environment.ts

@@ -1,6 +1,6 @@
 import {Value} from "@actions/languageservice/value-providers/config";
 import {Octokit} from "@octokit/rest";
-import {TTLCache} from "../utils/cache";
+import {TTLCache} from "../utils/cache.js";
 
 export async function getEnvironments(client: Octokit, cache: TTLCache, owner: string, name: string): Promise<Value[]> {
   const environments = await cache.get(`${owner}/${name}/environments`, undefined, () =>

languageserver/src/value-providers/runs-on.ts

@@ -2,8 +2,8 @@ import {log} from "@actions/languageservice/log";
 import {Value} from "@actions/languageservice/value-providers/config";
 import {DEFAULT_RUNNER_LABELS} from "@actions/languageservice/value-providers/default";
 import {Octokit} from "@octokit/rest";
-import {TTLCache} from "../utils/cache";
-import {errorMessage} from "../utils/error";
+import {TTLCache} from "../utils/cache.js";
+import {errorMessage} from "../utils/error.js";
 
 // Limitation: getRunnerLabels returns default hosted labels and labels for repository self-hosted runners.
 // It doesn't return labels for organization runners visible to the repository.

languageserver/tsconfig.build.json

@@ -5,6 +5,7 @@
     "declaration": true,
     "declarationMap": true,
     "noEmit": false,
-    "outDir": "./dist"
+    "outDir": "./dist",
+    "skipLibCheck": true
   }
 }

languageservice/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@actions/languageservice",
-  "version": "0.3.32",
+  "version": "0.3.47",
   "description": "Language service for GitHub Actions",
   "license": "MIT",
   "type": "module",
@@ -47,15 +47,15 @@
     "watch": "tsc --build tsconfig.build.json --watch"
   },
   "dependencies": {
-    "@actions/expressions": "^0.3.32",
-    "@actions/workflow-parser": "^0.3.32",
+    "@actions/expressions": "^0.3.47",
+    "@actions/workflow-parser": "^0.3.47",
     "vscode-languageserver-textdocument": "^1.0.7",
     "vscode-languageserver-types": "^3.17.2",
     "vscode-uri": "^3.0.8",
     "yaml": "^2.1.1"
   },
   "engines": {
-    "node": ">= 18"
+    "node": ">= 20"
   },
   "files": [
     "dist/**/*"
@@ -74,6 +74,6 @@
     "rimraf": "^3.0.2",
     "ts-jest": "^29.0.3",
     "ts-node": "^10.9.1",
-    "typescript": "^4.8.4"
+    "typescript": "^5.8.3"
   }
 }

languageservice/src/code-actions/code-actions.ts

@@ -0,0 +1,55 @@
+import {FeatureFlags} from "@actions/expressions";
+import {CodeAction, CodeActionKind, Diagnostic} from "vscode-languageserver-types";
+import {CodeActionContext, CodeActionProvider} from "./types.js";
+import {getQuickfixProviders} from "./quickfix/quickfix-providers.js";
+
+export interface CodeActionParams {
+  uri: string;
+  documentContent: string;
+  diagnostics: Diagnostic[];
+  only?: string[];
+  featureFlags?: FeatureFlags;
+}
+
+export function getCodeActions(params: CodeActionParams): CodeAction[] {
+  const actions: CodeAction[] = [];
+  const context: CodeActionContext = {
+    uri: params.uri,
+    documentContent: params.documentContent,
+    featureFlags: params.featureFlags
+  };
+
+  // Build providers map based on feature flags
+  const providersByKind: Map<string, CodeActionProvider[]> = new Map([
+    [CodeActionKind.QuickFix, getQuickfixProviders(params.featureFlags)]
+    // [CodeActionKind.Refactor, getRefactorProviders(params.featureFlags)],
+    // [CodeActionKind.Source, getSourceProviders(params.featureFlags)],
+    // etc
+  ]);
+
+  // Filter to requested kinds, or use all if none specified
+  const requestedKinds = params.only;
+  const kindsToCheck = requestedKinds
+    ? [...providersByKind.keys()].filter(kind => requestedKinds.some(requested => kind.startsWith(requested)))
+    : [...providersByKind.keys()];
+
+  for (const diagnostic of params.diagnostics) {
+    for (const kind of kindsToCheck) {
+      const providers = providersByKind.get(kind) ?? [];
+      for (const provider of providers) {
+        if (provider.diagnosticCodes.includes(diagnostic.code)) {
+          const action = provider.createCodeAction(context, diagnostic);
+          if (action) {
+            action.kind = kind;
+            action.diagnostics = [diagnostic];
+            actions.push(action);
+          }
+        }
+      }
+    }
+  }
+
+  return actions;
+}
+
+export type {CodeActionContext, CodeActionProvider} from "./types.js";

languageservice/src/code-actions/quickfix/add-missing-inputs.ts

@@ -0,0 +1,245 @@
+import {isMapping} from "@actions/workflow-parser";
+import {MappingToken} from "@actions/workflow-parser/templates/tokens/mapping-token";
+import {ScalarToken} from "@actions/workflow-parser/templates/tokens/scalar-token";
+import {TemplateToken} from "@actions/workflow-parser/templates/tokens/template-token";
+import {CodeAction, Position, TextEdit} from "vscode-languageserver-types";
+import {error} from "../../log.js";
+import {findToken} from "../../utils/find-token.js";
+import {getOrParseWorkflow} from "../../utils/workflow-cache.js";
+import {DiagnosticCode, MissingInputsDiagnosticData} from "../../validate-action-reference.js";
+import {CodeActionContext, CodeActionProvider} from "../types.js";
+
+/**
+ * Information extracted from a step token needed to generate edits
+ */
+interface StepInfo {
+  /** Column where step keys start (1-indexed), e.g., the column of "uses:" */
+  stepKeyColumn: number;
+  /** End line of the step (1-indexed) */
+  stepEndLine: number;
+  /** Detected indent size (spaces per level) */
+  indentSize: number;
+  /** Information about existing with: block, if present */
+  withInfo?: {
+    keyColumn: number;
+    keyEndLine: number;
+    valueEndLine: number;
+    hasChildren: boolean;
+    /** Column of first child input (1-indexed), for indentation detection */
+    firstChildColumn?: number;
+  };
+}
+
+export const addMissingInputsProvider: CodeActionProvider = {
+  diagnosticCodes: [DiagnosticCode.MissingRequiredInputs],
+
+  createCodeAction(context: CodeActionContext, diagnostic): CodeAction | undefined {
+    const data = diagnostic.data as MissingInputsDiagnosticData | undefined;
+    if (!data) {
+      return undefined;
+    }
+
+    // Parse the document to get the step token
+    const stepInfo = getStepInfo(context, diagnostic.range.start);
+    if (!stepInfo) {
+      return undefined;
+    }
+
+    const edits = createInputEdits(data.missingInputs, stepInfo);
+    if (!edits || edits.length === 0) {
+      return undefined;
+    }
+
+    const inputNames = data.missingInputs.map(i => i.name).join(", ");
+
+    return {
+      title: `Add missing input${data.missingInputs.length > 1 ? "s" : ""}: ${inputNames}`,
+      edit: {
+        changes: {
+          [context.uri]: edits
+        }
+      }
+    };
+  }
+};
+
+/**
+ * Parse the document and extract step information needed for generating edits.
+ * Returns undefined if parsing fails or the step token cannot be found.
+ */
+function getStepInfo(context: CodeActionContext, diagnosticPosition: Position): StepInfo | undefined {
+  // Parse the document (uses cache if available from validation)
+  const file = {name: context.uri, content: context.documentContent};
+  const parseResult = getOrParseWorkflow(file, context.uri);
+
+  if (!parseResult.value) {
+    error("Failed to parse workflow for missing inputs quickfix");
+    return undefined;
+  }
+
+  // Find the token at the diagnostic position
+  const {path} = findToken(diagnosticPosition, parseResult.value);
+
+  // Walk up the path to find the step token (regular-step)
+  const stepToken = findStepInPath(path);
+  if (!stepToken) {
+    error("Could not find step token for missing inputs quickfix");
+    return undefined;
+  }
+
+  return extractStepInfo(stepToken);
+}
+
+/**
+ * Find the step token (regular-step) in the token path
+ */
+function findStepInPath(path: TemplateToken[]): MappingToken | undefined {
+  // Walk backwards through path to find the step
+  for (let i = path.length - 1; i >= 0; i--) {
+    if (path[i].definition?.key === "regular-step" && isMapping(path[i])) {
+      return path[i] as MappingToken;
+    }
+  }
+  return undefined;
+}
+
+/**
+ * Extract position and indentation info from a step token
+ */
+function extractStepInfo(stepToken: MappingToken): StepInfo | undefined {
+  if (!stepToken.range) {
+    return undefined;
+  }
+
+  // Get the column of the first key in the step
+  let stepKeyColumn = stepToken.range.start.column;
+  if (stepToken.count > 0) {
+    const firstEntry = stepToken.get(0);
+    if (firstEntry?.key.range) {
+      stepKeyColumn = firstEntry.key.range.start.column;
+    }
+  }
+
+  // Find the with: block if present
+  let withKey: ScalarToken | undefined;
+  let withToken: TemplateToken | undefined;
+  for (const {key, value} of stepToken) {
+    if (key.toString() === "with") {
+      withKey = key;
+      withToken = value;
+      break;
+    }
+  }
+
+  // Calculate indent size
+  let indentSize = 2; // Default
+  let withInfo: StepInfo["withInfo"];
+
+  if (withKey?.range && withToken?.range) {
+    // Has with: block - extract its info
+    const hasChildren = isMapping(withToken) && withToken.count > 0;
+    let firstChildColumn: number | undefined;
+
+    if (hasChildren) {
+      const firstChild = (withToken as MappingToken).get(0);
+      if (firstChild?.key.range) {
+        firstChildColumn = firstChild.key.range.start.column;
+        // Detect indent size from with: children
+        indentSize = firstChildColumn - withKey.range.start.column;
+      }
+    }
+
+    withInfo = {
+      keyColumn: withKey.range.start.column,
+      keyEndLine: withKey.range.end.line,
+      valueEndLine: withToken.range.end.line,
+      hasChildren,
+      firstChildColumn
+    };
+  } else {
+    // No with: block - detect indent size using heuristics
+    // Based on the step key column position, estimate indent size
+    // 2-space indent files typically have step keys at column 7
+    // 4-space indent files typically have step keys at column 15
+    const zeroIndexedCol = stepKeyColumn - 1;
+    if (zeroIndexedCol >= 10) {
+      indentSize = 4;
+    }
+  }
+
+  return {
+    stepKeyColumn,
+    stepEndLine: stepToken.range.end.line,
+    indentSize,
+    withInfo
+  };
+}
+
+/**
+ * Generate text edits to add missing inputs
+ */
+function createInputEdits(missingInputs: MissingInputsDiagnosticData["missingInputs"], stepInfo: StepInfo): TextEdit[] {
+  const formatInputLines = (indent: string) =>
+    missingInputs.map(input => {
+      const value = input.default ?? '""';
+      return `${indent}${input.name}: ${value}`;
+    });
+
+  if (stepInfo.withInfo) {
+    // `with:` exists - add inputs to existing block
+    const withIndent = stepInfo.withInfo.keyColumn - 1; // 0-indexed
+    const inputIndentSize = stepInfo.withInfo.firstChildColumn
+      ? stepInfo.withInfo.firstChildColumn - stepInfo.withInfo.keyColumn
+      : stepInfo.indentSize;
+
+    const inputIndent = " ".repeat(withIndent + inputIndentSize);
+    const inputLines = formatInputLines(inputIndent);
+
+    // Calculate insert position
+    let insertLine: number;
+    if (stepInfo.withInfo.hasChildren) {
+      // Insert after the last child (at end of with: block)
+      // valueEndLine is 1-indexed, we want 0-indexed for Position
+      insertLine = stepInfo.withInfo.valueEndLine - 1;
+    } else {
+      // Empty with: block - insert on the next line after with:
+      // keyEndLine is 1-indexed, convert to 0-indexed and go to next line
+      insertLine = stepInfo.withInfo.keyEndLine;
+    }
+
+    const insertPosition: Position = {
+      line: insertLine,
+      character: 0
+    };
+
+    return [
+      {
+        range: {start: insertPosition, end: insertPosition},
+        newText: inputLines.map(line => line + "\n").join("")
+      }
+    ];
+  } else {
+    // No `with:` key - add `with:` at the same level as other step keys
+    const withKeyIndent = stepInfo.stepKeyColumn - 1; // 0-indexed (columns are 1-based)
+
+    const withIndent = " ".repeat(withKeyIndent);
+    const inputIndent = " ".repeat(withKeyIndent + stepInfo.indentSize);
+    const inputLines = formatInputLines(inputIndent);
+
+    const newText = `${withIndent}with:\n` + inputLines.map(line => `${line}\n`).join("");
+
+    // Insert at end of step
+    // stepEndLine is 1-indexed, we want 0-indexed and insert before the line after
+    const insertPosition: Position = {
+      line: stepInfo.stepEndLine - 1,
+      character: 0
+    };
+
+    return [
+      {
+        range: {start: insertPosition, end: insertPosition},
+        newText
+      }
+    ];
+  }
+}

languageservice/src/code-actions/quickfix/quickfix-providers.ts

@@ -0,0 +1,13 @@
+import {FeatureFlags} from "@actions/expressions";
+import {CodeActionProvider} from "../types.js";
+import {addMissingInputsProvider} from "./add-missing-inputs.js";
+
+export function getQuickfixProviders(featureFlags?: FeatureFlags): CodeActionProvider[] {
+  const providers: CodeActionProvider[] = [];
+
+  if (featureFlags?.isEnabled("missingInputsQuickfix")) {
+    providers.push(addMissingInputsProvider);
+  }
+
+  return providers;
+}

languageservice/src/code-actions/tests/runner.test.ts

@@ -0,0 +1,90 @@
+import * as path from "path";
+import {fileURLToPath} from "url";
+import {loadTestCases, runTestCase} from "./runner.js";
+import {ValidationConfig} from "../../validate.js";
+import {ActionMetadata, ActionReference} from "../../action.js";
+import {clearCache} from "../../utils/workflow-cache.js";
+
+// ESM-compatible __dirname
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+
+// Mock action metadata provider for tests
+const validationConfig: ValidationConfig = {
+  actionsMetadataProvider: {
+    fetchActionMetadata: (ref: ActionReference): Promise<ActionMetadata | undefined> => {
+      const key = `${ref.owner}/${ref.name}@${ref.ref}`;
+
+      const metadata: Record<string, ActionMetadata> = {
+        "actions/cache@v1": {
+          name: "Cache",
+          description: "Cache dependencies",
+          inputs: {
+            path: {
+              description: "A list of files to cache",
+              required: true
+            },
+            key: {
+              description: "Cache key",
+              required: true
+            },
+            "restore-keys": {
+              description: "Restore keys",
+              required: false
+            }
+          }
+        },
+        "actions/setup-node@v3": {
+          name: "Setup Node",
+          description: "Setup Node.js",
+          inputs: {
+            "node-version": {
+              description: "Node version",
+              required: true,
+              default: "16"
+            }
+          }
+        }
+      };
+
+      return Promise.resolve(metadata[key]);
+    }
+  }
+};
+
+// Point to the source testdata directory
+const testdataDir = path.join(__dirname, "testdata");
+
+beforeEach(() => {
+  clearCache();
+});
+
+describe("code action golden tests", () => {
+  const testCases = loadTestCases(testdataDir);
+
+  if (testCases.length === 0) {
+    it.todo("no test cases found - add .yml files to testdata/");
+    return;
+  }
+
+  for (const testCase of testCases) {
+    it(testCase.name, async () => {
+      const result = await runTestCase(testCase, validationConfig);
+
+      if (!result.passed) {
+        let errorMessage = result.error || "Test failed";
+
+        if (result.expected !== undefined && result.actual !== undefined) {
+          errorMessage += "\n\n";
+          errorMessage += "=== EXPECTED (golden file) ===\n";
+          errorMessage += result.expected;
+          errorMessage += "\n\n";
+          errorMessage += "=== ACTUAL ===\n";
+          errorMessage += result.actual;
+        }
+
+        throw new Error(errorMessage);
+      }
+    });
+  }
+});

languageservice/src/code-actions/tests/runner.ts

@@ -0,0 +1,231 @@
+import * as fs from "fs";
+import * as path from "path";
+import {TextEdit} from "vscode-languageserver-types";
+import {TextDocument} from "vscode-languageserver-textdocument";
+import {FeatureFlags} from "@actions/expressions";
+import {validate, ValidationConfig} from "../../validate.js";
+import {getCodeActions, CodeActionParams} from "../code-actions.js";
+
+// Marker pattern: # want "diagnostic message" fix="code-action-name"
+const MARKER_PATTERN = /#\s*want\s+"([^"]+)"(?:\s+fix="([^"]+)")?/;
+
+export interface TestCase {
+  name: string;
+  inputPath: string;
+  goldenPath: string;
+  input: string;
+  golden: string;
+  markers: Marker[];
+}
+
+export interface Marker {
+  line: number;
+  message: string;
+  fix?: string;
+}
+
+export interface TestResult {
+  name: string;
+  passed: boolean;
+  error?: string;
+  expected?: string;
+  actual?: string;
+}
+
+/**
+ * Parse markers from input file content
+ */
+export function parseMarkers(content: string): Marker[] {
+  const lines = content.split("\n");
+  const markers: Marker[] = [];
+
+  for (let i = 0; i < lines.length; i++) {
+    const match = lines[i].match(MARKER_PATTERN);
+    if (match) {
+      markers.push({
+        line: i,
+        message: match[1],
+        fix: match[2]
+      });
+    }
+  }
+
+  return markers;
+}
+
+/**
+ * Strip markers from content (for processing)
+ */
+export function stripMarkers(content: string): string {
+  return content
+    .split("\n")
+    .map(line => line.replace(MARKER_PATTERN, "").trimEnd())
+    .join("\n");
+}
+
+/**
+ * Load all test cases from a testdata directory
+ */
+export function loadTestCases(testdataDir: string): TestCase[] {
+  const testCases: TestCase[] = [];
+
+  function walkDir(dir: string) {
+    const entries = fs.readdirSync(dir, {withFileTypes: true});
+
+    for (const entry of entries) {
+      const fullPath = path.join(dir, entry.name);
+
+      if (entry.isDirectory()) {
+        walkDir(fullPath);
+      } else if (entry.isFile() && entry.name.endsWith(".yml") && !entry.name.endsWith(".golden.yml")) {
+        const goldenPath = fullPath.replace(".yml", ".golden.yml");
+
+        if (fs.existsSync(goldenPath)) {
+          const input = fs.readFileSync(fullPath, "utf-8");
+          const golden = fs.readFileSync(goldenPath, "utf-8");
+
+          testCases.push({
+            name: path.relative(testdataDir, fullPath),
+            inputPath: fullPath,
+            goldenPath,
+            input,
+            golden,
+            markers: parseMarkers(input)
+          });
+        }
+      }
+    }
+  }
+
+  walkDir(testdataDir);
+  return testCases;
+}
+
+/**
+ * Apply text edits to a document
+ */
+export function applyEdits(content: string, edits: TextEdit[]): string {
+  // Sort edits in reverse order by position to apply from bottom to top
+  const sortedEdits = [...edits].sort((a, b) => {
+    if (b.range.start.line !== a.range.start.line) {
+      return b.range.start.line - a.range.start.line;
+    }
+    return b.range.start.character - a.range.start.character;
+  });
+
+  const lines = content.split("\n");
+
+  for (const edit of sortedEdits) {
+    const startLine = edit.range.start.line;
+    const startChar = edit.range.start.character;
+    const endLine = edit.range.end.line;
+    const endChar = edit.range.end.character;
+
+    const before = lines[startLine].slice(0, startChar);
+    const after = lines[endLine].slice(endChar);
+
+    const newLines = edit.newText.split("\n");
+    newLines[0] = before + newLines[0];
+    newLines[newLines.length - 1] = newLines[newLines.length - 1] + after;
+
+    lines.splice(startLine, endLine - startLine + 1, ...newLines);
+  }
+
+  return lines.join("\n");
+}
+
+/**
+ * Run a single test case
+ */
+export async function runTestCase(testCase: TestCase, validationConfig: ValidationConfig): Promise<TestResult> {
+  const strippedInput = stripMarkers(testCase.input);
+  const document = TextDocument.create("file:///test.yml", "yaml", 1, strippedInput);
+
+  // 1. Validate and get diagnostics
+  const diagnostics = await validate(document, validationConfig);
+
+  // 2.  Verify all expected diagnostics are present
+  const missingDiagnostics: string[] = [];
+  for (const marker of testCase.markers) {
+    const found = diagnostics.find(d => d.range.start.line === marker.line && d.message.includes(marker.message));
+    if (!found) {
+      missingDiagnostics.push(`line ${marker.line}: "${marker.message}"`);
+    }
+  }
+
+  if (missingDiagnostics.length > 0) {
+    return {
+      name: testCase.name,
+      passed: false,
+      error: `Missing expected diagnostics:\n  ${missingDiagnostics.join(
+        "\n  "
+      )}\n\nActual diagnostics:\n  ${diagnostics.map(d => `line ${d.range.start.line}: "${d.message}"`).join("\n  ")}`
+    };
+  }
+
+  // 3.  Collect all edits from all matching code actions
+  const allEdits: TextEdit[] = [];
+
+  for (const marker of testCase.markers) {
+    if (!marker.fix) {
+      continue;
+    }
+
+    const diagnostic = diagnostics.find(d => d.range.start.line === marker.line && d.message.includes(marker.message));
+
+    if (!diagnostic) {
+      continue; // Already reported above
+    }
+
+    const params: CodeActionParams = {
+      uri: document.uri,
+      documentContent: strippedInput,
+      diagnostics: [diagnostic],
+      featureFlags: new FeatureFlags({all: true})
+    };
+
+    const actions = getCodeActions(params);
+    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion -- marker.fix is checked at the start of the loop
+    const matchingAction = actions.find(a => a.title.toLowerCase().includes(marker.fix!.toLowerCase()));
+
+    if (!matchingAction) {
+      return {
+        name: testCase.name,
+        passed: false,
+        error: `Code action "${marker.fix}" not found for diagnostic on line ${marker.line}.\nAvailable actions: ${
+          actions.map(a => a.title).join(", ") || "(none)"
+        }`
+      };
+    }
+
+    if (!matchingAction.edit?.changes) {
+      return {
+        name: testCase.name,
+        passed: false,
+        error: `Code action "${marker.fix}" has no edits`
+      };
+    }
+
+    const edits = matchingAction.edit.changes[document.uri] || [];
+    allEdits.push(...edits);
+  }
+
+  // 4. Apply all edits and compare to golden file
+  const actualOutput = applyEdits(strippedInput, allEdits);
+  const expectedOutput = testCase.golden;
+
+  if (actualOutput.trim() !== expectedOutput.trim()) {
+    return {
+      name: testCase.name,
+      passed: false,
+      error: "Output does not match golden file",
+      expected: expectedOutput,
+      actual: actualOutput
+    };
+  }
+
+  return {
+    name: testCase.name,
+    passed: true
+  };
+}

languageservice/src/code-actions/tests/testdata/quickfix/existing-with-key-without-inputs.golden.yml

@@ -0,0 +1,9 @@
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/cache@v1
+        with:
+          path: ""
+          key: ""

languageservice/src/code-actions/tests/testdata/quickfix/existing-with-key-without-inputs.yml

@@ -0,0 +1,7 @@
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/cache@v1
+        with: # want "Missing required inputs: `path`, `key`" fix="Add missing inputs: path, key"

languageservice/src/code-actions/tests/testdata/quickfix/existing-with-key.golden.yml

@@ -0,0 +1,10 @@
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/cache@v1
+        with:
+          restore-keys: ${{ runner.os }}-
+          path: ""
+          key: ""

languageservice/src/code-actions/tests/testdata/quickfix/existing-with-key.yml

@@ -0,0 +1,8 @@
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/cache@v1
+        with: # want "Missing required inputs: `path`, `key`" fix="Add missing inputs: path, key"
+          restore-keys: ${{ runner.os }}-

languageservice/src/code-actions/tests/testdata/quickfix/four-space-indent.golden.yml

@@ -0,0 +1,9 @@
+on: push
+jobs:
+    build:
+        runs-on: ubuntu-latest
+        steps:
+            - uses: actions/cache@v1
+              with:
+                  path: ""
+                  key: ""

languageservice/src/code-actions/tests/testdata/quickfix/four-space-indent.yml

@@ -0,0 +1,6 @@
+on: push
+jobs:
+    build:
+        runs-on: ubuntu-latest
+        steps:
+            - uses: actions/cache@v1 # want "Missing required inputs: `path`, `key`" fix="Add missing inputs: path, key"

languageservice/src/code-actions/tests/testdata/quickfix/no-with-key.golden.yml

@@ -0,0 +1,9 @@
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/cache@v1
+        with:
+          path: ""
+          key: ""

languageservice/src/code-actions/tests/testdata/quickfix/no-with-key.yml

@@ -0,0 +1,6 @@
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/cache@v1 # want "Missing required inputs: `path`, `key`" fix="Add missing inputs: path, key"

languageservice/src/code-actions/types.ts

@@ -0,0 +1,23 @@
+import {FeatureFlags} from "@actions/expressions";
+import {CodeAction, Diagnostic} from "vscode-languageserver-types";
+
+export interface CodeActionContext {
+  uri: string;
+  documentContent: string;
+  featureFlags?: FeatureFlags;
+}
+
+/**
+ * A provider that can produce a code action for a given diagnostic
+ */
+export interface CodeActionProvider {
+  /**
+   * The diagnostic codes this provider handles
+   */
+  diagnosticCodes: (string | number | undefined)[];
+
+  /**
+   * Create a code action for the diagnostic, if applicable
+   */
+  createCodeAction(context: CodeActionContext, diagnostic: Diagnostic): CodeAction | undefined;
+}

languageservice/src/complete-action.test.ts

@@ -134,6 +134,49 @@ runs:
       expect(labels).toContain("arch");
       expect(labels).toContain("temp");
     });
+
+    it("completes if expression value for composite run step", async () => {
+      const [doc, position] = createActionDocument(`name: My Action
+description: Test action
+runs:
+  using: composite
+  steps:
+    - if: |
+      run: echo "hello"
+      shell: bash`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      // Should show expression-related completions (status functions and contexts)
+      expect(labels).toContain("always");
+      expect(labels).toContain("success");
+      expect(labels).toContain("failure");
+      expect(labels).toContain("cancelled");
+      expect(labels).toContain("runner");
+      expect(labels).toContain("github");
+      expect(labels).toContain("inputs");
+      expect(labels).toContain("steps");
+    });
+
+    it("completes if expression value for composite uses step", async () => {
+      const [doc, position] = createActionDocument(`name: My Action
+description: Test action
+runs:
+  using: composite
+  steps:
+    - if: |
+      uses: actions/checkout@v4`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      // Should show expression-related completions
+      expect(labels).toContain("always");
+      expect(labels).toContain("success");
+      expect(labels).toContain("failure");
+      expect(labels).toContain("cancelled");
+      expect(labels).toContain("runner");
+      expect(labels).toContain("github");
+    });
   });
 
   describe("top-level completions", () => {
@@ -184,6 +227,186 @@ runs:
 
       expect(labels).toContain("using");
     });
+
+    it("filters runs keys for node20 actions", async () => {
+      const [doc, position] = createActionDocument(`name: Test
+description: Test
+runs:
+  using: node20
+  |`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      // Should show Node.js action keys
+      expect(labels).toContain("main");
+      expect(labels).toContain("pre");
+      expect(labels).toContain("post");
+      expect(labels).toContain("pre-if");
+      expect(labels).toContain("post-if");
+
+      // Should NOT show composite or docker keys
+      expect(labels).not.toContain("steps");
+      expect(labels).not.toContain("image");
+      expect(labels).not.toContain("entrypoint");
+    });
+
+    it("filters runs keys for node24 actions", async () => {
+      const [doc, position] = createActionDocument(`name: Test
+description: Test
+runs:
+  using: node24
+  |`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      // Should show Node.js action keys
+      expect(labels).toContain("main");
+      expect(labels).toContain("pre");
+      expect(labels).toContain("post");
+      expect(labels).toContain("pre-if");
+      expect(labels).toContain("post-if");
+
+      // Should NOT show composite or docker keys
+      expect(labels).not.toContain("steps");
+      expect(labels).not.toContain("image");
+      expect(labels).not.toContain("entrypoint");
+    });
+
+    it("completes pre-if expression value for node actions", async () => {
+      const [doc, position] = createActionDocument(`name: Test
+description: Test
+runs:
+  using: node24
+  main: index.js
+  pre: setup.js
+  pre-if: |`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      // Should show expression-related completions (context functions and namespaces)
+      expect(labels).toContain("always");
+      expect(labels).toContain("success");
+      expect(labels).toContain("failure");
+      expect(labels).toContain("cancelled");
+      expect(labels).toContain("runner");
+      expect(labels).toContain("github");
+      expect(labels).toContain("inputs");
+      expect(labels).toContain("hashFiles");
+    });
+
+    it("completes post-if expression value for node actions", async () => {
+      const [doc, position] = createActionDocument(`name: Test
+description: Test
+runs:
+  using: node24
+  main: index.js
+  post: cleanup.js
+  post-if: |`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      // Should show expression-related completions
+      expect(labels).toContain("always");
+      expect(labels).toContain("runner");
+      expect(labels).toContain("hashFiles");
+    });
+
+    it("completes pre-if expression value for docker actions", async () => {
+      const [doc, position] = createActionDocument(`name: Test
+description: Test
+runs:
+  using: docker
+  image: docker://alpine
+  pre-entrypoint: setup.sh
+  pre-if: |`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      // Should show expression-related completions
+      expect(labels).toContain("always");
+      expect(labels).toContain("runner");
+      expect(labels).toContain("github");
+      expect(labels).toContain("hashFiles");
+    });
+
+    it("filters runs keys for composite actions", async () => {
+      const [doc, position] = createActionDocument(`name: Test
+description: Test
+runs:
+  using: composite
+  |`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      // Should show composite action keys
+      expect(labels).toContain("steps");
+
+      // Should NOT show Node.js or docker keys
+      expect(labels).not.toContain("main");
+      expect(labels).not.toContain("pre");
+      expect(labels).not.toContain("post");
+      expect(labels).not.toContain("image");
+    });
+
+    it("filters runs keys for docker actions", async () => {
+      const [doc, position] = createActionDocument(`name: Test
+description: Test
+runs:
+  using: docker
+  |`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      // Should show Docker action keys
+      expect(labels).toContain("image");
+      expect(labels).toContain("args");
+      expect(labels).toContain("env");
+      expect(labels).toContain("entrypoint");
+      expect(labels).toContain("pre-entrypoint");
+      expect(labels).toContain("post-entrypoint");
+
+      // Should NOT show Node.js or composite keys
+      expect(labels).not.toContain("main");
+      expect(labels).not.toContain("steps");
+    });
+
+    it("prioritizes using when not set", async () => {
+      const [doc, position] = createActionDocument(`name: Test
+description: Test
+runs:
+  |`);
+      const completions = await complete(doc, position);
+
+      // Find the using completion
+      const usingCompletion = completions.find(c => c.label === "using");
+      expect(usingCompletion).toBeDefined();
+
+      // It should have a sortText that makes it sort after snippets
+      expect(usingCompletion?.sortText).toBe("9_using");
+    });
+
+    it("completes step keys inside composite action steps", async () => {
+      const [doc, position] = createActionDocument(`name: Test
+description: Test
+runs:
+  using: composite
+  steps:
+    - run: echo hello
+      shell: bash
+    - |`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      // Should show step keys, not filtered by runs-level logic
+      expect(labels).toContain("run");
+      expect(labels).toContain("uses");
+      expect(labels).toContain("shell");
+      expect(labels).toContain("id");
+      expect(labels).toContain("name");
+      expect(labels).toContain("if");
+      expect(labels).toContain("env");
+      expect(labels).toContain("working-directory");
+    });
   });
 
   describe("branding completions", () => {
@@ -251,6 +474,38 @@ runs:
       expect(labels).not.toContain("jobs");
     });
 
+    it("includes descriptions from schema for completion items", async () => {
+      const [doc, position] = createActionDocument(`|`, "file:///my-repo/action.yml");
+      const completions = await complete(doc, position);
+
+      const authorCompletion = completions.find(c => c.label === "author");
+      expect(authorCompletion).toBeDefined();
+      expect(authorCompletion?.documentation).toBeDefined();
+      expect((authorCompletion?.documentation as {value: string})?.value).toContain("author");
+    });
+
+    it("includes descriptions for branding completion", async () => {
+      const [doc, position] = createActionDocument(`|`, "file:///my-repo/action.yml");
+      const completions = await complete(doc, position);
+
+      const brandingCompletion = completions.find(c => c.label === "branding");
+      expect(brandingCompletion).toBeDefined();
+      expect(brandingCompletion?.documentation).toBeDefined();
+      expect((brandingCompletion?.documentation as {value: string})?.value).toContain("branding");
+    });
+
+    it("falls back to type description when property has no description", async () => {
+      // `inputs` uses shorthand form in schema: "inputs": "inputs-strict"
+      // So the property has no description, but the type `inputs-strict` does
+      const [doc, position] = createActionDocument(`|`, "file:///my-repo/action.yml");
+      const completions = await complete(doc, position);
+
+      const inputsCompletion = completions.find(c => c.label === "inputs");
+      expect(inputsCompletion).toBeDefined();
+      expect(inputsCompletion?.documentation).toBeDefined();
+      expect((inputsCompletion?.documentation as {value: string})?.value).toContain("Input parameters");
+    });
+
     it("does not route workflow files to action completion", async () => {
       const doc = TextDocument.create("file:///repo/.github/workflows/ci.yml", "yaml", 1, `o`);
       const completions = await complete(doc, {line: 0, character: 1});
@@ -260,4 +515,175 @@ runs:
       expect(labels).toContain("jobs");
     });
   });
+
+  describe("action scaffolding snippets", () => {
+    it("offers full scaffolding snippets in empty file", async () => {
+      const [doc, position] = createActionDocument(`|`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      expect(labels).toContain("Node.js Action");
+      expect(labels).toContain("Composite Action");
+      expect(labels).toContain("Docker Action");
+
+      // Verify they are snippets
+      const nodeSnippet = completions.find(c => c.label === "Node.js Action");
+      expect(nodeSnippet?.kind).toBe(15); // CompletionItemKind.Snippet
+      expect(nodeSnippet?.insertTextFormat).toBe(2); // InsertTextFormat.Snippet
+    });
+
+    it("offers full scaffolding snippets when no name or description exists", async () => {
+      const [doc, position] = createActionDocument(`author: me
+|`);
+      const completions = await complete(doc, position);
+
+      const nodeSnippet = completions.find(c => c.label === "Node.js Action");
+      expect(nodeSnippet).toBeDefined();
+      // Full snippet should include name:
+      expect((nodeSnippet?.textEdit as {newText: string})?.newText).toContain("name:");
+    });
+
+    it("offers runs-only snippets when name exists", async () => {
+      const [doc, position] = createActionDocument(`name: My Action
+|`);
+      const completions = await complete(doc, position);
+
+      const nodeSnippet = completions.find(c => c.label === "Node.js Action");
+      expect(nodeSnippet).toBeDefined();
+      // Runs-only snippet should start with inputs:, not name:
+      expect((nodeSnippet?.textEdit as {newText: string})?.newText).toMatch(/^inputs:/);
+      expect((nodeSnippet?.textEdit as {newText: string})?.newText).toContain("runs:");
+    });
+
+    it("offers runs-only snippets when description exists", async () => {
+      const [doc, position] = createActionDocument(`description: Does something
+|`);
+      const completions = await complete(doc, position);
+
+      const compositeSnippet = completions.find(c => c.label === "Composite Action");
+      expect(compositeSnippet).toBeDefined();
+      // Runs-only snippet should start with inputs:, not description:
+      expect((compositeSnippet?.textEdit as {newText: string})?.newText).toMatch(/^inputs:/);
+      expect((compositeSnippet?.textEdit as {newText: string})?.newText).toContain("runs:");
+    });
+
+    it("does not offer snippets when runs.using already exists", async () => {
+      const [doc, position] = createActionDocument(`name: My Action
+description: Test
+runs:
+  using: composite
+  |`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      expect(labels).not.toContain("Node.js Action");
+      expect(labels).not.toContain("Composite Action");
+      expect(labels).not.toContain("Docker Action");
+    });
+
+    it("offers snippets inside runs when using is not set", async () => {
+      const [doc, position] = createActionDocument(`name: My Action
+description: Test
+runs:
+  |`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      expect(labels).toContain("Node.js Action");
+      expect(labels).toContain("Composite Action");
+      expect(labels).toContain("Docker Action");
+    });
+
+    it("does not offer snippets at root level when runs exists", async () => {
+      const [doc, position] = createActionDocument(`name: My Action
+description: Test
+runs:
+  steps: []
+|`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      expect(labels).not.toContain("Node.js Action");
+      expect(labels).not.toContain("Composite Action");
+      expect(labels).not.toContain("Docker Action");
+    });
+
+    it("does not offer snippets when nested inside runs steps", async () => {
+      const [doc, position] = createActionDocument(`name: My Action
+description: Test
+runs:
+  using: composite
+  steps:
+    - |`);
+      const completions = await complete(doc, position);
+      const labels = completions.map(c => c.label);
+
+      expect(labels).not.toContain("Node.js Action");
+      expect(labels).not.toContain("Composite Action");
+      expect(labels).not.toContain("Docker Action");
+    });
+
+    it("Node.js snippet contains expected content", async () => {
+      const [doc, position] = createActionDocument(`|`);
+      const completions = await complete(doc, position);
+
+      const nodeSnippet = completions.find(c => c.label === "Node.js Action");
+      const text = (nodeSnippet?.textEdit as {newText: string})?.newText;
+
+      expect(text).toContain("using: node24");
+      expect(text).toContain("main:");
+      expect(text).toContain("inputs:");
+      expect(text).toContain("outputs:");
+    });
+
+    it("Composite snippet contains expected content", async () => {
+      const [doc, position] = createActionDocument(`|`);
+      const completions = await complete(doc, position);
+
+      const compositeSnippet = completions.find(c => c.label === "Composite Action");
+      const text = (compositeSnippet?.textEdit as {newText: string})?.newText;
+
+      expect(text).toContain("using: composite");
+      expect(text).toContain("steps:");
+      expect(text).toContain("shell: bash");
+    });
+
+    it("Docker snippet contains expected content", async () => {
+      const [doc, position] = createActionDocument(`|`);
+      const completions = await complete(doc, position);
+
+      const dockerSnippet = completions.find(c => c.label === "Docker Action");
+      const text = (dockerSnippet?.textEdit as {newText: string})?.newText;
+
+      expect(text).toContain("using: docker");
+      expect(text).toContain("image:");
+      expect(text).toContain("entrypoint:");
+    });
+
+    it("replaces typed text when selecting scaffolding snippet", async () => {
+      // User typed "compo" and then triggered completion
+      const [doc, position] = createActionDocument(`compo|`);
+      const completions = await complete(doc, position);
+
+      const compositeSnippet = completions.find(c => c.label === "Composite Action");
+      expect(compositeSnippet).toBeDefined();
+
+      // The textEdit should replace "compo", not insert after it
+      const textEdit = compositeSnippet?.textEdit as {range: {start: {character: number}; end: {character: number}}};
+      expect(textEdit.range.start.character).toBe(0); // Start of "compo"
+      expect(textEdit.range.end.character).toBe(5); // End of "compo"
+    });
+
+    it("handles empty file with no typed text", async () => {
+      const [doc, position] = createActionDocument(`|`);
+      const completions = await complete(doc, position);
+
+      const compositeSnippet = completions.find(c => c.label === "Composite Action");
+      const textEdit = compositeSnippet?.textEdit as {range: {start: {character: number}; end: {character: number}}};
+
+      // Zero-length range is fine when there's nothing to replace
+      expect(textEdit.range.start.character).toBe(0);
+      expect(textEdit.range.end.character).toBe(0);
+    });
+  });
 });

languageservice/src/complete-action.ts

@@ -0,0 +1,480 @@
+import {TemplateToken} from "@actions/workflow-parser/templates/tokens/index";
+import {MappingToken} from "@actions/workflow-parser/templates/tokens/mapping-token";
+import {Position} from "vscode-languageserver-textdocument";
+import {CompletionItem, CompletionItemKind, InsertTextFormat, Range, TextEdit} from "vscode-languageserver-types";
+import {Value} from "./value-providers/config.js";
+
+/**
+ * Valid keys for each action type under the `runs:` section.
+ * Source: https://github.com/actions/runner/blob/main/src/Runner.Worker/ActionManifestManager.cs
+ */
+const ACTION_NODE_KEYS = new Set(["using", "main", "pre", "post", "pre-if", "post-if"]);
+const ACTION_COMPOSITE_KEYS = new Set(["using", "steps"]);
+const ACTION_DOCKER_KEYS = new Set([
+  "using",
+  "image",
+  "args",
+  "env",
+  "entrypoint",
+  "pre-entrypoint",
+  "pre-if",
+  "post-entrypoint",
+  "post-if"
+]);
+
+/**
+ * Action scaffolding snippets.
+ *
+ * Full variants include name, description, inputs, outputs, and runs.
+ * Runs-only variants include just the runs block.
+ */
+const ACTION_SNIPPET_NODEJS_FULL = `name: '\${1:Action Name}'
+description: '\${2:What this action does}'
+
+inputs:
+  name:
+    description: 'Name to greet'
+    required: false
+    default: 'World'
+
+outputs:
+  greeting:
+    description: 'The greeting message'
+
+runs:
+  # For more on JavaScript actions (including @actions/toolkit), see:
+  # https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-javascript-action
+  using: node24
+  main: index.js
+  # Sample index.js (vanilla JS, no build required):
+  #
+  #   const fs = require('fs');
+  #   const name = process.env.INPUT_NAME || 'World';
+  #   const greeting = \\\`Hello \\\${name}\\\`;
+  #   console.log(greeting);
+  #   fs.appendFileSync(process.env.GITHUB_OUTPUT, \\\`greeting=\\\${greeting}\\\\n\\\`);
+`;
+
+const ACTION_SNIPPET_NODEJS_RUNS = `inputs:
+  name:
+    description: 'Name to greet'
+    required: false
+    default: 'World'
+
+outputs:
+  greeting:
+    description: 'The greeting message'
+
+runs:
+  # For more on JavaScript actions (including @actions/toolkit), see:
+  # https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-javascript-action
+  using: node24
+  main: index.js
+  # Sample index.js (vanilla JS, no build required):
+  #
+  #   const fs = require('fs');
+  #   const name = process.env.INPUT_NAME || 'World';
+  #   const greeting = \\\`Hello \\\${name}\\\`;
+  #   console.log(greeting);
+  #   fs.appendFileSync(process.env.GITHUB_OUTPUT, \\\`greeting=\\\${greeting}\\\\n\\\`);
+`;
+
+const ACTION_SNIPPET_NODEJS_USING = `# For more on JavaScript actions (including @actions/toolkit), see:
+# https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-javascript-action
+using: node24
+main: index.js
+# Sample index.js (vanilla JS, no build required):
+#
+#   console.log('Hello World');
+`;
+
+const ACTION_SNIPPET_COMPOSITE_FULL = `name: '\${1:Action Name}'
+description: '\${2:What this action does}'
+
+inputs:
+  name:
+    description: 'Name to greet'
+    required: false
+    default: 'World'
+
+outputs:
+  greeting:
+    description: 'The greeting message'
+    value: \\\${{ steps.greet.outputs.greeting }}
+
+runs:
+  # For more on composite actions, see:
+  # https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-composite-action
+  using: composite
+  steps:
+    - id: greet
+      shell: bash
+      env:
+        INPUT_NAME: \\\${{ inputs.name }}
+      run: |
+        GREETING="Hello \\$INPUT_NAME"
+        echo "\\$GREETING"
+        echo "greeting=\\$GREETING" >> \\$GITHUB_OUTPUT
+`;
+
+const ACTION_SNIPPET_COMPOSITE_RUNS = `inputs:
+  name:
+    description: 'Name to greet'
+    required: false
+    default: 'World'
+
+outputs:
+  greeting:
+    description: 'The greeting message'
+    value: \\\${{ steps.greet.outputs.greeting }}
+
+runs:
+  # For more on composite actions, see:
+  # https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-composite-action
+  using: composite
+  steps:
+    - id: greet
+      shell: bash
+      env:
+        INPUT_NAME: \\\${{ inputs.name }}
+      run: |
+        GREETING="Hello \\$INPUT_NAME"
+        echo "\\$GREETING"
+        echo "greeting=\\$GREETING" >> \\$GITHUB_OUTPUT
+`;
+
+const ACTION_SNIPPET_COMPOSITE_USING = `# For more on composite actions, see:
+# https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-composite-action
+using: composite
+steps:
+  - shell: bash
+    run: echo "Hello World"
+`;
+
+const ACTION_SNIPPET_DOCKER_FULL = `name: '\${1:Action Name}'
+description: '\${2:What this action does}'
+
+inputs:
+  name:
+    description: 'Name to greet'
+    required: false
+    default: 'World'
+
+outputs:
+  greeting:
+    description: 'The greeting message'
+
+runs:
+  # For more on Docker actions, see:
+  # https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-docker-container-action
+  using: docker
+  # 'docker://image:tag' uses pre-built image, 'Dockerfile' builds locally
+  image: '\${3:docker://alpine:3.20}'
+  env:
+    INPUT_NAME: \\\${{ inputs.name }}
+  entrypoint: '\${4:sh}'
+  args:
+    - -c
+    - |
+      GREETING="Hello \\$INPUT_NAME"
+      echo "\\$GREETING"
+      echo "greeting=\\$GREETING" >> \\$GITHUB_OUTPUT
+`;
+
+const ACTION_SNIPPET_DOCKER_RUNS = `inputs:
+  name:
+    description: 'Name to greet'
+    required: false
+    default: 'World'
+
+outputs:
+  greeting:
+    description: 'The greeting message'
+
+runs:
+  # For more on Docker actions, see:
+  # https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-docker-container-action
+  using: docker
+  # 'docker://image:tag' uses pre-built image, 'Dockerfile' builds locally
+  image: '\${1:docker://alpine:3.20}'
+  env:
+    INPUT_NAME: \\\${{ inputs.name }}
+  entrypoint: '\${2:sh}'
+  args:
+    - -c
+    - |
+      GREETING="Hello \\$INPUT_NAME"
+      echo "\\$GREETING"
+      echo "greeting=\\$GREETING" >> \\$GITHUB_OUTPUT
+`;
+
+const ACTION_SNIPPET_DOCKER_USING = `# For more on Docker actions, see:
+# https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-docker-container-action
+using: docker
+# 'docker://image:tag' uses pre-built image, 'Dockerfile' builds locally
+image: '\${1:docker://alpine:3.20}'
+entrypoint: '\${2:sh}'
+args:
+  - -c
+  - echo "Hello World"
+`;
+
+/**
+ * Filters action.yml `runs:` completions based on the `using:` value.
+ *
+ * When the user is completing keys under `runs:`:
+ * - If `using: node20` is set, only show Node.js action keys
+ * - If `using: composite` is set, only show composite action keys
+ * - If `using: docker` is set, only show Docker action keys
+ * - If `using:` is not set, show all keys but prioritize `using` first
+ */
+export function filterActionRunsCompletions(values: Value[], path: TemplateToken[], root: TemplateToken): Value[] {
+  // Find the runs mapping from the root
+  let runsMapping: MappingToken | undefined;
+  if (root instanceof MappingToken) {
+    for (let i = 0; i < root.count; i++) {
+      const {key, value} = root.get(i);
+      if (key.toString().toLowerCase() === "runs" && value instanceof MappingToken) {
+        runsMapping = value;
+        break;
+      }
+    }
+  }
+  if (!runsMapping) {
+    return values;
+  }
+
+  // Check if the runs mapping is in our path (meaning we're completing inside it)
+  const isInsideRuns = path.some(token => token === runsMapping);
+  if (!isInsideRuns) {
+    return values;
+  }
+
+  // Find where runsMapping is in the path
+  const runsMappingIndex = path.indexOf(runsMapping);
+  if (runsMappingIndex === -1) {
+    return values;
+  }
+
+  // Check if there's anything after runsMapping in the path
+  // If so, we're nested deeper (e.g., inside steps sequence or a step mapping)
+  if (runsMappingIndex < path.length - 1) {
+    return values;
+  }
+
+  // Get the using value from the runs mapping
+  let usingValue: string | undefined;
+  for (let i = 0; i < runsMapping.count; i++) {
+    const {key, value} = runsMapping.get(i);
+    if (key.toString().toLowerCase() === "using") {
+      usingValue = value.toString();
+      break;
+    }
+  }
+
+  // Determine which keys to allow
+  let allowedKeys: Set<string>;
+
+  if (!usingValue) {
+    // No using value set - show all keys but prioritize "using"
+    return values.map(v => {
+      if (v.label.toLowerCase() === "using") {
+        return {...v, sortText: "9_using"}; // Sort after snippets (0_, 1_, 2_)
+      }
+      return v;
+    });
+  } else if (usingValue.match(/^node\d+$/i)) {
+    allowedKeys = ACTION_NODE_KEYS;
+  } else if (usingValue.toLowerCase() === "composite") {
+    allowedKeys = ACTION_COMPOSITE_KEYS;
+  } else if (usingValue.toLowerCase() === "docker") {
+    allowedKeys = ACTION_DOCKER_KEYS;
+  } else {
+    // Unknown using value - show all
+    return values;
+  }
+
+  // Filter to only allowed keys
+  return values.filter(v => allowedKeys.has(v.label.toLowerCase()));
+}
+
+/**
+ * Gets action scaffolding snippet completions for action.yml files.
+ *
+ * Returns snippet completions when `runs.using` is not present, offering
+ * three action types: Node.js, Composite, and Docker.
+ *
+ * Three variants per type:
+ * - "_FULL": Full scaffold with name, description, inputs, outputs, and runs
+ * - "_RUNS": Inputs, outputs, and runs (when name/description already exists)
+ * - "_USING": Minimal runs content (when inside `runs:` mapping)
+ *
+ * Which variant is shown depends on context:
+ * - Inside `runs:` mapping → "_USING" variants
+ * - At root with name/description → "_RUNS" variants
+ * - At root without name/description → "_FULL" variants
+ */
+export function getActionScaffoldingSnippets(
+  root: TemplateToken | undefined,
+  path: TemplateToken[],
+  position: Position,
+  replaceRange?: Range
+): CompletionItem[] {
+  // Get the runs mapping from the root, if it exists
+  let runsMapping: MappingToken | undefined;
+  if (root instanceof MappingToken) {
+    for (let i = 0; i < root.count; i++) {
+      const {key, value} = root.get(i);
+      if (key.toString().toLowerCase() === "runs" && value instanceof MappingToken) {
+        runsMapping = value;
+        break;
+      }
+    }
+  }
+
+  // Check if runs.using already exists - if so, no scaffolding needed
+  if (runsMapping) {
+    for (let i = 0; i < runsMapping.count; i++) {
+      const {key} = runsMapping.get(i);
+      if (key.toString().toLowerCase() === "using") {
+        return [];
+      }
+    }
+  }
+
+  // Show "_USING" variants directly inside `runs`
+  const runsMappingIndex = runsMapping ? path.indexOf(runsMapping) : -1;
+  const isDirectlyInsideRuns = runsMappingIndex !== -1 && runsMappingIndex === path.length - 1;
+  if (isDirectlyInsideRuns) {
+    return [
+      createSnippetCompletion(
+        "Node.js Action",
+        "Scaffold a Node.js action\n\n[Documentation](https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-javascript-action)",
+        ACTION_SNIPPET_NODEJS_USING,
+        position,
+        "0_nodejs",
+        replaceRange
+      ),
+      createSnippetCompletion(
+        "Composite Action",
+        "Scaffold a composite action\n\n[Documentation](https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-composite-action)",
+        ACTION_SNIPPET_COMPOSITE_USING,
+        position,
+        "1_composite",
+        replaceRange
+      ),
+      createSnippetCompletion(
+        "Docker Action",
+        "Scaffold a Docker action\n\n[Documentation](https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-docker-container-action)",
+        ACTION_SNIPPET_DOCKER_USING,
+        position,
+        "2_docker",
+        replaceRange
+      )
+    ];
+  }
+
+  // Not at root or `runs` already exists?
+  const isAtRoot = path.length === 0 || (path.length === 1 && path[0] === root);
+  if (!isAtRoot || runsMapping) {
+    return [];
+  }
+
+  // Determine which variant to show based on existing root keys
+  let hasNameOrDescription = false;
+  if (root instanceof MappingToken) {
+    for (let i = 0; i < root.count; i++) {
+      const keyStr = root.get(i).key.toString().toLowerCase();
+      if (keyStr === "name" || keyStr === "description") {
+        hasNameOrDescription = true;
+        break;
+      }
+    }
+  }
+
+  // Show "_RUNS" variants (inputs, outputs, and runs block)
+  if (hasNameOrDescription) {
+    return [
+      createSnippetCompletion(
+        "Node.js Action",
+        "Scaffold a Node.js action\n\n[Documentation](https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-javascript-action)",
+        ACTION_SNIPPET_NODEJS_RUNS,
+        position,
+        "1_nodejs",
+        replaceRange
+      ),
+      createSnippetCompletion(
+        "Composite Action",
+        "Scaffold a composite action\n\n[Documentation](https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-composite-action)",
+        ACTION_SNIPPET_COMPOSITE_RUNS,
+        position,
+        "2_composite",
+        replaceRange
+      ),
+      createSnippetCompletion(
+        "Docker Action",
+        "Scaffold a Docker action\n\n[Documentation](https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-docker-container-action)",
+        ACTION_SNIPPET_DOCKER_RUNS,
+        position,
+        "3_docker",
+        replaceRange
+      )
+    ];
+  }
+
+  // Show "_FULL" variants (complete scaffold)
+  return [
+    createSnippetCompletion(
+      "Node.js Action",
+      "Scaffold a complete Node.js action\n\n[Documentation](https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-javascript-action)",
+      ACTION_SNIPPET_NODEJS_FULL,
+      position,
+      "1_nodejs",
+      replaceRange
+    ),
+    createSnippetCompletion(
+      "Composite Action",
+      "Scaffold a complete composite action\n\n[Documentation](https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-composite-action)",
+      ACTION_SNIPPET_COMPOSITE_FULL,
+      position,
+      "2_composite",
+      replaceRange
+    ),
+    createSnippetCompletion(
+      "Docker Action",
+      "Scaffold a complete Docker action\n\n[Documentation](https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-docker-container-action)",
+      ACTION_SNIPPET_DOCKER_FULL,
+      position,
+      "3_docker",
+      replaceRange
+    )
+  ];
+}
+
+/**
+ * Creates a snippet completion item.
+ */
+function createSnippetCompletion(
+  label: string,
+  description: string,
+  snippetText: string,
+  position: Position,
+  sortText: string,
+  replaceRange?: Range
+): CompletionItem {
+  // Use replace if we have a range, otherwise insert at position
+  const textEdit = replaceRange ? TextEdit.replace(replaceRange, snippetText) : TextEdit.insert(position, snippetText);
+
+  return {
+    label,
+    labelDetails: {description: "snippet"},
+    kind: CompletionItemKind.Snippet,
+    documentation: {
+      kind: "markdown",
+      value: description
+    },
+    insertTextFormat: InsertTextFormat.Snippet,
+    sortText,
+    textEdit
+  };
+}

languageservice/src/complete.expressions.test.ts

@@ -1,6 +1,6 @@
 /* eslint-disable @typescript-eslint/no-non-null-assertion */
-import {data, DescriptionDictionary} from "@actions/expressions";
-import {CompletionItem, CompletionItemKind} from "vscode-languageserver-types";
+import {data, DescriptionDictionary, FeatureFlags} from "@actions/expressions";
+import {CompletionItem, CompletionItemKind, MarkupContent} from "vscode-languageserver-types";
 import {complete, getExpressionInput} from "./complete.js";
 import {ContextProviderConfig} from "./context-providers/config.js";
 import {registerLogger} from "./log.js";
@@ -68,12 +68,16 @@ describe("expressions", () => {
   describe("top-level auto-complete", () => {
     it("single region", async () => {
       const input = "run-name: ${{ | }}";
-      const result = await complete(...getPositionFromCursor(input));
+      const result = await complete(...getPositionFromCursor(input), {
+        contextProviderConfig,
+        featureFlags: new FeatureFlags({allowCaseFunction: true})
+      });
 
       expect(result.map(x => x.label)).toEqual([
         "github",
         "inputs",
         "vars",
+        "case",
         "contains",
         "endsWith",
         "format",
@@ -108,12 +112,16 @@ describe("expressions", () => {
 
     it("single region with existing input", async () => {
       const input = "run-name: ${{ g| }}";
-      const result = await complete(...getPositionFromCursor(input), {contextProviderConfig});
+      const result = await complete(...getPositionFromCursor(input), {
+        contextProviderConfig,
+        featureFlags: new FeatureFlags({allowCaseFunction: true})
+      });
 
       expect(result.map(x => x.label)).toEqual([
         "github",
         "inputs",
         "vars",
+        "case",
         "contains",
         "endsWith",
         "format",
@@ -126,12 +134,16 @@ describe("expressions", () => {
 
     it("single region with existing condition", async () => {
       const input = "run-name: ${{ g| == 'test' }}";
-      const result = await complete(...getPositionFromCursor(input), {contextProviderConfig});
+      const result = await complete(...getPositionFromCursor(input), {
+        contextProviderConfig,
+        featureFlags: new FeatureFlags({allowCaseFunction: true})
+      });
 
       expect(result.map(x => x.label)).toEqual([
         "github",
         "inputs",
         "vars",
+        "case",
         "contains",
         "endsWith",
         "format",
@@ -144,12 +156,16 @@ describe("expressions", () => {
 
     it("multiple regions with partial function", async () => {
       const input = "run-name: Run a ${{ inputs.test }} one-line script ${{ from|('test') == inputs.name }}";
-      const result = await complete(...getPositionFromCursor(input), {contextProviderConfig});
+      const result = await complete(...getPositionFromCursor(input), {
+        contextProviderConfig,
+        featureFlags: new FeatureFlags({allowCaseFunction: true})
+      });
 
       expect(result.map(x => x.label)).toEqual([
         "github",
         "inputs",
         "vars",
+        "case",
         "contains",
         "endsWith",
         "format",
@@ -162,12 +178,16 @@ describe("expressions", () => {
 
     it("multiple regions - first region", async () => {
       const input = "run-name: test-${{ git| == 1 }}-${{ github.event }}";
-      const result = await complete(...getPositionFromCursor(input), {contextProviderConfig});
+      const result = await complete(...getPositionFromCursor(input), {
+        contextProviderConfig,
+        featureFlags: new FeatureFlags({allowCaseFunction: true})
+      });
 
       expect(result.map(x => x.label)).toEqual([
         "github",
         "inputs",
         "vars",
+        "case",
         "contains",
         "endsWith",
         "format",
@@ -180,12 +200,16 @@ describe("expressions", () => {
 
     it("multiple regions", async () => {
       const input = "run-name: test-${{ github }}-${{ | }}";
-      const result = await complete(...getPositionFromCursor(input), {contextProviderConfig});
+      const result = await complete(...getPositionFromCursor(input), {
+        contextProviderConfig,
+        featureFlags: new FeatureFlags({allowCaseFunction: true})
+      });
 
       expect(result.map(x => x.label)).toEqual([
         "github",
         "inputs",
         "vars",
+        "case",
         "contains",
         "endsWith",
         "format",
@@ -395,6 +419,36 @@ jobs:
 
         expect(result.map(x => x.label)).toEqual(["event"]);
       });
+
+      it("includes both contexts and extension functions", async () => {
+        const input = `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - run: echo
+      if: |`;
+        const result = await complete(...getPositionFromCursor(input), {contextProviderConfig});
+        const labels = result.map(x => x.label);
+
+        // Context namespaces should be present
+        expect(labels).toContain("github");
+        expect(labels).toContain("runner");
+        expect(labels).toContain("env");
+        expect(labels).toContain("steps");
+
+        // Extension functions should be present (from schema context array)
+        expect(labels).toContain("hashFiles");
+        expect(labels).toContain("always");
+        expect(labels).toContain("success");
+        expect(labels).toContain("failure");
+        expect(labels).toContain("cancelled");
+
+        // Built-in functions should be present
+        expect(labels).toContain("toJson");
+        expect(labels).toContain("fromJson");
+        expect(labels).toContain("contains");
+      });
     });
   });
 
@@ -1126,7 +1180,10 @@ jobs:
         run: echo hi
 `;
 
-      const result = await complete(...getPositionFromCursor(input), {contextProviderConfig});
+      const result = await complete(...getPositionFromCursor(input), {
+        contextProviderConfig,
+        featureFlags: new FeatureFlags({allowCaseFunction: true})
+      });
       expect(result.map(x => x.label)).toEqual([
         "env",
         "github",
@@ -1139,6 +1196,7 @@ jobs:
         "steps",
         "strategy",
         "vars",
+        "case",
         "contains",
         "endsWith",
         "format",
@@ -1250,6 +1308,7 @@ jobs:
     expect(hashFiles).toBeDefined();
     expect(hashFiles!.kind).toBe(CompletionItemKind.Function);
     expect(hashFiles!.insertText).toBe("hashFiles()");
+    expect((hashFiles!.documentation as MarkupContent)?.value).toContain("Returns a single hash for the set of files");
 
     // Not a function
     const github = result.find(x => x.label === "github");

languageservice/src/complete.test.ts

@@ -6,6 +6,7 @@ import {getPositionFromCursor} from "./test-utils/cursor-position.js";
 import {TestLogger} from "./test-utils/logger.js";
 import {clearCache} from "./utils/workflow-cache.js";
 import {ValueProviderConfig, ValueProviderKind} from "./value-providers/config.js";
+import {FeatureFlags} from "@actions/expressions/features";
 
 registerLogger(new TestLogger());
 
@@ -19,8 +20,8 @@ describe("completion", () => {
     const result = await complete(...getPositionFromCursor(input));
 
     expect(result).not.toBeUndefined();
-    // 12 runner labels + 2 escape hatches (switch to list, switch to full syntax)
-    expect(result.length).toEqual(14);
+    // 28 runner labels + 2 escape hatches (switch to list, switch to full syntax)
+    expect(result.length).toEqual(30);
     const labels = result.map(x => x.label);
     expect(labels).toContain("macos-latest");
     expect(labels).toContain("(switch to list)");
@@ -59,7 +60,7 @@ jobs:
     const result = await complete(...getPositionFromCursor(input));
 
     expect(result).not.toBeUndefined();
-    expect(result.length).toEqual(11);
+    expect(result.length).toEqual(27);
 
     const labels = result.map(x => x.label);
     expect(labels).toContain("macos-latest");
@@ -723,16 +724,28 @@ jobs:
       expect(switchToList!.sortText).toEqual("zzz_switch_1");
       expect(switchToFull!.sortText).toEqual("zzz_switch_2");
 
-      // Escape hatches should have textEdit that restructures the YAML
+      // Escape hatches should have textEdit at cursor position (for VS Code filtering compatibility)
       const listEdit = switchToList!.textEdit as TextEdit;
       const fullEdit = switchToFull!.textEdit as TextEdit;
 
-      expect(listEdit.newText).toEqual("runs-on:\n  - ");
-      expect(fullEdit.newText).toEqual("runs-on:\n  ");
+      // Main textEdit inserts newline and indented content at cursor position
+      expect(listEdit.newText).toEqual("\n  - ");
+      expect(fullEdit.newText).toEqual("\n  ");
 
-      // TextEdit range should cover from key start to cursor position
-      expect(listEdit.range.start).toEqual({line: 3, character: 4});
-      expect(fullEdit.range.start).toEqual({line: 3, character: 4});
+      // TextEdit range should be at cursor position (empty range)
+      expect(listEdit.range.start).toEqual({line: 3, character: 13});
+      expect(listEdit.range.end).toEqual({line: 3, character: 13});
+      expect(fullEdit.range.start).toEqual({line: 3, character: 13});
+      expect(fullEdit.range.end).toEqual({line: 3, character: 13});
+
+      // additionalTextEdits should clean up the key portion
+      expect(switchToList!.additionalTextEdits).toHaveLength(1);
+      expect(switchToList!.additionalTextEdits![0].range.start).toEqual({line: 3, character: 4});
+      expect(switchToList!.additionalTextEdits![0].range.end).toEqual({line: 3, character: 13});
+      expect(switchToList!.additionalTextEdits![0].newText).toEqual("runs-on:");
+
+      expect(switchToFull!.additionalTextEdits).toHaveLength(1);
+      expect(switchToFull!.additionalTextEdits![0].newText).toEqual("runs-on:");
     });
 
     it("permissions shows only switch to full syntax (no sequence form)", async () => {
@@ -824,9 +837,16 @@ jobs:
 
       const switchToList = result.find(x => x.label === "(switch to list)");
       const textEdit = switchToList!.textEdit as TextEdit;
+      const additionalEdits = switchToList!.additionalTextEdits!;
 
-      // Applying this edit to "runs-on: " should produce "runs-on:\n  - "
-      expect(textEdit.newText).toEqual("runs-on:\n  - ");
+      // Main textEdit inserts newline content at cursor
+      expect(textEdit.newText).toEqual("\n  - ");
+
+      // additionalTextEdits replaces "runs-on: " with "runs-on:"
+      expect(additionalEdits).toHaveLength(1);
+      expect(additionalEdits[0].newText).toEqual("runs-on:");
+
+      // Combined result when applied: "runs-on:\n  - "
     });
   });
 
@@ -876,4 +896,150 @@ jobs:
       expect(result.some(x => x.label === "macos-latest")).toBe(true);
     });
   });
+
+  describe("expression completions", () => {
+    it("include case function when enabled", async () => {
+      const input = "on: push\njobs:\n  build:\n    runs-on: ${{ c|";
+      const result = await complete(...getPositionFromCursor(input), {
+        featureFlags: new FeatureFlags({allowCaseFunction: true})
+      });
+
+      expect(result).not.toBeUndefined();
+      // Expression completions starting with 'c': case, contains
+      const labels = result.map(x => x.label);
+      expect(labels).toContain("case");
+      expect(labels).toContain("contains");
+    });
+
+    it("exclude case function when disabled", async () => {
+      const input = "on: push\njobs:\n  build:\n    runs-on: ${{ c|";
+      const result = await complete(...getPositionFromCursor(input), {
+        featureFlags: new FeatureFlags({allowCaseFunction: false})
+      });
+
+      expect(result).not.toBeUndefined();
+      // Expression completions starting with 'c': contains
+      const labels = result.map(x => x.label);
+      expect(labels).not.toContain("case");
+      expect(labels).toContain("contains");
+    });
+  });
+});
+
+describe("schedule timezone completion", () => {
+  it("includes timezone when allowCronTimezone is enabled", async () => {
+    const input = `on:
+  schedule:
+    - |`;
+    const result = await complete(...getPositionFromCursor(input), {
+      featureFlags: new FeatureFlags({allowCronTimezone: true})
+    });
+
+    expect(result).not.toBeUndefined();
+    const labels = result.map(x => x.label);
+    expect(labels).toContain("cron");
+    expect(labels).toContain("timezone");
+  });
+
+  it("excludes timezone when allowCronTimezone is disabled", async () => {
+    const input = `on:
+  schedule:
+    - |`;
+    const result = await complete(...getPositionFromCursor(input), {
+      featureFlags: new FeatureFlags({allowCronTimezone: false})
+    });
+
+    expect(result).not.toBeUndefined();
+    const labels = result.map(x => x.label);
+    expect(labels).toContain("cron");
+    expect(labels).not.toContain("timezone");
+  });
+
+  it("excludes timezone when no feature flags are provided", async () => {
+    const input = `on:
+  schedule:
+    - |`;
+    const result = await complete(...getPositionFromCursor(input));
+
+    expect(result).not.toBeUndefined();
+    const labels = result.map(x => x.label);
+    expect(labels).toContain("cron");
+    expect(labels).not.toContain("timezone");
+  });
+});
+
+describe("permissions copilot-requests completion", () => {
+  it("includes copilot-requests when allowCopilotRequestsPermission is enabled", async () => {
+    const input = `on: push
+permissions:
+  |`;
+    const result = await complete(...getPositionFromCursor(input), {
+      featureFlags: new FeatureFlags({allowCopilotRequestsPermission: true})
+    });
+
+    expect(result).not.toBeUndefined();
+    const labels = result.map(x => x.label);
+    expect(labels).toContain("actions");
+    expect(labels).toContain("copilot-requests");
+  });
+
+  it("excludes copilot-requests when allowCopilotRequestsPermission is disabled", async () => {
+    const input = `on: push
+permissions:
+  |`;
+    const result = await complete(...getPositionFromCursor(input), {
+      featureFlags: new FeatureFlags({allowCopilotRequestsPermission: false})
+    });
+
+    expect(result).not.toBeUndefined();
+    const labels = result.map(x => x.label);
+    expect(labels).toContain("actions");
+    expect(labels).not.toContain("copilot-requests");
+  });
+
+  it("excludes copilot-requests when no feature flags are provided", async () => {
+    const input = `on: push
+permissions:
+  |`;
+    const result = await complete(...getPositionFromCursor(input));
+
+    expect(result).not.toBeUndefined();
+    const labels = result.map(x => x.label);
+    expect(labels).toContain("actions");
+    expect(labels).not.toContain("copilot-requests");
+  });
+
+  it("includes copilot-requests in job-level permissions when allowCopilotRequestsPermission is enabled", async () => {
+    const input = `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    permissions:
+      |`;
+    const result = await complete(...getPositionFromCursor(input), {
+      featureFlags: new FeatureFlags({allowCopilotRequestsPermission: true})
+    });
+
+    expect(result).not.toBeUndefined();
+    const labels = result.map(x => x.label);
+    expect(labels).toContain("actions");
+    expect(labels).toContain("copilot-requests");
+  });
+
+  it("excludes copilot-requests from job-level permissions when allowCopilotRequestsPermission is disabled", async () => {
+    const input = `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    permissions:
+      |`;
+    const result = await complete(...getPositionFromCursor(input), {
+      featureFlags: new FeatureFlags({allowCopilotRequestsPermission: false})
+    });
+
+    expect(result).not.toBeUndefined();
+    const labels = result.map(x => x.label);
+    expect(labels).toContain("actions");
+    expect(labels).not.toContain("copilot-requests");
+  });
 });

languageservice/src/complete.ts

@@ -1,8 +1,10 @@
-import {complete as completeExpression, DescriptionDictionary} from "@actions/expressions";
+import {complete as completeExpression, DescriptionDictionary, FeatureFlags} from "@actions/expressions";
 import {CompletionItem as ExpressionCompletionItem} from "@actions/expressions/completion";
+import {FunctionInfo} from "@actions/expressions/funcs/info";
 import {isBasicExpression, isSequence, isString} from "@actions/workflow-parser";
 import {getActionSchema} from "@actions/workflow-parser/actions/action-schema";
 import {ErrorPolicy} from "@actions/workflow-parser/model/convert";
+import {splitAllowedContext} from "@actions/workflow-parser/templates/allowed-context";
 import {DefinitionType} from "@actions/workflow-parser/templates/schema/definition-type";
 import {OneOfDefinition} from "@actions/workflow-parser/templates/schema/one-of-definition";
 import {TemplateSchema} from "@actions/workflow-parser/templates/schema/template-schema";
@@ -16,8 +18,10 @@ import {FileProvider} from "@actions/workflow-parser/workflows/file-provider";
 import {getWorkflowSchema} from "@actions/workflow-parser/workflows/workflow-schema";
 import {Position, TextDocument} from "vscode-languageserver-textdocument";
 import {CompletionItem, CompletionItemKind, CompletionItemTag, Range, TextEdit} from "vscode-languageserver-types";
+import {filterActionRunsCompletions, getActionScaffoldingSnippets} from "./complete-action.js";
 import {ContextProviderConfig} from "./context-providers/config.js";
 import {getActionExpressionContext, getWorkflowExpressionContext, Mode} from "./context-providers/default.js";
+import {getFunctionDescription} from "./context-providers/descriptions.js";
 import {ActionContext, getActionContext} from "./context/action-context.js";
 import {getWorkflowContext, WorkflowContext} from "./context/workflow-context.js";
 import {validatorFunctions} from "./expression-validation/functions.js";
@@ -54,6 +58,7 @@ export type CompletionConfig = {
   valueProviderConfig?: ValueProviderConfig;
   contextProviderConfig?: ContextProviderConfig;
   fileProvider?: FileProvider;
+  featureFlags?: FeatureFlags;
 };
 
 export async function complete(
@@ -119,25 +124,31 @@ export async function complete(
   }
 
   // Expression completions
-  if (token && (isBasicExpression(token) || isPotentiallyExpression(token))) {
+  if (token && (isBasicExpression(token) || isPotentiallyExpression(token, isAction))) {
     const allowedContext = token.definitionInfo?.allowedContext || [];
+    const {namedContexts, functions: extensionFunctions} = splitAllowedContext(allowedContext);
     const context = isAction
-      ? getActionExpressionContext(allowedContext, config?.contextProviderConfig, actionContext, Mode.Completion)
+      ? getActionExpressionContext(namedContexts, config?.contextProviderConfig, actionContext, Mode.Completion)
       : await getWorkflowExpressionContext(
-          allowedContext,
+          namedContexts,
           config?.contextProviderConfig,
           workflowContext,
           Mode.Completion
         );
 
-    return getExpressionCompletionItems(token, context, newPos);
+    // Populate function descriptions for completion display
+    for (const func of extensionFunctions) {
+      func.description = getFunctionDescription(func.name);
+    }
+
+    return getExpressionCompletionItems(token, context, extensionFunctions, newPos, config?.featureFlags);
   }
 
   const indentation = guessIndentation(newDoc, 2, true); // Use 2 spaces as default and most common for YAML
   const indentString = " ".repeat(indentation.tabSize);
 
   // YAML key/value completions
-  const values = await getValues(
+  let values = await getValues(
     token,
     keyToken,
     parent,
@@ -147,6 +158,24 @@ export async function complete(
     schema
   );
 
+  // Filter action.yml `runs:` completions based on `using:` value
+  if (isAction && parsedTemplate.value) {
+    values = filterActionRunsCompletions(values, path, parsedTemplate.value);
+  }
+
+  // Filter `timezone` from schedule completions when the feature flag is disabled
+  if (!config?.featureFlags?.isEnabled("allowCronTimezone") && parent?.definition?.key === "schedule") {
+    values = values.filter(v => v.label !== "timezone");
+  }
+
+  // Filter `copilot-requests` from permissions completions when the feature flag is disabled
+  if (
+    !config?.featureFlags?.isEnabled("allowCopilotRequestsPermission") &&
+    parent?.definition?.key === "permissions-mapping"
+  ) {
+    values = values.filter(v => v.label !== "copilot-requests");
+  }
+
   // Offer "(switch to list)" / "(switch to mapping)" when the schema allows alternative forms
   const escapeHatches = getEscapeHatchCompletions(token, keyToken, indentString, newPos, schema);
   values.push(...escapeHatches);
@@ -178,8 +207,14 @@ export async function complete(
     }
   }
 
+  // Get action scaffolding snippets if applicable
+  let actionSnippets: CompletionItem[] = [];
+  if (isAction) {
+    actionSnippets = getActionScaffoldingSnippets(parsedTemplate.value, path, position, replaceRange);
+  }
+
   // Convert values to LSP CompletionItems
-  return values.map(value => {
+  const completionItems = values.map(value => {
     const newText = value.insertText || value.label;
 
     // Escape hatches provide their own textEdit to restructure the YAML
@@ -192,6 +227,12 @@ export async function complete(
       textEdit = TextEdit.insert(position, newText);
     }
 
+    // Convert additionalTextEdits if present
+    let additionalTextEdits: TextEdit[] | undefined;
+    if (value.additionalTextEdits) {
+      additionalTextEdits = value.additionalTextEdits.map(edit => TextEdit.replace(edit.range, edit.newText));
+    }
+
     const item: CompletionItem = {
       label: value.label,
       labelDetails: value.labelDetail ? {description: value.labelDetail} : undefined,
@@ -202,11 +243,15 @@ export async function complete(
         value: value.description
       },
       tags: value.deprecated ? [CompletionItemTag.Deprecated] : undefined,
-      textEdit
+      textEdit,
+      additionalTextEdits
     };
 
     return item;
   });
+
+  // Add action scaffolding snippets if available
+  return [...completionItems, ...actionSnippets];
 }
 
 /**
@@ -388,9 +433,19 @@ function getEscapeHatchCompletions(
     return [];
   }
 
-  // Calculate the range from key start to current position
-  // This covers "key: " so we can replace it with "key:\n  - " or "key:\n  "
-  const editRange = {
+  // For VS Code compatibility, we use a cursor-position range for the main textEdit
+  // and additionalTextEdits to clean up the key portion. This prevents VS Code from
+  // filtering out escape hatches based on the key text (e.g., "runs-on: ").
+  //
+  // Main textEdit: insert at cursor position (newline + indented content)
+  // additionalTextEdits: replace "key: " with "key:" (removes trailing space)
+  const cursorRange = {
+    start: {line: position.line, character: position.character},
+    end: {line: position.line, character: position.character}
+  };
+
+  // Range from key start to cursor - used to replace "key: " with "key:" in additionalTextEdits
+  const keyToCursorRange = {
     start: {line: keyRange.start.line - 1, character: keyRange.start.column - 1},
     end: {line: position.line, character: position.character}
   };
@@ -400,9 +455,15 @@ function getEscapeHatchCompletions(
       label: "(switch to list)",
       sortText: "zzz_switch_1",
       textEdit: {
-        range: editRange,
-        newText: `${keyName}:\n${indentation}- `
-      }
+        range: cursorRange,
+        newText: `\n${indentation}- `
+      },
+      additionalTextEdits: [
+        {
+          range: keyToCursorRange,
+          newText: `${keyName}:`
+        }
+      ]
     });
   }
 
@@ -411,9 +472,15 @@ function getEscapeHatchCompletions(
       label: "(switch to mapping)",
       sortText: "zzz_switch_2",
       textEdit: {
-        range: editRange,
-        newText: `${keyName}:\n${indentation}`
-      }
+        range: cursorRange,
+        newText: `\n${indentation}`
+      },
+      additionalTextEdits: [
+        {
+          range: keyToCursorRange,
+          newText: `${keyName}:`
+        }
+      ]
     });
   }
 
@@ -476,7 +543,9 @@ export function getExistingValues(token: TemplateToken | null, parent: TemplateT
 function getExpressionCompletionItems(
   token: TemplateToken,
   context: DescriptionDictionary,
-  pos: Position
+  extensionFunctions: FunctionInfo[],
+  pos: Position,
+  featureFlags?: FeatureFlags
 ): CompletionItem[] {
   if (!token.range) {
     return [];
@@ -495,8 +564,8 @@ function getExpressionCompletionItems(
   const expressionInput = (getExpressionInput(currentInput, cursorOffset) || "").trim();
 
   try {
-    return completeExpression(expressionInput, context, [], validatorFunctions).map(item =>
-      mapExpressionCompletionItem(item, currentInput[cursorOffset])
+    return completeExpression(expressionInput, context, extensionFunctions, validatorFunctions, featureFlags).map(
+      item => mapExpressionCompletionItem(item, currentInput[cursorOffset])
     );
   } catch (e) {
     error(`Error while completing expression: '${(e as Error)?.message || "<no details>"}'`);

languageservice/src/context-providers/default.ts

@@ -198,9 +198,13 @@ function getDefaultActionContext(
     case "runner":
       return getRunnerContext();
 
-    case "env":
-      // Actions can access env but we don't have runtime values
-      return new DescriptionDictionary();
+    case "env": {
+      // Actions can access env but we don't know what env vars the calling workflow defines
+      // Mark as incomplete to avoid false positive "Context access might be invalid" warnings
+      const envContext = new DescriptionDictionary();
+      envContext.complete = false;
+      return envContext;
+    }
 
     case "job": {
       // https://docs.github.com/en/actions/learn-github-actions/contexts#job-context
@@ -218,9 +222,13 @@ function getDefaultActionContext(
     case "strategy":
       return getStrategyContext();
 
-    case "matrix":
-      // Actions can access matrix context at runtime
-      return new DescriptionDictionary();
+    case "matrix": {
+      // Actions can access matrix context at runtime but we don't know the calling workflow's matrix
+      // Mark as incomplete to avoid false positive "Context access might be invalid" warnings
+      const matrixContext = new DescriptionDictionary();
+      matrixContext.complete = false;
+      return matrixContext;
+    }
   }
 
   return undefined;

languageservice/src/hover-action.test.ts

@@ -53,6 +53,20 @@ ru|ns:
       expect(result).not.toBeNull();
       expect(result?.contents).toContain("runs");
     });
+
+    it("shows description for author key", async () => {
+      const [doc, position] = createActionDocument(`name: My Action
+description: Test
+au|thor: Me
+runs:
+  using: node20
+  main: index.js`);
+      const result = await hover(doc, position);
+
+      expect(result).not.toBeNull();
+      expect(result?.contents).toContain("author");
+      expect(result?.contents).toContain("Documentation");
+    });
   });
 
   describe("runs properties", () => {
@@ -145,6 +159,7 @@ brand|ing:
 
       expect(result).not.toBeNull();
       expect(result?.contents).toContain("brand");
+      expect(result?.contents).toContain("Documentation");
     });
 
     it("shows description for icon key", async () => {

languageservice/src/hover.test.ts

@@ -110,8 +110,7 @@ jobs:
 `;
     const result = await hover(...getPositionFromCursor(input));
     expect(result).not.toBeUndefined();
-    // Cron description is now shown via diagnostics, not hover
-    expect(result?.contents).toEqual("");
+    expect(result?.contents).toEqual("Runs at 0 and 30 minutes past the hour, at 00:00 and 12:00");
   });
 
   it("on a cron mapping key", async () => {
@@ -121,7 +120,9 @@ jobs:
 `;
     const result = await hover(...getPositionFromCursor(input));
     expect(result).not.toBeUndefined();
-    expect(result?.contents).toEqual("");
+    expect(result?.contents).toEqual(
+      "A cron expression that represents a schedule. A scheduled workflow will run at most once every 5 minutes."
+    );
   });
 
   it("on an invalid cron schedule", async () => {
@@ -131,7 +132,9 @@ jobs:
 `;
     const result = await hover(...getPositionFromCursor(input));
     expect(result).not.toBeUndefined();
-    expect(result?.contents).toEqual("");
+    expect(result?.contents).toEqual(
+      "A cron expression that represents a schedule. A scheduled workflow will run at most once every 5 minutes."
+    );
   });
 
   it("shows context inherited from parent nodes", async () => {
@@ -196,7 +199,7 @@ jobs:
     const result = await hover(...getPositionFromCursor(input), testHoverConfig("uses", "step-uses", undefined));
     expect(result).not.toBeUndefined();
     expect(result?.contents).toEqual(
-      "Selects an action to run as part of a step in your job. An action is a reusable unit of code. You can use an action defined in the same repository as the workflow, a public repository, or in a published Docker container image."
+      "Selects an action to run as part of a step in your job. An action is a reusable unit of code. You can use an action defined in the same repository as the workflow, a public repository, a [private repository with access enabled](https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/enabling-features-for-your-repository/managing-github-actions-settings-for-a-repository#allowing-access-to-components-in-a-private-repository), or in a published Docker container image."
     );
   });
 });

languageservice/src/hover.ts

@@ -2,6 +2,8 @@ import {data, DescriptionDictionary, Parser} from "@actions/expressions";
 import {FunctionDefinition, FunctionInfo} from "@actions/expressions/funcs/info";
 import {Lexer} from "@actions/expressions/lexer";
 import {parseAction} from "@actions/workflow-parser/actions/action-parser";
+import {isString} from "@actions/workflow-parser";
+import {getCronDescription} from "@actions/workflow-parser/model/converter/cron";
 import {ErrorPolicy} from "@actions/workflow-parser/model/convert";
 import {splitAllowedContext} from "@actions/workflow-parser/templates/allowed-context";
 import {TemplateToken} from "@actions/workflow-parser/templates/tokens/template-token";
@@ -69,7 +71,7 @@ export async function hover(document: TextDocument, position: Position, config?:
   // Early exit if there's nothing to provide hover for
   const hoverToken = token || keyToken;
   const isExpressionHover =
-    token && tokenDefinitionInfo && (isBasicExpression(token) || isPotentiallyExpression(token));
+    token && tokenDefinitionInfo && (isBasicExpression(token) || isPotentiallyExpression(token, isAction));
   if (!isExpressionHover && !hoverToken?.definition) {
     return null;
   }
@@ -134,6 +136,17 @@ export async function hover(document: TextDocument, position: Position, config?:
   // Non-expression hover: show the schema description for the YAML key or value
   info(`Calculating hover for token with definition ${hoverToken.definition.key}`);
 
+  // Check for cron expression hover
+  if (isString(hoverToken) && hoverToken.definition.key === "cron-pattern") {
+    const cronDescription = getCronDescription(hoverToken.value);
+    if (cronDescription) {
+      return {
+        contents: cronDescription,
+        range: mapRange(hoverToken.range)
+      };
+    }
+  }
+
   let description: string;
   if (!isAction && tokenResult.parent && isReusableWorkflowJobInput(tokenResult)) {
     // Reusable workflow call: fetch the called workflow's input descriptions

languageservice/src/index.ts

@@ -1,4 +1,4 @@
-export {complete} from "./complete.js";
+export {complete, CompletionConfig} from "./complete.js";
 export {ContextProviderConfig} from "./context-providers/config.js";
 export {documentLinks} from "./document-links.js";
 export {hover} from "./hover.js";
@@ -6,3 +6,4 @@ export {getInlayHints} from "./inlay-hints.js";
 export {Logger, LogLevel, registerLogger, setLogLevel} from "./log.js";
 export {validate, ValidationConfig, ActionsMetadataProvider} from "./validate.js";
 export {ValueProviderConfig, ValueProviderKind} from "./value-providers/config.js";
+export {getCodeActions, CodeActionParams} from "./code-actions/code-actions.js";

languageservice/src/utils/expression-detection.test.ts

@@ -0,0 +1,170 @@
+import {isPotentiallyExpression} from "./expression-detection.js";
+import {TemplateToken} from "@actions/workflow-parser/templates/tokens/template-token";
+import {TokenType} from "@actions/workflow-parser/templates/tokens/types";
+import {Definition} from "@actions/workflow-parser/templates/schema/definition";
+
+// Helper to create a mock TemplateToken with the properties we need to test
+function createMockToken(options: {value?: string; definitionKey?: string; isString?: boolean}): TemplateToken {
+  const {value = "", definitionKey, isString = true} = options;
+
+  const mockDefinition = definitionKey ? ({key: definitionKey} as Definition) : undefined;
+
+  return {
+    value: isString ? value : undefined,
+    definition: mockDefinition,
+    templateTokenType: isString ? TokenType.String : TokenType.Mapping,
+    // Required by isString type guard (isLiteral checks isLiteral property)
+    isLiteral: isString,
+    isScalar: isString
+  } as unknown as TemplateToken;
+}
+
+describe("isPotentiallyExpression", () => {
+  describe("expression markers", () => {
+    it("returns true when token value contains ${{", () => {
+      const token = createMockToken({value: "${{ github.actor }}"});
+      expect(isPotentiallyExpression(token, false)).toBe(true);
+      expect(isPotentiallyExpression(token, true)).toBe(true);
+    });
+
+    it("returns true when token value contains embedded ${{", () => {
+      const token = createMockToken({value: "Hello ${{ github.actor }}!"});
+      expect(isPotentiallyExpression(token, false)).toBe(true);
+      expect(isPotentiallyExpression(token, true)).toBe(true);
+    });
+
+    it("returns false when token value does not contain ${{", () => {
+      const token = createMockToken({value: "plain text"});
+      expect(isPotentiallyExpression(token, false)).toBe(false);
+      expect(isPotentiallyExpression(token, true)).toBe(false);
+    });
+
+    it("returns false for non-string tokens without expression marker", () => {
+      const token = createMockToken({isString: false});
+      expect(isPotentiallyExpression(token, false)).toBe(false);
+      expect(isPotentiallyExpression(token, true)).toBe(false);
+    });
+  });
+
+  describe("workflow schema if-conditions", () => {
+    it("returns true for job-if definition in workflow", () => {
+      const token = createMockToken({value: "success()", definitionKey: "job-if"});
+      expect(isPotentiallyExpression(token, false)).toBe(true);
+    });
+
+    it("returns false for job-if definition in action (not valid in action schema)", () => {
+      const token = createMockToken({value: "success()", definitionKey: "job-if"});
+      expect(isPotentiallyExpression(token, true)).toBe(false);
+    });
+
+    it("returns true for step-if definition in workflow", () => {
+      const token = createMockToken({value: "failure()", definitionKey: "step-if"});
+      expect(isPotentiallyExpression(token, false)).toBe(true);
+    });
+
+    it("returns true for snapshot-if definition in workflow", () => {
+      const token = createMockToken({value: "always()", definitionKey: "snapshot-if"});
+      expect(isPotentiallyExpression(token, false)).toBe(true);
+    });
+
+    it("returns false for snapshot-if definition in action (not valid in action schema)", () => {
+      const token = createMockToken({value: "always()", definitionKey: "snapshot-if"});
+      expect(isPotentiallyExpression(token, true)).toBe(false);
+    });
+  });
+
+  describe("action schema if-conditions", () => {
+    describe("composite action step if (run and uses)", () => {
+      it("returns true for step-if definition in action", () => {
+        const token = createMockToken({value: "success()", definitionKey: "step-if"});
+        expect(isPotentiallyExpression(token, true)).toBe(true);
+      });
+
+      it("returns true for step-if with run step condition", () => {
+        // Composite action run step: if condition
+        const token = createMockToken({value: "github.event_name == 'push'", definitionKey: "step-if"});
+        expect(isPotentiallyExpression(token, true)).toBe(true);
+      });
+
+      it("returns true for step-if with uses step condition", () => {
+        // Composite action uses step: if condition
+        const token = createMockToken({value: "runner.os == 'Linux'", definitionKey: "step-if"});
+        expect(isPotentiallyExpression(token, true)).toBe(true);
+      });
+    });
+
+    describe("pre-if and post-if (node/docker actions)", () => {
+      it("returns true for runs-if definition in action (pre-if)", () => {
+        const token = createMockToken({value: "runner.os == 'Linux'", definitionKey: "runs-if"});
+        expect(isPotentiallyExpression(token, true)).toBe(true);
+      });
+
+      it("returns true for runs-if definition in action (post-if)", () => {
+        const token = createMockToken({value: "always()", definitionKey: "runs-if"});
+        expect(isPotentiallyExpression(token, true)).toBe(true);
+      });
+
+      it("returns false for runs-if definition in workflow (not valid in workflow schema)", () => {
+        const token = createMockToken({value: "always()", definitionKey: "runs-if"});
+        expect(isPotentiallyExpression(token, false)).toBe(false);
+      });
+    });
+  });
+
+  describe("mixed scenarios", () => {
+    it("returns true when expression marker present even if definition is not if-related", () => {
+      const token = createMockToken({value: "${{ github.actor }}", definitionKey: "some-other-definition"});
+      expect(isPotentiallyExpression(token, false)).toBe(true);
+      expect(isPotentiallyExpression(token, true)).toBe(true);
+    });
+
+    it("returns true when both expression marker and if definition present", () => {
+      const token = createMockToken({value: "${{ success() }}", definitionKey: "step-if"});
+      expect(isPotentiallyExpression(token, false)).toBe(true);
+      expect(isPotentiallyExpression(token, true)).toBe(true);
+    });
+
+    it("returns false for plain text with non-if definition", () => {
+      const token = createMockToken({value: "plain text", definitionKey: "string"});
+      expect(isPotentiallyExpression(token, false)).toBe(false);
+      expect(isPotentiallyExpression(token, true)).toBe(false);
+    });
+
+    it("returns false when token has no definition and no expression marker", () => {
+      const token = createMockToken({value: "plain text"});
+      expect(isPotentiallyExpression(token, false)).toBe(false);
+      expect(isPotentiallyExpression(token, true)).toBe(false);
+    });
+  });
+
+  describe("edge cases", () => {
+    it("handles empty string value", () => {
+      const token = createMockToken({value: ""});
+      expect(isPotentiallyExpression(token, false)).toBe(false);
+      expect(isPotentiallyExpression(token, true)).toBe(false);
+    });
+
+    it("handles expression marker as if-condition value", () => {
+      const token = createMockToken({value: "${{ always() }}", definitionKey: "job-if"});
+      expect(isPotentiallyExpression(token, false)).toBe(true);
+      // For action, job-if is not valid, but ${{ is present
+      expect(isPotentiallyExpression(token, true)).toBe(true);
+    });
+
+    it("handles partial expression marker", () => {
+      const token = createMockToken({value: "${incomplete"});
+      expect(isPotentiallyExpression(token, false)).toBe(false);
+      expect(isPotentiallyExpression(token, true)).toBe(false);
+    });
+
+    it("handles ${{ at different positions", () => {
+      const startToken = createMockToken({value: "${{ foo }} bar"});
+      const middleToken = createMockToken({value: "bar ${{ foo }} baz"});
+      const endToken = createMockToken({value: "bar ${{ foo }}"});
+
+      expect(isPotentiallyExpression(startToken, false)).toBe(true);
+      expect(isPotentiallyExpression(middleToken, false)).toBe(true);
+      expect(isPotentiallyExpression(endToken, false)).toBe(true);
+    });
+  });
+});

languageservice/src/utils/expression-detection.ts

@@ -2,10 +2,36 @@ import {isString} from "@actions/workflow-parser";
 import {OPEN_EXPRESSION} from "@actions/workflow-parser/templates/template-constants";
 import {TemplateToken} from "@actions/workflow-parser/templates/tokens/index";
 
-export function isPotentiallyExpression(token: TemplateToken): boolean {
-  const containsExpression = isString(token) && token.value != null && token.value.indexOf(OPEN_EXPRESSION) >= 0;
-  // If conditions are always expressions (job-if, step-if, snapshot-if)
-  const definitionKey = token.definition?.key;
-  const isIfCondition = definitionKey === "job-if" || definitionKey === "step-if" || definitionKey === "snapshot-if";
-  return containsExpression || isIfCondition;
+/**
+ * Workflow schema if-condition definition keys.
+ * - job-if: job level if condition
+ * - step-if: step level if condition
+ * - snapshot-if: snapshot if condition
+ */
+const WORKFLOW_IF_DEFINITIONS = new Set(["job-if", "step-if", "snapshot-if"]);
+
+/**
+ * Action schema if-condition definition keys.
+ * - step-if: composite action step if condition (run-step and uses-step)
+ * - runs-if: pre-if and post-if at the runs level (node/docker actions)
+ */
+const ACTION_IF_DEFINITIONS = new Set(["step-if", "runs-if"]);
+
+export function isPotentiallyExpression(token: TemplateToken, isAction: boolean): boolean {
+  // Check if token contains expression syntax
+  if (isString(token) && token.value != null && token.value.indexOf(OPEN_EXPRESSION) >= 0) {
+    return true;
+  }
+
+  // Check if token is an if-condition (always treated as expressions)
+  if (!token.definition?.key) {
+    return false;
+  }
+
+  // Definition keys differ between workflow and action schemas
+  if (isAction) {
+    return ACTION_IF_DEFINITIONS.has(token.definition.key);
+  } else {
+    return WORKFLOW_IF_DEFINITIONS.has(token.definition.key);
+  }
 }

languageservice/src/utils/validate-if.ts

@@ -0,0 +1,65 @@
+/**
+ * Shared validation utilities for `if` condition literal text detection.
+ * Used by both workflow and action validation.
+ */
+
+import {data} from "@actions/expressions";
+import {Expr, FunctionCall, Literal, Logical} from "@actions/expressions/ast";
+
+/**
+ * Checks if a format function contains literal text in its format string.
+ * This indicates user confusion about how expressions work.
+ *
+ * Example: format('push == {0}', github.event_name)
+ * The literal text "push == " will always evaluate to truthy.
+ *
+ * @param expr The expression to check
+ * @returns true if the expression is a format() call with literal text
+ */
+export function hasFormatWithLiteralText(expr: Expr): boolean {
+  // If this is a logical AND expression (from ensureStatusFunction wrapping)
+  // check the right side for the format call
+  if (expr instanceof Logical && expr.operator.lexeme === "&&" && expr.args.length === 2) {
+    return hasFormatWithLiteralText(expr.args[1]);
+  }
+
+  if (!(expr instanceof FunctionCall)) {
+    return false;
+  }
+
+  // Check if this is a format function
+  if (expr.functionName.lexeme.toLowerCase() !== "format") {
+    return false;
+  }
+
+  // Check if the first argument is a string literal
+  if (expr.args.length < 1) {
+    return false;
+  }
+
+  const firstArg = expr.args[0];
+  if (!(firstArg instanceof Literal) || firstArg.literal.kind !== data.Kind.String) {
+    return false;
+  }
+
+  // Get the format string and trim whitespace
+  const formatString = firstArg.literal.coerceString();
+  const trimmed = formatString.trim();
+
+  // Check if there's literal text (non-replacement tokens) after trimming
+  let inToken = false;
+  for (let i = 0; i < trimmed.length; i++) {
+    if (!inToken && trimmed[i] === "{") {
+      inToken = true;
+    } else if (inToken && trimmed[i] === "}") {
+      inToken = false;
+    } else if (inToken && trimmed[i] >= "0" && trimmed[i] <= "9") {
+      // OK - this is a replacement token like {0}, {1}, etc.
+    } else {
+      // Found literal text
+      return true;
+    }
+  }
+
+  return false;
+}

languageservice/src/utils/validate-uses.ts

@@ -0,0 +1,118 @@
+/**
+ * Shared validation utilities for step `uses` field format.
+ * Used by both workflow and action validation.
+ */
+
+import {Diagnostic, DiagnosticSeverity} from "vscode-languageserver-types";
+import {StringToken} from "@actions/workflow-parser/templates/tokens/string-token";
+import {mapRange} from "./range.js";
+
+// Matches a short SHA (7-8 hex characters) that looks like it should be a full SHA
+const SHORT_SHA_PATTERN = /^[0-9a-f]{7,8}$/i;
+const SHORT_SHA_DOCS_URL =
+  "https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-third-party-actions";
+
+/**
+ * Checks if a ref looks like a short SHA and adds a warning if so.
+ * Returns true if a warning was added.
+ */
+export function warnIfShortSha(diagnostics: Diagnostic[], token: StringToken, ref: string): boolean {
+  if (SHORT_SHA_PATTERN.test(ref)) {
+    diagnostics.push({
+      message: `The provided ref '${ref}' may be a shortened commit SHA. If so, please use the full 40-character commit SHA instead, as short SHAs are not supported.`,
+      severity: DiagnosticSeverity.Warning,
+      range: mapRange(token.range),
+      code: "short-sha-ref",
+      codeDescription: {
+        href: SHORT_SHA_DOCS_URL
+      }
+    });
+    return true;
+  }
+  return false;
+}
+
+/**
+ * Validates the format of a step's `uses` field.
+ *
+ * Valid formats:
+ * - docker://image:tag
+ * - ./local/path
+ * - .\local\path (Windows)
+ * - {owner}/{repo}@{ref}
+ * - {owner}/{repo}/{path}@{ref}
+ */
+export function validateStepUsesFormat(diagnostics: Diagnostic[], token: StringToken): void {
+  const uses = token.value;
+
+  // Empty uses value
+  if (!uses) {
+    diagnostics.push({
+      message: "'uses' value in action cannot be blank",
+      severity: DiagnosticSeverity.Error,
+      range: mapRange(token.range),
+      code: "invalid-uses-format"
+    });
+    return;
+  }
+
+  // Docker image reference - always valid format
+  if (uses.startsWith("docker://")) {
+    return;
+  }
+
+  // Local action path - always valid format
+  if (uses.startsWith("./") || uses.startsWith(".\\")) {
+    return;
+  }
+
+  // Remote action: must be {owner}/{repo}[/path]@{ref}
+  const atSegments = uses.split("@");
+
+  // Must have exactly one @
+  if (atSegments.length !== 2) {
+    addStepUsesFormatError(diagnostics, token);
+    return;
+  }
+
+  const [repoPath, gitRef] = atSegments;
+
+  // Ref cannot be empty
+  if (!gitRef) {
+    addStepUsesFormatError(diagnostics, token);
+    return;
+  }
+
+  // Split by / or \ to get path segments
+  const pathSegments = repoPath.split(/[\\/]/);
+
+  // Must have at least owner and repo (both non-empty)
+  if (pathSegments.length < 2 || !pathSegments[0] || !pathSegments[1]) {
+    addStepUsesFormatError(diagnostics, token);
+    return;
+  }
+
+  // Check if this is a reusable workflow reference (should be at job level, not step)
+  // Path would be like: owner/repo/.github/workflows/file.yml
+  if (pathSegments.length >= 4 && pathSegments[2] === ".github" && pathSegments[3] === "workflows") {
+    diagnostics.push({
+      message: "Reusable workflows should be referenced at the top-level `jobs.<job_id>.uses` key, not within steps",
+      severity: DiagnosticSeverity.Error,
+      range: mapRange(token.range),
+      code: "invalid-uses-format"
+    });
+    return;
+  }
+
+  // Warn if ref looks like a short SHA
+  warnIfShortSha(diagnostics, token, gitRef);
+}
+
+function addStepUsesFormatError(diagnostics: Diagnostic[], token: StringToken): void {
+  diagnostics.push({
+    message: `Expected format {owner}/{repo}[/path]@{ref}. Actual '${token.value}'`,
+    severity: DiagnosticSeverity.Error,
+    range: mapRange(token.range),
+    code: "invalid-uses-format"
+  });
+}

languageservice/src/validate-action-reference.test.ts

@@ -249,7 +249,21 @@ jobs:
             line: 7
           }
         },
-        severity: DiagnosticSeverity.Error
+        severity: DiagnosticSeverity.Error,
+        code: "missing-required-inputs",
+        data: {
+          action: {
+            name: "cache",
+            owner: "actions",
+            ref: "v1"
+          },
+          missingInputs: [
+            {
+              default: undefined,
+              name: "path"
+            }
+          ]
+        }
       }
     ]);
   });
@@ -294,7 +308,25 @@ jobs:
             line: 7
           }
         },
-        severity: DiagnosticSeverity.Error
+        severity: DiagnosticSeverity.Error,
+        code: "missing-required-inputs",
+        data: {
+          action: {
+            name: "cache",
+            owner: "actions",
+            ref: "v1"
+          },
+          missingInputs: [
+            {
+              default: undefined,
+              name: "path"
+            },
+            {
+              default: undefined,
+              name: "key"
+            }
+          ]
+        }
       }
     ]);
   });
@@ -323,7 +355,25 @@ jobs:
             line: 6
           }
         },
-        severity: DiagnosticSeverity.Error
+        severity: DiagnosticSeverity.Error,
+        code: "missing-required-inputs",
+        data: {
+          action: {
+            name: "cache",
+            owner: "actions",
+            ref: "v1"
+          },
+          missingInputs: [
+            {
+              default: undefined,
+              name: "path"
+            },
+            {
+              default: undefined,
+              name: "key"
+            }
+          ]
+        }
       }
     ]);
   });

languageservice/src/validate-action-reference.ts

@@ -4,10 +4,22 @@ import {Step} from "@actions/workflow-parser/model/workflow-template";
 import {ScalarToken} from "@actions/workflow-parser/templates/tokens/scalar-token";
 import {TemplateToken} from "@actions/workflow-parser/templates/tokens/template-token";
 import {Diagnostic, DiagnosticSeverity} from "vscode-languageserver-types";
-import {parseActionReference} from "./action.js";
+import {ActionReference, parseActionReference} from "./action.js";
 import {mapRange} from "./utils/range.js";
 import {ValidationConfig} from "./validate.js";
 
+export const DiagnosticCode = {
+  MissingRequiredInputs: "missing-required-inputs"
+} as const;
+
+export interface MissingInputsDiagnosticData {
+  action: ActionReference;
+  missingInputs: Array<{
+    name: string;
+    default?: string;
+  }>;
+}
+
 /**
  * Validates action references in workflow steps, checking for valid inputs and required inputs.
  */
@@ -94,10 +106,22 @@ export async function validateActionReference(
       missingRequiredInputs.length === 1
         ? `Missing required input \`${missingRequiredInputs[0][0]}\``
         : `Missing required inputs: ${missingRequiredInputs.map(input => `\`${input[0]}\``).join(", ")}`;
+
+    // Build minimal diagnostic data - position calculation happens in the quickfix
+    const diagnosticData: MissingInputsDiagnosticData = {
+      action,
+      missingInputs: missingRequiredInputs.map(([name, input]) => ({
+        name,
+        default: input.default
+      }))
+    };
+
     diagnostics.push({
       severity: DiagnosticSeverity.Error,
-      range: mapRange((withKey || stepToken).range), // Highlight the whole step if we don't have a with key
-      message: message
+      range: mapRange((withKey || stepToken).range),
+      message: message,
+      code: DiagnosticCode.MissingRequiredInputs,
+      data: diagnosticData
     });
   }
 }

languageservice/src/validate-action.test.ts

@@ -347,4 +347,919 @@ runs:
       expect(diagnostics).toEqual([]);
     });
   });
+
+  describe("invalid key combinations based on using type", () => {
+    it("reports error for node20 action with steps", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid - node20 with steps
+runs:
+  using: node20
+  main: index.js
+  steps:
+    - run: echo "hello"
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.length).toBeGreaterThan(0);
+      // Schema reports "Unexpected value 'steps'" for invalid keys
+      expect(diagnostics.some(d => d.message.includes("steps"))).toBe(true);
+    });
+
+    it("reports error for composite action with main", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid - composite with main
+runs:
+  using: composite
+  steps:
+    - run: echo "hello"
+      shell: bash
+  main: index.js
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.length).toBeGreaterThan(0);
+      // Schema reports "Unexpected value 'main'" for invalid keys
+      expect(diagnostics.some(d => d.message.includes("main"))).toBe(true);
+    });
+
+    it("reports error for docker action with steps", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid - docker with steps
+runs:
+  using: docker
+  image: Dockerfile
+  steps:
+    - run: echo "hello"
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.length).toBeGreaterThan(0);
+      // Schema reports "Unexpected value 'steps'" for invalid keys
+      expect(diagnostics.some(d => d.message.includes("steps"))).toBe(true);
+    });
+
+    it("reports error for docker action with main", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid - docker with main
+runs:
+  using: docker
+  image: Dockerfile
+  main: index.js
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.length).toBeGreaterThan(0);
+      // Schema reports "Unexpected value 'main'" for invalid keys
+      expect(diagnostics.some(d => d.message.includes("main"))).toBe(true);
+    });
+
+    it("reports error for node20 action missing main", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid - node20 without main
+runs:
+  using: node20
+  pre: setup.js
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.length).toBeGreaterThan(0);
+      expect(diagnostics.some(d => d.message.includes("main"))).toBe(true);
+    });
+
+    it("reports error for node24 action missing main", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid - node24 without main
+runs:
+  using: node24
+  pre: setup.js
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message === "'main' is required for Node.js actions (using: node24)")).toBe(true);
+      // Should NOT have duplicate schema error
+      expect(diagnostics.filter(d => d.message.includes("main")).length).toBe(1);
+    });
+
+    it("reports error for node24 action with only using (no narrowing key)", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid - node24 without main
+runs:
+  using: node24
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message === "'main' is required for Node.js actions (using: node24)")).toBe(true);
+      // Should NOT have the generic "not enough info" schema error
+      expect(diagnostics.some(d => d.message.includes("There's not enough info"))).toBe(false);
+    });
+
+    it("reports error for composite action missing steps", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid - composite without steps
+runs:
+  using: composite
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message === "'steps' is required for composite actions (using: composite)")).toBe(
+        true
+      );
+      // Should NOT have duplicate schema error
+      expect(diagnostics.some(d => d.message.includes("There's not enough info"))).toBe(false);
+    });
+
+    it("reports error for docker action missing image", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid - docker without image
+runs:
+  using: docker
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message === "'image' is required for Docker actions (using: docker)")).toBe(true);
+      // Should NOT have duplicate schema error
+      expect(diagnostics.some(d => d.message.includes("There's not enough info"))).toBe(false);
+    });
+
+    it("reports error for docker action with entrypoint but missing image", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid - docker without image
+runs:
+  using: docker
+  entrypoint: /entrypoint.sh
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message === "'image' is required for Docker actions (using: docker)")).toBe(true);
+      // Should NOT have duplicate "Required property is missing: image" schema error
+      expect(diagnostics.filter(d => d.message.includes("image")).length).toBe(1);
+    });
+
+    it("lets schema handle missing using", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid - no using
+runs:
+  main: index.js
+`);
+      const diagnostics = await validate(doc);
+      // Should have schema error about not enough info or unexpected value
+      expect(diagnostics.length).toBeGreaterThan(0);
+      // Should NOT have custom validation error (can't determine action type)
+      expect(diagnostics.some(d => d.message.includes("is required for"))).toBe(false);
+    });
+
+    it("lets schema handle invalid using value", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid - bad using value
+runs:
+  using: not-supported
+  main: index.js
+`);
+      const diagnostics = await validate(doc);
+      // Should have schema error about unexpected value
+      expect(diagnostics.length).toBeGreaterThan(0);
+      // Should NOT have custom validation error (unknown action type)
+      expect(diagnostics.some(d => d.message.includes("is required for"))).toBe(false);
+      expect(diagnostics.some(d => d.message.includes("is not valid for"))).toBe(false);
+    });
+  });
+
+  describe("composite step uses format validation", () => {
+    it("validates valid uses format with version", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Uses another action
+runs:
+  using: composite
+  steps:
+    - uses: actions/checkout@v4
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "invalid-uses-format")).toBe(false);
+    });
+
+    it("validates docker:// uses format", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Uses docker image
+runs:
+  using: composite
+  steps:
+    - uses: docker://alpine:3.14
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "invalid-uses-format")).toBe(false);
+    });
+
+    it("validates local ./ uses format", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Uses local action
+runs:
+  using: composite
+  steps:
+    - uses: ./local-action
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "invalid-uses-format")).toBe(false);
+    });
+
+    it("errors on missing @ref", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Missing version
+runs:
+  using: composite
+  steps:
+    - uses: actions/checkout
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "invalid-uses-format")).toBe(true);
+      expect(diagnostics.some(d => d.message.includes("Expected format"))).toBe(true);
+    });
+
+    it("errors on invalid format", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid format
+runs:
+  using: composite
+  steps:
+    - uses: invalid-format
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "invalid-uses-format")).toBe(true);
+    });
+
+    it("warns on short SHA", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Short SHA
+runs:
+  using: composite
+  steps:
+    - uses: actions/checkout@a1b2c3d
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "short-sha-ref")).toBe(true);
+      expect(diagnostics.some(d => d.message.includes("shortened commit SHA"))).toBe(true);
+    });
+
+    it("allows full SHA", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Full SHA
+runs:
+  using: composite
+  steps:
+    - uses: actions/checkout@a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "short-sha-ref")).toBe(false);
+    });
+
+    it("errors on reusable workflow in step uses", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Wrong workflow reference
+runs:
+  using: composite
+  steps:
+    - uses: owner/repo/.github/workflows/build.yml@main
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Reusable workflows should be referenced"))).toBe(true);
+    });
+  });
+
+  describe("composite step if literal text validation", () => {
+    it("errors when literal text mixed with embedded expression", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Literal text in if
+runs:
+  using: composite
+  steps:
+    - if: push == \${{ github.event_name }}
+      run: echo hi
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "expression-literal-text-in-condition")).toBe(true);
+      expect(diagnostics.some(d => d.message.includes("literal text outside replacement tokens"))).toBe(true);
+    });
+
+    it("allows valid expression in if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Valid if expression
+runs:
+  using: composite
+  steps:
+    - if: \${{ github.event_name == 'push' }}
+      run: echo hi
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "expression-literal-text-in-condition")).toBe(false);
+    });
+
+    it("allows if without expression markers (auto-wrapped)", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: If without markers
+runs:
+  using: composite
+  steps:
+    - if: github.event_name == 'push'
+      run: echo hi
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "expression-literal-text-in-condition")).toBe(false);
+    });
+
+    it("allows success() function", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Success function
+runs:
+  using: composite
+  steps:
+    - if: success()
+      run: echo hi
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "expression-literal-text-in-condition")).toBe(false);
+    });
+
+    it("errors on format with literal text in if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Format with literal text
+runs:
+  using: composite
+  steps:
+    - if: \${{ format('event is {0}', github.event_name) }}
+      run: echo hi
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "expression-literal-text-in-condition")).toBe(true);
+    });
+
+    it("allows format with only replacement tokens", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Format with only tokens
+runs:
+  using: composite
+  steps:
+    - if: \${{ format('{0}', github.event_name) }}
+      run: echo hi
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "expression-literal-text-in-condition")).toBe(false);
+    });
+
+    it("validates if in uses-step", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: If in uses step
+runs:
+  using: composite
+  steps:
+    - if: push == \${{ github.event_name }}
+      uses: actions/checkout@v4
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "expression-literal-text-in-condition")).toBe(true);
+    });
+  });
+
+  describe("pre-if and post-if validation", () => {
+    it("errors on explicit expression with literal text in pre-if for node action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Literal text in pre-if
+runs:
+  using: node20
+  main: index.js
+  pre: setup.js
+  pre-if: push == \${{ github.event_name }}
+`);
+      const diagnostics = await validate(doc);
+      // Explicit ${{ }} syntax is not allowed for pre-if, so we get that error
+      expect(diagnostics.some(d => d.code === "explicit-expression-not-allowed")).toBe(true);
+    });
+
+    it("errors on explicit expression with literal text in post-if for node action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Literal text in post-if
+runs:
+  using: node20
+  main: index.js
+  post: cleanup.js
+  post-if: event == \${{ github.event_name }}
+`);
+      const diagnostics = await validate(doc);
+      // Explicit ${{ }} syntax is not allowed for post-if, so we get that error
+      expect(diagnostics.some(d => d.code === "explicit-expression-not-allowed")).toBe(true);
+    });
+
+    it("errors on explicit expression with literal text in pre-if for docker action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Literal text in pre-if
+runs:
+  using: docker
+  image: Dockerfile
+  pre-entrypoint: /setup.sh
+  pre-if: push == \${{ github.event_name }}
+`);
+      const diagnostics = await validate(doc);
+      // Explicit ${{ }} syntax is not allowed for pre-if, so we get that error
+      expect(diagnostics.some(d => d.code === "explicit-expression-not-allowed")).toBe(true);
+    });
+
+    it("errors on explicit expression with literal text in post-if for docker action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Literal text in post-if
+runs:
+  using: docker
+  image: Dockerfile
+  post-entrypoint: /cleanup.sh
+  post-if: event == \${{ github.event_name }}
+`);
+      const diagnostics = await validate(doc);
+      // Explicit ${{ }} syntax is not allowed for post-if, so we get that error
+      expect(diagnostics.some(d => d.code === "explicit-expression-not-allowed")).toBe(true);
+    });
+
+    it("allows valid expression in pre-if for node action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Valid pre-if
+runs:
+  using: node20
+  main: index.js
+  pre: setup.js
+  pre-if: success()
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "expression-literal-text-in-condition")).toBe(false);
+    });
+
+    it("allows valid expression in post-if for node action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Valid post-if
+runs:
+  using: node20
+  main: index.js
+  post: cleanup.js
+  post-if: always()
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "expression-literal-text-in-condition")).toBe(false);
+    });
+
+    it("errors on explicit expression syntax in pre-if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Explicit expression in pre-if
+runs:
+  using: node20
+  main: index.js
+  pre: setup.js
+  pre-if: \${{ runner.os == 'Windows' }}
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "explicit-expression-not-allowed")).toBe(true);
+      expect(diagnostics.some(d => d.message.includes("pre-if"))).toBe(true);
+    });
+
+    it("errors on explicit expression syntax in post-if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Explicit expression in post-if
+runs:
+  using: node20
+  main: index.js
+  post: cleanup.js
+  post-if: \${{ always() }}
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "explicit-expression-not-allowed")).toBe(true);
+      expect(diagnostics.some(d => d.message.includes("post-if"))).toBe(true);
+    });
+
+    it("allows expression with failure() in post-if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Valid post-if
+runs:
+  using: node20
+  main: index.js
+  post: cleanup.js
+  post-if: failure()
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "expression-literal-text-in-condition")).toBe(false);
+    });
+
+    it("allows expression with cancelled() in post-if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Valid post-if
+runs:
+  using: node20
+  main: index.js
+  post: cleanup.js
+  post-if: cancelled()
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "expression-literal-text-in-condition")).toBe(false);
+    });
+  });
+
+  describe("format string validation", () => {
+    it("errors on format() with too few arguments in composite step if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Format mismatch
+runs:
+  using: composite
+  steps:
+    - if: format('{0} {1}', 'only-one')
+      run: echo hi
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "format-arg-count-mismatch")).toBe(true);
+    });
+
+    it("errors on invalid format string in composite step if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Invalid format
+runs:
+  using: composite
+  steps:
+    - if: format('{', 'arg')
+      run: echo hi
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "invalid-format-string")).toBe(true);
+    });
+
+    it("errors on format() with too few arguments in pre-if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Format mismatch in pre-if
+runs:
+  using: node20
+  main: index.js
+  pre: setup.js
+  pre-if: format('{0} {1}', 'only-one')
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "format-arg-count-mismatch")).toBe(true);
+    });
+
+    it("errors on format() with too few arguments in post-if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Format mismatch in post-if
+runs:
+  using: node20
+  main: index.js
+  post: cleanup.js
+  post-if: format('{0} {1} {2}', 'a', 'b')
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "format-arg-count-mismatch")).toBe(true);
+    });
+
+    it("allows valid format() call in composite step if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Valid format
+runs:
+  using: composite
+  steps:
+    - if: format('{0} {1}', 'a', 'b') == 'a b'
+      run: echo hi
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "format-arg-count-mismatch")).toBe(false);
+      expect(diagnostics.some(d => d.code === "invalid-format-string")).toBe(false);
+    });
+
+    it("allows valid format() call in pre-if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Valid format in pre-if
+runs:
+  using: node20
+  main: index.js
+  pre: setup.js
+  pre-if: format('{0}', runner.os) == 'Linux'
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "format-arg-count-mismatch")).toBe(false);
+      expect(diagnostics.some(d => d.code === "invalid-format-string")).toBe(false);
+    });
+
+    it("errors on format() with too few arguments in run expression", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Format mismatch in run
+runs:
+  using: composite
+  steps:
+    - run: echo \${{ format('{0} {1}', 'only-one') }}
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "format-arg-count-mismatch")).toBe(true);
+    });
+
+    it("errors on format() with too few arguments in input default", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Format mismatch in input default
+inputs:
+  greeting:
+    description: Greeting message
+    default: \${{ format('{0} {1}', 'hello') }}
+runs:
+  using: node20
+  main: index.js
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.code === "format-arg-count-mismatch")).toBe(true);
+    });
+  });
+
+  describe("if condition context validation", () => {
+    it("warns on unknown context in composite step if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Unknown context in if
+runs:
+  using: composite
+  steps:
+    - if: foo == bar
+      run: echo hi
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized named-value"))).toBe(true);
+    });
+
+    it("warns on unknown context in pre-if for node action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Unknown context in pre-if
+runs:
+  using: node20
+  main: index.js
+  pre: setup.js
+  pre-if: foo == bar
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized named-value"))).toBe(true);
+    });
+
+    it("warns on unknown context in post-if for node action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Unknown context in post-if
+runs:
+  using: node20
+  main: index.js
+  post: cleanup.js
+  post-if: foo == bar
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized named-value"))).toBe(true);
+    });
+
+    it("warns on unknown context in pre-if for docker action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Unknown context in pre-if
+runs:
+  using: docker
+  image: Dockerfile
+  pre-entrypoint: /setup.sh
+  pre-if: foo == bar
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized named-value"))).toBe(true);
+    });
+
+    it("warns on unknown context in post-if for docker action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Unknown context in post-if
+runs:
+  using: docker
+  image: Dockerfile
+  post-entrypoint: /cleanup.sh
+  post-if: foo == bar
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized named-value"))).toBe(true);
+    });
+
+    it("allows valid contexts in composite step if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Valid context in if
+runs:
+  using: composite
+  steps:
+    - if: github.event_name == 'push'
+      run: echo hi
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized named-value"))).toBe(false);
+    });
+
+    it("allows valid contexts in pre-if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Valid context in pre-if
+runs:
+  using: node20
+  main: index.js
+  pre: setup.js
+  pre-if: runner.os == 'Linux'
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized named-value"))).toBe(false);
+    });
+
+    it("allows valid contexts in post-if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Valid context in post-if
+runs:
+  using: node20
+  main: index.js
+  post: cleanup.js
+  post-if: runner.os == 'Linux'
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized named-value"))).toBe(false);
+    });
+
+    it("allows hashFiles function in composite step if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: hashFiles in if
+runs:
+  using: composite
+  steps:
+    - if: hashFiles('**/package-lock.json') != ''
+      run: echo hi
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized"))).toBe(false);
+    });
+
+    it("allows success, failure, always, cancelled functions in composite step if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Status functions in if
+runs:
+  using: composite
+  steps:
+    - if: success() && !cancelled()
+      run: echo success
+      shell: bash
+    - if: failure()
+      run: echo failure
+      shell: bash
+    - if: always()
+      run: echo always
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized"))).toBe(false);
+    });
+
+    it("allows hashFiles function in pre-if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: hashFiles in pre-if
+runs:
+  using: node20
+  main: index.js
+  pre: setup.js
+  pre-if: hashFiles('**/package-lock.json') != ''
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized"))).toBe(false);
+    });
+
+    it("allows status functions in post-if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Status functions in post-if
+runs:
+  using: node20
+  main: index.js
+  post: cleanup.js
+  post-if: always() || failure()
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized"))).toBe(false);
+    });
+
+    it("errors on unknown function in composite step if", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Unknown function in if
+runs:
+  using: composite
+  steps:
+    - if: unknownFunc()
+      run: echo hi
+      shell: bash
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized function"))).toBe(true);
+    });
+
+    it("errors on unknown function in pre-if for node action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Unknown function in pre-if
+runs:
+  using: node20
+  main: index.js
+  pre: setup.js
+  pre-if: unknownFunc()
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized function"))).toBe(true);
+    });
+
+    it("errors on unknown function in post-if for node action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Unknown function in post-if
+runs:
+  using: node20
+  main: index.js
+  post: cleanup.js
+  post-if: unknownFunc()
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized function"))).toBe(true);
+    });
+
+    it("errors on unknown function in pre-if for docker action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Unknown function in pre-if
+runs:
+  using: docker
+  image: Dockerfile
+  pre-entrypoint: /setup.sh
+  pre-if: unknownFunc()
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized function"))).toBe(true);
+    });
+
+    it("errors on unknown function in post-if for docker action", async () => {
+      const doc = createActionDocument(`
+name: My Action
+description: Unknown function in post-if
+runs:
+  using: docker
+  image: Dockerfile
+  post-entrypoint: /cleanup.sh
+  post-if: unknownFunc()
+`);
+      const diagnostics = await validate(doc);
+      expect(diagnostics.some(d => d.message.includes("Unrecognized function"))).toBe(true);
+    });
+  });
 });

languageservice/src/validate-action.ts

@@ -2,20 +2,58 @@
  * Validation for action.yml / action.yaml manifest files
  */
 
-import {isMapping} from "@actions/workflow-parser";
+import {Lexer, Parser} from "@actions/expressions";
+import {Expr} from "@actions/expressions/ast";
+import {isMapping, isString} from "@actions/workflow-parser";
 import {isActionStep} from "@actions/workflow-parser/model/type-guards";
 import {ErrorPolicy} from "@actions/workflow-parser/model/convert";
+import {ActionTemplate} from "@actions/workflow-parser/actions/action-template";
+import {ensureStatusFunction} from "@actions/workflow-parser/model/converter/if-condition";
+import {splitAllowedContext} from "@actions/workflow-parser/templates/allowed-context";
+import {BasicExpressionToken} from "@actions/workflow-parser/templates/tokens/basic-expression-token";
+import {MappingToken} from "@actions/workflow-parser/templates/tokens/mapping-token";
 import {SequenceToken} from "@actions/workflow-parser/templates/tokens/sequence-token";
+import {StringToken} from "@actions/workflow-parser/templates/tokens/string-token";
 import {TemplateToken} from "@actions/workflow-parser/templates/tokens/template-token";
+import {TokenRange} from "@actions/workflow-parser/templates/tokens/token-range";
+import {TemplateValidationError} from "@actions/workflow-parser/templates/template-validation-error";
 import {File} from "@actions/workflow-parser/workflows/file";
 import {TextDocument} from "vscode-languageserver-textdocument";
 import {Diagnostic, DiagnosticSeverity} from "vscode-languageserver-types";
 import {error} from "./log.js";
 import {mapRange} from "./utils/range.js";
+import {hasFormatWithLiteralText} from "./utils/validate-if.js";
+import {validateStepUsesFormat} from "./utils/validate-uses.js";
 import {getOrConvertActionTemplate, getOrParseAction} from "./utils/workflow-cache.js";
 import {validateActionReference} from "./validate-action-reference.js";
+import {validateFormatCalls} from "./validate-format-string.js";
 import {ValidationConfig} from "./validate.js";
 
+/**
+ * Valid keys for each action type under the `runs:` section.
+ * Source: https://github.com/actions/runner/blob/main/src/Runner.Worker/ActionManifestManager.cs
+ */
+const NODE_KEYS = new Set(["using", "main", "pre", "post", "pre-if", "post-if"]);
+const COMPOSITE_KEYS = new Set(["using", "steps"]);
+const DOCKER_KEYS = new Set([
+  "using",
+  "image",
+  "args",
+  "env",
+  "entrypoint",
+  "pre-entrypoint",
+  "pre-if",
+  "post-entrypoint",
+  "post-if"
+]);
+
+/**
+ * Required keys for each action type (besides 'using').
+ */
+const NODE_REQUIRED_KEYS = ["main"];
+const COMPOSITE_REQUIRED_KEYS = ["steps"];
+const DOCKER_REQUIRED_KEYS = ["image"];
+
 /**
  * Validates an action.yml file
  *
@@ -38,8 +76,24 @@ export async function validateAction(textDocument: TextDocument, config?: Valida
       return [];
     }
 
-    // Map parser errors to diagnostics
-    for (const err of result.context.errors.getErrors()) {
+    // Convert the action template (this may add validation errors for pre-if/post-if)
+    let template: ActionTemplate | undefined;
+    if (result.value) {
+      template = getOrConvertActionTemplate(result.context, result.value, textDocument.uri, {
+        errorPolicy: ErrorPolicy.TryConversion
+      });
+    }
+
+    // Get schema and conversion errors (must be after conversion to include conversion errors)
+    const schemaErrors = result.context.errors.getErrors();
+
+    // Run custom runs key validation, which also filters redundant schema errors in place
+    if (result.value) {
+      diagnostics.push(...validateRunsKeysAndFilterErrors(result.value, schemaErrors));
+    }
+
+    // Map remaining schema errors to diagnostics
+    for (const err of schemaErrors) {
       const range = mapRange(err.range);
 
       // Determine severity based on error type
@@ -58,13 +112,9 @@ export async function validateAction(textDocument: TextDocument, config?: Valida
     }
 
     // Validate composite action steps if we have a parsed result
-    if (result.value) {
-      const template = getOrConvertActionTemplate(result.context, result.value, textDocument.uri, {
-        errorPolicy: ErrorPolicy.TryConversion
-      });
-
+    if (result.value && template) {
       // Only composite actions have steps to validate
-      if (template?.runs?.using === "composite") {
+      if (template.runs?.using === "composite") {
         const steps = template.runs.steps ?? [];
 
         // Find the steps sequence token from the raw parsed result
@@ -79,9 +129,17 @@ export async function validateAction(textDocument: TextDocument, config?: Valida
             if (isActionStep(step) && isMapping(stepToken)) {
               await validateActionReference(diagnostics, stepToken, step, config);
             }
+
+            // Validate step uses format
+            if (isMapping(stepToken)) {
+              validateStepUsesField(diagnostics, stepToken);
+            }
           }
         }
       }
+
+      // Single traversal for all expression validation (like workflow's additionalValidations)
+      validateAllTokens(diagnostics, result.value);
     }
   } catch (e) {
     error(`Unhandled error while validating action file: ${(e as Error).message}`);
@@ -90,6 +148,196 @@ export async function validateAction(textDocument: TextDocument, config?: Valida
   return diagnostics;
 }
 
+/**
+ * Validates the `uses` field format in a composite action step.
+ */
+function validateStepUsesField(diagnostics: Diagnostic[], stepToken: MappingToken): void {
+  for (let i = 0; i < stepToken.count; i++) {
+    const {key, value} = stepToken.get(i);
+    const keyStr = isString(key) ? key.value.toLowerCase() : "";
+
+    if (keyStr === "uses" && isString(value)) {
+      validateStepUsesFormat(diagnostics, value);
+    }
+  }
+}
+
+/**
+ * Single traversal validation for all tokens in the action template.
+ * This follows the same pattern as workflow validation's additionalValidations:
+ * - For BasicExpressionToken: validate format() calls
+ * - For StringToken on if conditions: validate literal text detection and format() calls
+ * - For pre-if/post-if with explicit ${{ }}: report error (not supported by runner)
+ *
+ * Context validation (unknown named values) is handled by workflow-parser during conversion.
+ */
+function validateAllTokens(diagnostics: Diagnostic[], root: TemplateToken): void {
+  for (const [parent, token] of TemplateToken.traverse(root)) {
+    const definitionKey = token.definition?.key;
+
+    // Validate all BasicExpressionToken instances for format() calls
+    if (token instanceof BasicExpressionToken && token.range) {
+      // Check for literal text in if conditions (format with literal text)
+      if (definitionKey === "step-if") {
+        validateIfLiteralText(diagnostics, token);
+      }
+
+      // Validate format() calls for all expressions
+      for (const expression of token.originalExpressions || [token]) {
+        validateExpressionFormatCalls(diagnostics, expression);
+      }
+
+      // Check for explicit ${{ }} in pre-if/post-if (not supported by runner)
+      if (definitionKey === "runs-if" && parent instanceof MappingToken) {
+        // Resolve the key name (pre-if or post-if) from parent mapping
+        let keyName: string | undefined;
+        for (let i = 0; i < parent.count; i++) {
+          const {key, value} = parent.get(i);
+          if (value === token) {
+            keyName = key.toString().toLowerCase();
+            break;
+          }
+        }
+
+        if (keyName) {
+          diagnostics.push({
+            message: `Explicit expression syntax \${{ }} is not supported for '${keyName}'. Remove the \${{ }} markers and use the expression directly.`,
+            range: mapRange(token.range),
+            severity: DiagnosticSeverity.Error,
+            code: "explicit-expression-not-allowed"
+          });
+        }
+      }
+    }
+
+    // Handle implicit if conditions (StringToken without ${{ }})
+    // These allow expression syntax without the markers
+    if (isString(token) && token.range) {
+      if (definitionKey === "step-if" || definitionKey === "runs-if") {
+        validateImplicitIfCondition(diagnostics, token);
+      }
+    }
+  }
+}
+
+const LITERAL_TEXT_IN_CONDITION_MESSAGE =
+  "Conditional expression contains literal text outside replacement tokens. This will cause the expression to always evaluate to truthy. Did you mean to put the entire expression inside ${{ }}?";
+const LITERAL_TEXT_IN_CONDITION_CODE = "expression-literal-text-in-condition";
+
+/**
+ * Validates an implicit if condition (StringToken without ${{ }}).
+ * Checks for literal text detection and validates format() calls.
+ */
+function validateImplicitIfCondition(diagnostics: Diagnostic[], token: StringToken): void {
+  const condition = token.value.trim();
+  if (!condition) {
+    return;
+  }
+
+  const allowedContext = token.definitionInfo?.allowedContext || [];
+  const {namedContexts, functions} = splitAllowedContext(allowedContext);
+
+  // Ensure the condition has a status function, wrapping if needed
+  const finalCondition = ensureStatusFunction(condition, token.definitionInfo);
+
+  try {
+    const l = new Lexer(finalCondition);
+    const lr = l.lex();
+    const p = new Parser(lr.tokens, namedContexts, functions);
+    const expr = p.parse();
+
+    // Check for literal text in the expression (format with literal text)
+    if (hasFormatWithLiteralText(expr)) {
+      diagnostics.push({
+        message: LITERAL_TEXT_IN_CONDITION_MESSAGE,
+        range: mapRange(token.range),
+        severity: DiagnosticSeverity.Error,
+        code: LITERAL_TEXT_IN_CONDITION_CODE
+      });
+    }
+
+    // Validate format() function calls
+    validateFormatCallsAndAddDiagnostics(diagnostics, expr, token.range);
+  } catch {
+    // Ignore parse errors - they'll be caught by schema validation or workflow-parser
+  }
+}
+
+/**
+ * Validates a BasicExpressionToken for literal text in if conditions.
+ */
+function validateIfLiteralText(diagnostics: Diagnostic[], token: BasicExpressionToken): void {
+  const allowedContext = token.definitionInfo?.allowedContext || [];
+  const {namedContexts, functions} = splitAllowedContext(allowedContext);
+
+  try {
+    const l = new Lexer(token.expression);
+    const lr = l.lex();
+    const p = new Parser(lr.tokens, namedContexts, functions);
+    const expr = p.parse();
+
+    if (hasFormatWithLiteralText(expr)) {
+      diagnostics.push({
+        message: LITERAL_TEXT_IN_CONDITION_MESSAGE,
+        range: mapRange(token.range),
+        severity: DiagnosticSeverity.Error,
+        code: LITERAL_TEXT_IN_CONDITION_CODE
+      });
+    }
+  } catch {
+    // Ignore parse errors - they'll be caught by schema validation or workflow-parser
+  }
+}
+
+/**
+ * Validates format() function calls in an expression token.
+ */
+function validateExpressionFormatCalls(diagnostics: Diagnostic[], token: BasicExpressionToken): void {
+  const allowedContext = token.definitionInfo?.allowedContext || [];
+  const {namedContexts, functions} = splitAllowedContext(allowedContext);
+
+  try {
+    const l = new Lexer(token.expression);
+    const lr = l.lex();
+    const p = new Parser(lr.tokens, namedContexts, functions);
+    const expr = p.parse();
+
+    validateFormatCallsAndAddDiagnostics(diagnostics, expr, token.range);
+  } catch {
+    // Ignore parse errors - they'll be caught by schema validation
+  }
+}
+
+/**
+ * Helper to validate format() function calls and add diagnostics.
+ */
+function validateFormatCallsAndAddDiagnostics(
+  diagnostics: Diagnostic[],
+  expr: Expr,
+  range: TokenRange | undefined
+): void {
+  const formatErrors = validateFormatCalls(expr);
+  for (const formatError of formatErrors) {
+    if (formatError.type === "invalid-syntax") {
+      diagnostics.push({
+        message: `Invalid format string: ${formatError.message}`,
+        range: mapRange(range),
+        severity: DiagnosticSeverity.Error,
+        code: "invalid-format-string"
+      });
+    } else if (formatError.type === "arg-count-mismatch") {
+      diagnostics.push({
+        message: `Format string references argument {${formatError.expected - 1}} but only ${
+          formatError.provided
+        } argument(s) provided`,
+        range: mapRange(range),
+        severity: DiagnosticSeverity.Error,
+        code: "format-arg-count-mismatch"
+      });
+    }
+  }
+}
+
 /**
  * Find the steps sequence token from the raw action template.
  * Traverses the token tree looking for the "composite-steps" definition.
@@ -102,3 +350,133 @@ function findStepsSequence(root: TemplateToken): SequenceToken | undefined {
   }
   return undefined;
 }
+
+/**
+ * Validates that the keys under `runs:` are valid for the specified `using:` type.
+ * Also filters out schema errors (in place) that this validation replaces with more specific messages.
+ */
+function validateRunsKeysAndFilterErrors(
+  root: TemplateToken,
+  schemaErrors: TemplateValidationError[] // mutated: redundant errors are removed
+): Diagnostic[] {
+  const diagnostics: Diagnostic[] = [];
+
+  // Find the runs mapping from the root
+  let runsMapping: MappingToken | undefined;
+  if (root instanceof MappingToken) {
+    for (let i = 0; i < root.count; i++) {
+      const {key, value} = root.get(i);
+      if (key.toString().toLowerCase() === "runs" && value instanceof MappingToken) {
+        runsMapping = value;
+        break;
+      }
+    }
+  }
+  if (!runsMapping) {
+    return diagnostics;
+  }
+
+  // Get the using value from the runs mapping
+  let usingValue: string | undefined;
+  for (let i = 0; i < runsMapping.count; i++) {
+    const {key, value} = runsMapping.get(i);
+    if (key.toString().toLowerCase() === "using") {
+      usingValue = value.toString();
+      break;
+    }
+  }
+  if (!usingValue) {
+    return diagnostics; // No using value, let schema validation handle it
+  }
+
+  // Determine allowed keys, required keys, and action type name
+  let allowedKeys: Set<string>;
+  let requiredKeys: string[];
+  let actionType: string;
+
+  if (usingValue.match(/^node\d+$/i)) {
+    allowedKeys = NODE_KEYS;
+    requiredKeys = NODE_REQUIRED_KEYS;
+    actionType = "Node.js";
+  } else if (usingValue.toLowerCase() === "composite") {
+    allowedKeys = COMPOSITE_KEYS;
+    requiredKeys = COMPOSITE_REQUIRED_KEYS;
+    actionType = "composite";
+  } else if (usingValue.toLowerCase() === "docker") {
+    allowedKeys = DOCKER_KEYS;
+    requiredKeys = DOCKER_REQUIRED_KEYS;
+    actionType = "Docker";
+  } else {
+    return diagnostics; // Unknown type, let schema validation handle it
+  }
+
+  // Get all present keys
+  const presentKeys = new Set<string>();
+  for (let i = 0; i < runsMapping.count; i++) {
+    const {key} = runsMapping.get(i);
+    presentKeys.add(key.toString().toLowerCase());
+  }
+
+  // Check for invalid keys
+  for (let i = 0; i < runsMapping.count; i++) {
+    const {key} = runsMapping.get(i);
+    const keyStr = key.toString().toLowerCase();
+
+    if (!allowedKeys.has(keyStr)) {
+      diagnostics.push({
+        severity: DiagnosticSeverity.Error,
+        range: mapRange(key.range),
+        message: `'${key.toString()}' is not valid for ${actionType} actions (using: ${usingValue})`
+      });
+    }
+  }
+
+  // Check for missing required keys
+  for (const requiredKey of requiredKeys) {
+    if (!presentKeys.has(requiredKey)) {
+      // Find the 'using' key to report the error location
+      let usingKeyRange = runsMapping.range;
+      for (let i = 0; i < runsMapping.count; i++) {
+        const {key} = runsMapping.get(i);
+        if (key.toString().toLowerCase() === "using") {
+          usingKeyRange = key.range;
+          break;
+        }
+      }
+
+      diagnostics.push({
+        severity: DiagnosticSeverity.Error,
+        range: mapRange(usingKeyRange),
+        message: `'${requiredKey}' is required for ${actionType} actions (using: ${usingValue})`
+      });
+    }
+  }
+
+  // Remove schema errors that we're replacing with more specific messages (mutate in place)
+  for (let i = schemaErrors.length - 1; i >= 0; i--) {
+    const err = schemaErrors[i];
+
+    // Keep errors not at the runs section start
+    if (
+      err.range?.start.line !== runsMapping.range?.start.line ||
+      err.range?.start.column !== runsMapping.range?.start.column
+    ) {
+      continue;
+    }
+
+    // Check if this is an error we're replacing
+    const isOneOfAmbiguity = err.rawMessage.startsWith("There's not enough info to determine");
+    const isRequiredKey = /^Required property is missing: (main|steps|image)$/.test(err.rawMessage);
+
+    if (!isOneOfAmbiguity && !isRequiredKey) {
+      continue; // Keep errors we're not replacing
+    }
+
+    // Remove only if we have custom diagnostics for this
+    if (diagnostics.length > 0) {
+      schemaErrors.splice(i, 1);
+    }
+  }
+
+  return diagnostics;
+}

languageservice/src/validate-format-string.ts

@@ -0,0 +1,199 @@
+/**
+ * Format string validation for format() function calls.
+ * Port of Go's format_validator.go from actions-workflow-parser.
+ */
+
+import {Expr, FunctionCall, Literal, Binary, Unary, Logical, Grouping, IndexAccess} from "@actions/expressions/ast";
+import {Kind} from "@actions/expressions/data/expressiondata";
+
+/**
+ * Error types for format string validation
+ */
+export type FormatStringError =
+  | {type: "invalid-syntax"; message: string}
+  | {type: "arg-count-mismatch"; expected: number; provided: number};
+
+/**
+ * Validates a format string and returns the maximum placeholder index.
+ * Port of Go's validateFormatString from format_validator.go.
+ *
+ * @param formatString The format string to validate
+ * @returns { valid: boolean, maxArgIndex: number } where maxArgIndex is -1 if no placeholders
+ */
+export function validateFormatString(formatString: string): {valid: boolean; maxArgIndex: number} {
+  let maxIndex = -1;
+  let i = 0;
+
+  while (i < formatString.length) {
+    // Find next left brace
+    let lbrace = -1;
+    for (let j = i; j < formatString.length; j++) {
+      if (formatString[j] === "{") {
+        lbrace = j;
+        break;
+      }
+    }
+
+    // Find next right brace
+    let rbrace = -1;
+    for (let j = i; j < formatString.length; j++) {
+      if (formatString[j] === "}") {
+        rbrace = j;
+        break;
+      }
+    }
+
+    // No more braces
+    if (lbrace < 0 && rbrace < 0) {
+      break;
+    }
+
+    // Left brace comes first (or only left brace exists)
+    if (lbrace >= 0 && (rbrace < 0 || lbrace < rbrace)) {
+      // Check if it's escaped
+      if (lbrace + 1 < formatString.length && formatString[lbrace + 1] === "{") {
+        // Escaped left brace
+        i = lbrace + 2;
+        continue;
+      }
+
+      // This is a placeholder opening - find the closing brace
+      rbrace = -1;
+      for (let j = lbrace + 1; j < formatString.length; j++) {
+        if (formatString[j] === "}") {
+          rbrace = j;
+          break;
+        }
+      }
+
+      if (rbrace < 0) {
+        // Missing closing brace
+        return {valid: false, maxArgIndex: -1};
+      }
+
+      // Validate placeholder content (must be digits only)
+      if (rbrace === lbrace + 1) {
+        // Empty placeholder {}
+        return {valid: false, maxArgIndex: -1};
+      }
+
+      // Parse the index and validate it's all digits
+      let index = 0;
+      for (let j = lbrace + 1; j < rbrace; j++) {
+        const c = formatString[j];
+        if (c < "0" || c > "9") {
+          // Non-numeric character
+          return {valid: false, maxArgIndex: -1};
+        }
+        index = index * 10 + (c.charCodeAt(0) - "0".charCodeAt(0));
+      }
+
+      if (index > maxIndex) {
+        maxIndex = index;
+      }
+
+      i = rbrace + 1;
+      continue;
+    }
+
+    // Right brace comes first (or only right brace exists)
+    // Check if it's escaped
+    if (rbrace + 1 < formatString.length && formatString[rbrace + 1] === "}") {
+      // Escaped right brace
+      i = rbrace + 2;
+      continue;
+    }
+
+    // Unescaped right brace outside of placeholder
+    return {valid: false, maxArgIndex: -1};
+  }
+
+  return {valid: true, maxArgIndex: maxIndex};
+}
+
+/**
+ * Walks an expression AST to find and validate all format() function calls.
+ *
+ * @param expr The expression AST to validate
+ * @returns Array of validation errors found
+ */
+export function validateFormatCalls(expr: Expr): FormatStringError[] {
+  const errors: FormatStringError[] = [];
+  const stack: Expr[] = [expr];
+
+  while (stack.length > 0) {
+    const node = stack.pop();
+    if (!node) {
+      continue;
+    }
+
+    if (node instanceof FunctionCall) {
+      if (node.functionName.lexeme.toLowerCase() === "format") {
+        const error = validateSingleFormatCall(node);
+        if (error) {
+          errors.push(error);
+        }
+      }
+      // Push args for further processing (to find nested format calls)
+      for (const arg of node.args) {
+        stack.push(arg);
+      }
+    } else if (node instanceof Binary) {
+      stack.push(node.left, node.right);
+    } else if (node instanceof Unary) {
+      stack.push(node.expr);
+    } else if (node instanceof Logical) {
+      for (const arg of node.args) {
+        stack.push(arg);
+      }
+    } else if (node instanceof Grouping) {
+      stack.push(node.group);
+    } else if (node instanceof IndexAccess) {
+      stack.push(node.expr, node.index);
+    }
+    // Literal, ContextAccess - no children to process
+  }
+
+  return errors;
+}
+
+/**
+ * Validates a single format() function call.
+ *
+ * @param fc The FunctionCall AST node
+ * @returns Validation error if found, undefined if valid
+ */
+function validateSingleFormatCall(fc: FunctionCall): FormatStringError | undefined {
+  // Must have at least one argument (the format string)
+  if (fc.args.length < 1) {
+    return undefined;
+  }
+
+  // First argument must be a string literal
+  const firstArg = fc.args[0];
+  if (!(firstArg instanceof Literal) || firstArg.literal.kind !== Kind.String) {
+    return undefined; // Can't validate dynamic format strings
+  }
+
+  const formatString = firstArg.literal.coerceString();
+  const numArgs = fc.args.length - 1; // Subtract 1 for format string itself
+
+  const {valid, maxArgIndex} = validateFormatString(formatString);
+
+  if (!valid) {
+    return {
+      type: "invalid-syntax",
+      message: "Format string has invalid syntax (missing closing brace, unescaped braces, or invalid placeholder)"
+    };
+  }
+
+  if (maxArgIndex >= numArgs) {
+    return {
+      type: "arg-count-mismatch",
+      expected: maxArgIndex + 1, // Convert 0-based index to count
+      provided: numArgs
+    };
+  }
+
+  return undefined;
+}

languageservice/src/validate.block-scalar-warning.test.ts

@@ -0,0 +1,835 @@
+import {FeatureFlags} from "@actions/expressions";
+import {DiagnosticSeverity} from "vscode-languageserver-types";
+import {registerLogger} from "./log.js";
+import {createDocument} from "./test-utils/document.js";
+import {TestLogger} from "./test-utils/logger.js";
+import {clearCache} from "./utils/workflow-cache.js";
+import {validate, ValidationConfig} from "./validate.js";
+
+registerLogger(new TestLogger());
+
+const configWithFlag: ValidationConfig = {
+  featureFlags: new FeatureFlags({blockScalarChompingWarning: true})
+};
+
+beforeEach(() => {
+  clearCache();
+});
+
+describe("block scalar chomping - warning cases", () => {
+  describe("step-level env values", () => {
+    it("warns with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo $VAR
+        env:
+          VAR: |
+            \${{ matrix.value }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+
+    it("does not warn with keep chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo $VAR
+        env:
+          VAR: |+
+            \${{ matrix.value }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+
+    it("does not warn with strip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo $VAR
+        env:
+          VAR: |-
+            \${{ matrix.value }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+
+    it("uses > indicator in warning message for folded scalars", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo $VAR
+        env:
+          VAR: >
+            \${{ matrix.value }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '>' implicitly adds a trailing newline that may be unintentional. Use '>-' to remove it, or '>+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+
+    it("warns for plain string env value with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo $VAR
+        env:
+          VAR: |
+            hello world
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+  });
+
+  describe("job-level env values", () => {
+    it("warns with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      MY_VAR: |
+        some value
+    steps:
+      - run: echo done
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+  });
+
+  describe("workflow-level env values", () => {
+    it("warns with clip chomping", async () => {
+      const input = `
+on: push
+env:
+  GLOBAL_VAR: |
+    some value
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo done
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+  });
+
+  describe("container env values", () => {
+    it("warns with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: node:18
+      env:
+        CONTAINER_VAR: |
+          some value
+    steps:
+      - run: echo done
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+  });
+
+  describe("service container env values", () => {
+    it("warns with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    services:
+      redis:
+        image: redis
+        env:
+          REDIS_PASSWORD: |
+            secret123
+    steps:
+      - run: echo done
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+  });
+
+  describe("action input (with)", () => {
+    it("warns with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          script: |
+            \${{ matrix.value }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+
+    it("does not warn with keep chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          script: |+
+            \${{ matrix.value }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+
+    it("does not warn with strip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          script: |-
+            \${{ matrix.value }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+  });
+
+  describe("reusable workflow inputs (with)", () => {
+    it("warns with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  call-workflow:
+    uses: ./.github/workflows/reusable.yml
+    with:
+      my-input: |
+        some value
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+  });
+
+  describe("reusable workflow secrets", () => {
+    it("warns with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  call-workflow:
+    uses: ./.github/workflows/reusable.yml
+    secrets:
+      my-secret: |
+        \${{ secrets.TOKEN }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+  });
+
+  describe("job outputs", () => {
+    it("warns with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    outputs:
+      my_output: |
+        \${{ steps.test.outputs.value }}
+    steps:
+      - id: test
+        run: echo "value=test" >> $GITHUB_OUTPUT
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+
+    it("does not warn with strip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    outputs:
+      my_output: |-
+        \${{ steps.test.outputs.value }}
+    steps:
+      - id: test
+        run: echo "value=test" >> $GITHUB_OUTPUT
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+  });
+
+  describe("matrix values", () => {
+    it("warns for matrix vector value with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        config:
+          - |
+            value1
+          - value2
+    steps:
+      - run: echo \${{ matrix.config }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+
+    it("does not warn with strip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        config:
+          - |-
+            value1
+          - value2
+    steps:
+      - run: echo \${{ matrix.config }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+
+    it("warns for matrix include value with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+        include:
+          - os: |
+              windows-latest
+            special: true
+    steps:
+      - run: echo \${{ matrix.os }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+
+    it("warns for matrix exclude value with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest]
+        node: [16, 18]
+        exclude:
+          - os: |
+              windows-latest
+            node: 16
+    steps:
+      - run: echo \${{ matrix.os }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+
+    it("warns for deeply nested matrix value with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        config:
+          - foo:
+              bar: |
+                baz
+    steps:
+      - run: echo \${{ matrix.config }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+
+    it("warns for deeply nested matrix include value with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+        include:
+          - os: ubuntu-latest
+            config:
+              nested: |
+                value
+    steps:
+      - run: echo \${{ matrix.config }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+
+    it("warns for deeply nested matrix exclude value with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest]
+        exclude:
+          - os: windows-latest
+            config:
+              nested: |
+                value
+    steps:
+      - run: echo \${{ matrix.os }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+  });
+
+  describe("concurrency", () => {
+    it("warns for concurrency string with clip chomping", async () => {
+      const input = `
+on: push
+concurrency: |
+  my-group-\${{ github.ref }}
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo done
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+
+    it("does not warn for concurrency with strip chomping", async () => {
+      const input = `
+on: push
+concurrency: |-
+  my-group-\${{ github.ref }}
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo done
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+
+    it("warns for concurrency.group with clip chomping", async () => {
+      const input = `
+on: push
+concurrency:
+  group: |
+    my-group-\${{ github.ref }}
+  cancel-in-progress: true
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo done
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+
+    it("warns for job-level concurrency with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    concurrency: |
+      job-group-\${{ github.ref }}
+    steps:
+      - run: echo done
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          message:
+            "Block scalar '|' implicitly adds a trailing newline that may be unintentional. Use '|-' to remove it, or '|+' to explicitly keep it.",
+          code: "block-scalar-chomping",
+          severity: DiagnosticSeverity.Warning
+        })
+      );
+    });
+  });
+});
+
+describe("block scalar chomping - no warning cases", () => {
+  describe("fields trimmed server-side", () => {
+    it("does not warn for job-if with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    if: |
+      github.ref == 'refs/heads/main'
+    steps:
+      - run: echo done
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+
+    it("does not warn for step-if with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo done
+        if: |
+          github.ref == 'refs/heads/main'
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+
+    it("does not warn for runs-on with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: |
+      ubuntu-latest
+    steps:
+      - run: echo done
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+
+    it("does not warn for job name with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    name: |
+      My Job
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo done
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+
+    it("does not warn for step name with clip chomping", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: |
+          My Step
+        run: echo done
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+  });
+
+  describe("run field (intentionally allowed)", () => {
+    it("does not warn for step run field", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          echo hello
+          echo world
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+
+    it("does not warn for run field with expression", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          echo \${{ github.ref }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+  });
+
+  describe("non-block scalars", () => {
+    it("does not warn for quoted strings", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo $VAR
+        env:
+          VAR: "hello world"
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+
+    it("does not warn for flow scalars", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo $VAR
+        env:
+          VAR: hello world
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+
+    it("does not warn for inline expressions", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo $VAR
+        env:
+          VAR: \${{ matrix.value }}
+`;
+      const result = await validate(createDocument("wf.yaml", input), configWithFlag);
+
+      expect(result.filter(d => d.code === "block-scalar-chomping")).toEqual([]);
+    });
+  });
+});

languageservice/src/validate.expressions-literal-text.test.ts

@@ -160,6 +160,21 @@ jobs:
         })
       );
     });
+
+    it("errors on unknown context in plain string if condition", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - if: foo == bar
+        run: echo hi
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+
+      expect(result.some(d => d.message.includes("Unrecognized named-value"))).toBe(true);
+    });
   });
 
   describe("snapshot-if", () => {
@@ -211,4 +226,104 @@ jobs:
       );
     });
   });
+
+  // https://github.com/github/vscode-github-actions/issues/542
+  describe("YAML-quoted expressions", () => {
+    it("allows double-quoted expression in job-if", async () => {
+      // Quotes are needed when the expression contains a colon
+      const input = `
+on: push
+jobs:
+  publish:
+    if: "\${{ startsWith(github.event.head_commit.message, 'chore: release') }}"
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo hi
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+
+      expect(result).not.toContainEqual(
+        expect.objectContaining({
+          code: "expression-literal-text-in-condition"
+        })
+      );
+    });
+
+    it("allows single-quoted expression in job-if", async () => {
+      const input = `
+on: push
+jobs:
+  publish:
+    if: '\${{ startsWith(github.event.head_commit.message, "chore: release") }}'
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo hi
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+
+      expect(result).not.toContainEqual(
+        expect.objectContaining({
+          code: "expression-literal-text-in-condition"
+        })
+      );
+    });
+
+    it("allows double-quoted expression in step-if", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - if: "\${{ contains(github.event.head_commit.message, 'skip: ci') }}"
+        run: echo hi
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+
+      expect(result).not.toContainEqual(
+        expect.objectContaining({
+          code: "expression-literal-text-in-condition"
+        })
+      );
+    });
+
+    it("still errors when there is actual literal text outside expression", async () => {
+      // Even with quotes, if there's literal text outside ${{ }}, it should error
+      const input = `
+on: push
+jobs:
+  build:
+    if: "push == \${{ github.event_name }}"
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo hi
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          code: "expression-literal-text-in-condition"
+        })
+      );
+    });
+
+    it("errors on multiple expressions with literal text between them", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    if: "\${{ true }} and \${{ false }}"
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo hi
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          code: "expression-literal-text-in-condition"
+        })
+      );
+    });
+  });
 });

languageservice/src/validate.format-string.test.ts

@@ -0,0 +1,273 @@
+import {DiagnosticSeverity} from "vscode-languageserver-types";
+import {createDocument} from "./test-utils/document.js";
+import {validate} from "./validate.js";
+import {clearCache} from "./utils/workflow-cache.js";
+import {validateFormatString} from "./validate-format-string.js";
+
+beforeEach(() => {
+  clearCache();
+});
+
+describe("format string validation", () => {
+  describe("validateFormatString unit tests", () => {
+    it("returns valid for simple placeholder", () => {
+      const result = validateFormatString("{0}");
+      expect(result).toEqual({valid: true, maxArgIndex: 0});
+    });
+
+    it("returns valid for multiple placeholders", () => {
+      const result = validateFormatString("{0} {1} {2}");
+      expect(result).toEqual({valid: true, maxArgIndex: 2});
+    });
+
+    it("returns valid for text with placeholder", () => {
+      const result = validateFormatString("hello {0} world");
+      expect(result).toEqual({valid: true, maxArgIndex: 0});
+    });
+
+    it("returns valid for escaped left braces", () => {
+      const result = validateFormatString("{{0}} {0}");
+      expect(result).toEqual({valid: true, maxArgIndex: 0});
+    });
+
+    it("returns valid for escaped right braces", () => {
+      const result = validateFormatString("{0}}}");
+      expect(result).toEqual({valid: true, maxArgIndex: 0});
+    });
+
+    it("returns valid for no placeholders", () => {
+      const result = validateFormatString("hello world");
+      expect(result).toEqual({valid: true, maxArgIndex: -1});
+    });
+
+    it("returns invalid for missing closing brace", () => {
+      const result = validateFormatString("{0");
+      expect(result).toEqual({valid: false, maxArgIndex: -1});
+    });
+
+    it("returns invalid for empty placeholder", () => {
+      const result = validateFormatString("{}");
+      expect(result).toEqual({valid: false, maxArgIndex: -1});
+    });
+
+    it("returns invalid for non-numeric placeholder", () => {
+      const result = validateFormatString("{abc}");
+      expect(result).toEqual({valid: false, maxArgIndex: -1});
+    });
+
+    it("returns invalid for unescaped closing brace", () => {
+      const result = validateFormatString("text } more");
+      expect(result).toEqual({valid: false, maxArgIndex: -1});
+    });
+
+    it("handles out-of-order placeholders", () => {
+      const result = validateFormatString("{2} {0} {1}");
+      expect(result).toEqual({valid: true, maxArgIndex: 2});
+    });
+
+    it("handles repeated placeholders", () => {
+      const result = validateFormatString("{0} {0} {0}");
+      expect(result).toEqual({valid: true, maxArgIndex: 0});
+    });
+  });
+
+  describe("InvalidFormatString workflow validation", () => {
+    it("errors on missing closing brace", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo \${{ format('{0', github.event_name) }}
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          code: "invalid-format-string",
+          severity: DiagnosticSeverity.Error
+        })
+      );
+    });
+
+    it("errors on empty braces", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo \${{ format('{}', github.event_name) }}
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          code: "invalid-format-string"
+        })
+      );
+    });
+
+    it("errors on non-numeric placeholder", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo \${{ format('{abc}', github.event_name) }}
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          code: "invalid-format-string"
+        })
+      );
+    });
+
+    it("allows valid format strings", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo \${{ format('{0} {1}', github.event_name, github.ref) }}
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+      expect(result).not.toContainEqual(
+        expect.objectContaining({
+          code: "invalid-format-string"
+        })
+      );
+    });
+
+    it("allows escaped braces", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo \${{ format('{{0}} {0}', github.event_name) }}
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+      expect(result).not.toContainEqual(
+        expect.objectContaining({
+          code: "invalid-format-string"
+        })
+      );
+    });
+  });
+
+  describe("FormatArgCountMismatch workflow validation", () => {
+    it("errors when placeholder exceeds arg count", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo \${{ format('{2}', 'arg0', 'arg1') }}
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          code: "format-arg-count-mismatch",
+          severity: DiagnosticSeverity.Error
+        })
+      );
+    });
+
+    it("errors when referencing arg 0 with no args", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo \${{ format('{0}') }}
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          code: "format-arg-count-mismatch"
+        })
+      );
+    });
+
+    it("allows when arg count matches", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo \${{ format('{0} {1} {2}', 'a', 'b', 'c') }}
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+      expect(result).not.toContainEqual(
+        expect.objectContaining({
+          code: "format-arg-count-mismatch"
+        })
+      );
+    });
+
+    it("handles no placeholders correctly", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo \${{ format('hello world') }}
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+      expect(result).not.toContainEqual(
+        expect.objectContaining({
+          code: "format-arg-count-mismatch"
+        })
+      );
+    });
+
+    it("skips validation for dynamic format strings", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo \${{ format(env.FORMAT_STRING, 'arg') }}
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+      // Should not have format errors since we can't validate dynamic strings
+      expect(result).not.toContainEqual(
+        expect.objectContaining({
+          code: "invalid-format-string"
+        })
+      );
+      expect(result).not.toContainEqual(
+        expect.objectContaining({
+          code: "format-arg-count-mismatch"
+        })
+      );
+    });
+
+    it("validates nested format calls", async () => {
+      const input = `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo \${{ format('{0}', format('{2}', 'a')) }}
+`;
+      const result = await validate(createDocument("wf.yaml", input));
+      // The inner format call has an error
+      expect(result).toContainEqual(
+        expect.objectContaining({
+          code: "format-arg-count-mismatch"
+        })
+      );
+    });
+  });
+});

languageservice/src/validate.ts

@@ -1,5 +1,5 @@
-import {Lexer, Parser, data} from "@actions/expressions";
-import {Expr, FunctionCall, Literal, Logical} from "@actions/expressions/ast";
+import {FeatureFlags, Lexer, Parser} from "@actions/expressions";
+import {Expr} from "@actions/expressions/ast";
 import {TemplateParseResult, WorkflowTemplate, isBasicExpression, isMapping, isString} from "@actions/workflow-parser";
 import {ErrorPolicy} from "@actions/workflow-parser/model/convert";
 import {getCronDescription, hasCronIntervalLessThan5Minutes} from "@actions/workflow-parser/model/converter/cron";
@@ -24,9 +24,12 @@ import {error} from "./log.js";
 import {isActionDocument} from "./utils/document-type.js";
 import {findToken} from "./utils/find-token.js";
 import {mapRange} from "./utils/range.js";
+import {hasFormatWithLiteralText} from "./utils/validate-if.js";
+import {validateStepUsesFormat, warnIfShortSha} from "./utils/validate-uses.js";
 import {getOrConvertWorkflowTemplate, getOrParseWorkflow} from "./utils/workflow-cache.js";
 import {validateActionReference} from "./validate-action-reference.js";
 import {validateAction} from "./validate-action.js";
+import {validateFormatCalls} from "./validate-format-string.js";
 import {ValueProviderConfig, ValueProviderKind} from "./value-providers/config.js";
 import {defaultValueProviders} from "./value-providers/default.js";
 
@@ -38,6 +41,7 @@ export type ValidationConfig = {
   contextProviderConfig?: ContextProviderConfig;
   actionsMetadataProvider?: ActionsMetadataProvider;
   fileProvider?: FileProvider;
+  featureFlags?: FeatureFlags;
 };
 
 export type ActionsMetadataProvider = {
@@ -80,11 +84,12 @@ async function validateWorkflow(textDocument: TextDocument, config?: ValidationC
       // Errors will be updated in the context. Attempt to do the conversion anyway in order to give the user more information
       const template = await getOrConvertWorkflowTemplate(result.context, result.value, textDocument.uri, config, {
         fetchReusableWorkflowDepth: config?.fileProvider ? 1 : 0,
-        errorPolicy: ErrorPolicy.TryConversion
+        errorPolicy: ErrorPolicy.TryConversion,
+        featureFlags: config?.featureFlags
       });
 
       // Validate expressions and value providers
-      await additionalValidations(diagnostics, textDocument.uri, template, result.value, config);
+      await additionalValidations(diagnostics, textDocument.uri, template, result.value, config, config?.featureFlags);
     }
 
     // For now map parser errors directly to diagnostics
@@ -108,9 +113,10 @@ async function additionalValidations(
   documentUri: URI,
   template: WorkflowTemplate,
   root: TemplateToken,
-  config?: ValidationConfig
+  config?: ValidationConfig,
+  featureFlags?: FeatureFlags
 ) {
-  for (const [parent, token, key] of TemplateToken.traverse(root)) {
+  for (const [parent, token, key, ancestors] of TemplateToken.traverse(root)) {
     // If  the token is a value in a pair, use the key definition for validation
     // If the token has a parent (map, sequence, etc), use this definition for validation
     const validationToken = key || parent || token;
@@ -128,7 +134,12 @@ async function additionalValidations(
       );
     }
 
-    // If this is a job-if, step-if, or snapshot-if field (which are strings that should be treated as expressions), validate it
+    // Validate block scalar chomping for expressions and strings
+    if (featureFlags?.isEnabled("blockScalarChompingWarning")) {
+      validateBlockScalarChomping(diagnostics, token, parent, key, ancestors);
+    }
+
+    // `if` conditions allow omitting ${{ }}, so validate strings in these fields as expressions
     const definitionKey = token.definition?.key;
     if (
       isString(token) &&
@@ -148,7 +159,9 @@ async function additionalValidations(
           finalCondition,
           token.definitionInfo,
           undefined,
-          token.source
+          token.source,
+          undefined,
+          token.blockScalarHeader
         );
 
         await validateExpression(
@@ -275,116 +288,6 @@ function validateCronExpression(diagnostics: Diagnostic[], token: StringToken):
   }
 }
 
-// Matches a short SHA (7-8 hex characters) that looks like it should be a full SHA
-const SHORT_SHA_PATTERN = /^[0-9a-f]{7,8}$/i;
-const SHORT_SHA_DOCS_URL =
-  "https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-third-party-actions";
-
-/**
- * Checks if a ref looks like a short SHA and adds a warning if so.
- * Returns true if a warning was added.
- */
-function warnIfShortSha(diagnostics: Diagnostic[], token: StringToken, ref: string): boolean {
-  if (SHORT_SHA_PATTERN.test(ref)) {
-    diagnostics.push({
-      message: `The provided ref '${ref}' may be a shortened commit SHA. If so, please use the full 40-character commit SHA instead, as short SHAs are not supported.`,
-      severity: DiagnosticSeverity.Warning,
-      range: mapRange(token.range),
-      code: "short-sha-ref",
-      codeDescription: {
-        href: SHORT_SHA_DOCS_URL
-      }
-    });
-    return true;
-  }
-  return false;
-}
-
-/**
- * Validates the format of a step's `uses` field.
- *
- * Valid formats:
- * - docker://image:tag
- * - ./local/path
- * - .\local\path (Windows)
- * - {owner}/{repo}@{ref}
- * - {owner}/{repo}/{path}@{ref}
- */
-function validateStepUsesFormat(diagnostics: Diagnostic[], token: StringToken): void {
-  const uses = token.value;
-
-  // Empty uses value
-  if (!uses) {
-    diagnostics.push({
-      message: "`uses' value in action cannot be blank",
-      severity: DiagnosticSeverity.Error,
-      range: mapRange(token.range),
-      code: "invalid-uses-format"
-    });
-    return;
-  }
-
-  // Docker image reference - always valid format
-  if (uses.startsWith("docker://")) {
-    return;
-  }
-
-  // Local action path - always valid format
-  if (uses.startsWith("./") || uses.startsWith(".\\")) {
-    return;
-  }
-
-  // Remote action: must be {owner}/{repo}[/path]@{ref}
-  const atSegments = uses.split("@");
-
-  // Must have exactly one @
-  if (atSegments.length !== 2) {
-    addStepUsesFormatError(diagnostics, token);
-    return;
-  }
-
-  const [repoPath, gitRef] = atSegments;
-
-  // Ref cannot be empty
-  if (!gitRef) {
-    addStepUsesFormatError(diagnostics, token);
-    return;
-  }
-
-  // Split by / or \ to get path segments
-  const pathSegments = repoPath.split(/[\\/]/);
-
-  // Must have at least owner and repo (both non-empty)
-  if (pathSegments.length < 2 || !pathSegments[0] || !pathSegments[1]) {
-    addStepUsesFormatError(diagnostics, token);
-    return;
-  }
-
-  // Check if this is a reusable workflow reference (should be at job level, not step)
-  // Path would be like: owner/repo/.github/workflows/file.yml
-  if (pathSegments.length >= 4 && pathSegments[2] === ".github" && pathSegments[3] === "workflows") {
-    diagnostics.push({
-      message: "Reusable workflows should be referenced at the top-level `jobs.<job_id>.uses` key, not within steps",
-      severity: DiagnosticSeverity.Error,
-      range: mapRange(token.range),
-      code: "invalid-uses-format"
-    });
-    return;
-  }
-
-  // Warn if ref looks like a short SHA
-  warnIfShortSha(diagnostics, token, gitRef);
-}
-
-function addStepUsesFormatError(diagnostics: Diagnostic[], token: StringToken): void {
-  diagnostics.push({
-    message: `Expected format {owner}/{repo}[/path]@{ref}. Actual '${token.value}'`,
-    severity: DiagnosticSeverity.Error,
-    range: mapRange(token.range),
-    code: "invalid-uses-format"
-  });
-}
-
 /**
  * Validates the format of a job's `uses` field (reusable workflow reference).
  *
@@ -629,64 +532,6 @@ function getProviderContext(
   return getWorkflowContext(documentUri, template, path);
 }
 
-/**
- * Checks if a format function contains literal text in its format string.
- * This indicates user confusion about how expressions work.
- *
- * Example: format('push == {0}', github.event_name)
- * The literal text "push == " will always evaluate to truthy.
- *
- * @param expr The expression to check
- * @returns true if the expression is a format() call with literal text
- */
-function hasFormatWithLiteralText(expr: Expr): boolean {
-  // If this is a logical AND expression (from ensureStatusFunction wrapping)
-  // check the right side for the format call
-  if (expr instanceof Logical && expr.operator.lexeme === "&&" && expr.args.length === 2) {
-    return hasFormatWithLiteralText(expr.args[1]);
-  }
-
-  if (!(expr instanceof FunctionCall)) {
-    return false;
-  }
-
-  // Check if this is a format function
-  if (expr.functionName.lexeme.toLowerCase() !== "format") {
-    return false;
-  }
-
-  // Check if the first argument is a string literal
-  if (expr.args.length < 1) {
-    return false;
-  }
-
-  const firstArg = expr.args[0];
-  if (!(firstArg instanceof Literal) || firstArg.literal.kind !== data.Kind.String) {
-    return false;
-  }
-
-  // Get the format string and trim whitespace
-  const formatString = firstArg.literal.coerceString();
-  const trimmed = formatString.trim();
-
-  // Check if there's literal text (non-replacement tokens) after trimming
-  let inToken = false;
-  for (let i = 0; i < trimmed.length; i++) {
-    if (!inToken && trimmed[i] === "{") {
-      inToken = true;
-    } else if (inToken && trimmed[i] === "}") {
-      inToken = false;
-    } else if (inToken && trimmed[i] >= "0" && trimmed[i] <= "9") {
-      // OK - this is a replacement token like {0}, {1}, etc.
-    } else {
-      // Found literal text
-      return true;
-    }
-  }
-
-  return false;
-}
-
 async function validateExpression(
   diagnostics: Diagnostic[],
   token: BasicExpressionToken,
@@ -735,6 +580,28 @@ async function validateExpression(
       continue;
     }
 
+    // Validate format() function calls
+    const formatErrors = validateFormatCalls(expr);
+    for (const formatError of formatErrors) {
+      if (formatError.type === "invalid-syntax") {
+        diagnostics.push({
+          message: `Invalid format string: ${formatError.message}`,
+          range: mapRange(expression.range),
+          severity: DiagnosticSeverity.Error,
+          code: "invalid-format-string"
+        });
+      } else if (formatError.type === "arg-count-mismatch") {
+        diagnostics.push({
+          message: `Format string references argument {${formatError.expected - 1}} but only ${
+            formatError.provided
+          } argument(s) provided`,
+          range: mapRange(expression.range),
+          severity: DiagnosticSeverity.Error,
+          code: "format-arg-count-mismatch"
+        });
+      }
+    }
+
     const context = await getWorkflowExpressionContext(
       namedContexts,
       contextProviderConfig,
@@ -822,3 +689,90 @@ function getStaticConcurrencyGroup(token: TemplateToken | undefined): StringToke
 
   return undefined;
 }
+
+/**
+ * Validates YAML block scalar chomping.
+ *
+ * Block scalars (| and >) implicitly add a trailing newline by default ("clip" chomping).
+ * This is often unintended by the workflow author and can cause unexpected behavior.
+ * This function warns on certain fields when clip chomping is used (implicit trailing newline)
+ * and suggests they explicitly use strip (|-) or keep (|+) to clarify intent.
+ *
+ * Only specific fields are validated - those where trailing newlines may cause
+ * issues but aren't automatically trimmed server-side. For example env, inputs, outputs, etc.
+ *
+ * Skipped fields:
+ * - run: Multi-line scripts commonly have trailing newlines
+ * - Fields trimmed server-side: name, uses, shell, if, etc.
+ */
+function validateBlockScalarChomping(
+  diagnostics: Diagnostic[],
+  token: TemplateToken,
+  parent: TemplateToken | undefined,
+  key: TemplateToken | undefined,
+  ancestors: TemplateToken[]
+): void {
+  // Not an expression or string?
+  if (!isBasicExpression(token) && !isString(token)) {
+    return;
+  }
+
+  // Not a block scalar?
+  const header = token.blockScalarHeader;
+  if (!header) {
+    return;
+  }
+
+  // Not "clip" chomp style?
+  if (header.includes("+") || header.includes("-")) {
+    return;
+  }
+
+  // Check if we should warn
+  let shouldWarn = false;
+  const parentDefinitionName = parent?.definition?.key;
+  const tokenDefinitionName = token.definition?.key;
+  const keyName = key && isString(key) ? key.value : undefined;
+  if (
+    parentDefinitionName &&
+    [
+      "workflow-env",
+      "job-env",
+      "step-env",
+      "container-env",
+      "step-with",
+      "job-outputs",
+      "workflow-job-with",
+      "workflow-job-secrets"
+    ].includes(parentDefinitionName)
+  ) {
+    // env, with, outputs, or secrets fields
+    shouldWarn = true;
+  } else if (
+    ancestors.some(ancestor => {
+      const ancestorKey = ancestor.definition?.key;
+      return ancestorKey === "matrix" || ancestorKey === "matrix-filter" || ancestorKey === "matrix-filter-item";
+    })
+  ) {
+    // Matrix values (vectors, include, exclude)
+    shouldWarn = true;
+  } else if (tokenDefinitionName && ["workflow-concurrency", "job-concurrency"].includes(tokenDefinitionName)) {
+    // Concurrency shorthand
+    shouldWarn = true;
+  } else if (keyName === "group" && parentDefinitionName === "concurrency-mapping") {
+    // Concurrency group field
+    shouldWarn = true;
+  }
+
+  if (!shouldWarn) {
+    return;
+  }
+
+  const blockIndicator = header.startsWith("|") ? "|" : ">";
+  diagnostics.push({
+    message: `Block scalar '${blockIndicator}' implicitly adds a trailing newline that may be unintentional. Use '${blockIndicator}-' to remove it, or '${blockIndicator}+' to explicitly keep it.`,
+    range: mapRange(token.range),
+    severity: DiagnosticSeverity.Warning,
+    code: "block-scalar-chomping"
+  });
+}

languageservice/src/validate.uses-format.test.ts

@@ -295,7 +295,7 @@ jobs:
 `;
       const result = await validate(createDocument("wf.yaml", input));
       expect(result).toContainEqual({
-        message: "`uses' value in action cannot be blank",
+        message: "'uses' value in action cannot be blank",
         severity: DiagnosticSeverity.Error,
         range: {
           start: {line: 5, character: 12},

languageservice/src/value-providers/config.ts

@@ -27,6 +27,12 @@ export interface Value {
     range: {start: {line: number; character: number}; end: {line: number; character: number}};
     newText: string;
   };
+
+  /** Additional text edits to apply after the main edit (e.g., cleanup edits) */
+  additionalTextEdits?: {
+    range: {start: {line: number; character: number}; end: {line: number; character: number}};
+    newText: string;
+  }[];
 }
 
 export enum ValueProviderKind {

languageservice/src/value-providers/default.ts

@@ -4,19 +4,36 @@ import {reusableJobInputs} from "./reusable-job-inputs.js";
 import {reusableJobSecrets} from "./reusable-job-secrets.js";
 import {stringsToValues} from "./strings-to-values.js";
 
+// Refer to: https://github.com/actions/runner-images?tab=readme-ov-file#available-images
 export const DEFAULT_RUNNER_LABELS = [
-  "ubuntu-latest",
-  "ubuntu-24.04",
-  "ubuntu-22.04",
-  "ubuntu-20.04",
-  "ubuntu-slim",
-  "windows-latest",
-  "windows-2022",
-  "windows-2019",
-  "macos-latest",
-  "macos-15",
+  "codespaces-prebuild",
+  "macos-13",
+  "macos-13-large",
+  "macos-13-xlarge",
   "macos-14",
-  "self-hosted"
+  "macos-14-large",
+  "macos-14-xlarge",
+  "macos-15",
+  "macos-15-intel",
+  "macos-15-large",
+  "macos-15-xlarge",
+  "macos-26",
+  "macos-26-large",
+  "macos-26-xlarge",
+  "macos-latest",
+  "macos-latest-large",
+  "macos-latest-xlarge",
+  "self-hosted",
+  "ubuntu-22.04",
+  "ubuntu-22.04-arm",
+  "ubuntu-24.04",
+  "ubuntu-24.04-arm",
+  "ubuntu-latest",
+  "ubuntu-slim",
+  "windows-2022",
+  "windows-2025",
+  "windows-2025-vs2026",
+  "windows-latest"
 ];
 
 const runsOnValueProvider = {

languageservice/src/value-providers/definition.ts

@@ -107,10 +107,14 @@ function mappingValues(
   for (const [key, value] of Object.entries(mappingDefinition.properties)) {
     let insertText: string | undefined;
 
-    let description: string | undefined;
+    // Prefer the property's own description (from the schema's property definition),
+    // fall back to the type definition's description if the property doesn't have one
+    let description: string | undefined = value.description;
     if (value.type) {
       const typeDef = definitions[value.type];
-      description = typeDef?.description;
+      if (!description) {
+        description = typeDef?.description;
+      }
 
       if (typeDef) {
         switch (typeDef.definitionType) {

lerna.json

@@ -6,5 +6,5 @@
     "languageservice",
     "languageserver"
   ],
-  "version": "0.3.32"
+  "version": "0.3.47"
 }

package.json

@@ -9,10 +9,7 @@
     "./languageserver"
   ],
   "devDependencies": {
-    "lerna": "^8.2.2",
+    "lerna": "^9.0.0",
     "typescript": "5.8.3"
-  },
-  "overrides": {
-    "typescript": "$typescript"
   }
 }

workflow-parser/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@actions/workflow-parser",
-  "version": "0.3.32",
+  "version": "0.3.47",
   "license": "MIT",
   "type": "module",
   "source": "./src/index.ts",
@@ -48,12 +48,12 @@
     "watch": "tsc --build tsconfig.build.json --watch"
   },
   "dependencies": {
-    "@actions/expressions": "^0.3.32",
+    "@actions/expressions": "^0.3.47",
     "cronstrue": "^2.21.0",
     "yaml": "^2.0.0-8"
   },
   "engines": {
-    "node": ">= 18"
+    "node": ">= 20"
   },
   "files": [
     "dist/**/*"
@@ -69,6 +69,6 @@
     "prettier": "^2.8.3",
     "rimraf": "^3.0.2",
     "ts-jest": "^29.0.3",
-    "typescript": "^4.8.4"
+    "typescript": "^5.8.3"
   }
 }

workflow-parser/src/action-v1.0.json

@@ -137,6 +137,24 @@
       ],
       "string": {}
     },
+    "runs-if": {
+      "description": "Condition to control when this action's pre or post script runs.",
+      "context": [
+        "runner",
+        "github",
+        "job",
+        "strategy",
+        "matrix",
+        "env",
+        "inputs",
+        "always(0,0)",
+        "success(0,0)",
+        "failure(0,0)",
+        "cancelled(0,0)",
+        "hashFiles(1,255)"
+      ],
+      "string": {}
+    },
     "runs": {
       "one-of": [
         "container-runs",
@@ -242,7 +260,7 @@
             "description": "Allows you to run a script before the entrypoint action begins.\n\n[Documentation](https://docs.github.com/actions/creating-actions/metadata-syntax-for-github-actions#runspre-entrypoint)"
           },
           "pre-if": {
-            "type": "non-empty-string",
+            "type": "runs-if",
             "description": "Allows you to define conditions for the pre: action execution. The pre: action will only run if the conditions in pre-if are met. If not set, then pre-if defaults to always().\n\n[Documentation](https://docs.github.com/actions/creating-actions/metadata-syntax-for-github-actions#runspre-if)"
           },
           "post-entrypoint": {
@@ -250,7 +268,7 @@
             "description": "Allows you to run a cleanup script once the runs.entrypoint action has completed.\n\n[Documentation](https://docs.github.com/actions/creating-actions/metadata-syntax-for-github-actions#runspost-entrypoint)"
           },
           "post-if": {
-            "type": "non-empty-string",
+            "type": "runs-if",
             "description": "Allows you to define conditions for the post: action execution. The post: action will only run if the conditions in post-if are met. If not set, then post-if defaults to always().\n\n[Documentation](https://docs.github.com/actions/creating-actions/metadata-syntax-for-github-actions#runspost-if)"
           }
         }
@@ -267,6 +285,7 @@
           },
           "main": {
             "type": "non-empty-string",
+            "required": true,
             "description": "The file that contains your action code. The runtime specified in using executes this file.\n\n[Documentation](https://docs.github.com/actions/creating-actions/metadata-syntax-for-github-actions#runsmain)"
           },
           "pre": {
@@ -274,7 +293,7 @@
             "description": "Allows you to run a script at the start of a job, before the main: action begins. You can use pre: to run prerequisite setup scripts.\n\n[Documentation](https://docs.github.com/actions/creating-actions/metadata-syntax-for-github-actions#runspre)"
           },
           "pre-if": {
-            "type": "non-empty-string",
+            "type": "runs-if",
             "description": "Allows you to define conditions for the pre: action execution. The pre: action will only run if the conditions in pre-if are met. If not set, then pre-if defaults to always().\n\n[Documentation](https://docs.github.com/actions/creating-actions/metadata-syntax-for-github-actions#runspre-if)"
           },
           "post": {
@@ -282,7 +301,7 @@
             "description": "Allows you to run a script at the end of a job, once the main: action has completed. You can use post: to run cleanup scripts.\n\n[Documentation](https://docs.github.com/actions/creating-actions/metadata-syntax-for-github-actions#runspost)"
           },
           "post-if": {
-            "type": "non-empty-string",
+            "type": "runs-if",
             "description": "Allows you to define conditions for the post: action execution. The post: action will only run if the conditions in post-if are met. If not set, then post-if defaults to always().\n\n[Documentation](https://docs.github.com/actions/creating-actions/metadata-syntax-for-github-actions#runspost-if)"
           }
         }

workflow-parser/src/actions/action-parser.test.ts

@@ -317,4 +317,53 @@ runs:
       }
     }
   });
+
+  it("reports error for invalid context in pre-if", () => {
+    const content = `
+name: Node Action
+description: A node action
+runs:
+  using: node20
+  main: dist/index.js
+  pre: dist/setup.js
+  pre-if: foo == bar`;
+
+    const result = parseAction({name: "action.yml", content}, nullTrace);
+    expect(result.value).toBeDefined();
+    if (!result.value) return;
+
+    // Should have no errors before conversion
+    expect(result.context.errors.count).toBe(0);
+
+    // Convert the template - this should add the validation error
+    convertActionTemplate(result.context, result.value);
+
+    // Should have an error now about invalid context
+    expect(result.context.errors.count).toBeGreaterThan(0);
+    const errors = result.context.errors.getErrors();
+    expect(errors.some(e => e.rawMessage.includes("foo"))).toBe(true);
+  });
+
+  it("accepts valid context in pre-if", () => {
+    const content = `
+name: Node Action
+description: A node action
+runs:
+  using: node20
+  main: dist/index.js
+  pre: dist/setup.js
+  pre-if: runner.os == 'Linux'`;
+
+    const result = parseAction({name: "action.yml", content}, nullTrace);
+    expect(result.value).toBeDefined();
+    if (!result.value) return;
+
+    const template = convertActionTemplate(result.context, result.value);
+
+    // Should have no errors
+    expect(result.context.errors.count).toBe(0);
+    if (template.runs.using === "node20") {
+      expect(template.runs.preIf).toBe("runner.os == 'Linux'");
+    }
+  });
 });

workflow-parser/src/actions/action-template.ts

@@ -9,7 +9,7 @@ import {TemplateContext} from "../templates/template-context.js";
 import {isBoolean, isMapping, isScalar, isSequence, isString} from "../templates/tokens/type-guards.js";
 import {ErrorPolicy} from "../model/convert.js";
 import {Step} from "../model/workflow-template.js";
-import {convertToIfCondition} from "../model/converter/if-condition.js";
+import {convertToIfCondition, validateRunsIfCondition} from "../model/converter/if-condition.js";
 
 /**
  * Represents a parsed and converted action.yml file
@@ -310,7 +310,7 @@ function convertRuns(context: TemplateContext, token: TemplateToken): ActionRuns
 
       case "pre-if":
         if (isString(item.value)) {
-          preIf = item.value.value;
+          preIf = validateRunsIfCondition(context, item.value, item.value.value);
         }
         break;
 
@@ -322,7 +322,7 @@ function convertRuns(context: TemplateContext, token: TemplateToken): ActionRuns
 
       case "post-if":
         if (isString(item.value)) {
-          postIf = item.value.value;
+          postIf = validateRunsIfCondition(context, item.value, item.value.value);
         }
         break;
 

workflow-parser/src/expressions.test.ts

@@ -201,4 +201,355 @@ jobs:
       throw new Error("expected if to be a string (will be converted to expression later)");
     }
   });
+
+  describe("Block scalar chomp style preservation", () => {
+    it("preserves clip chomping (|) for literal block scalar", () => {
+      const result = parseWorkflow(
+        {
+          name: "test.yaml",
+          content: `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      TEST: |
+        \${{ github.event_name }}
+    steps:
+      - run: echo hi`
+        },
+        nullTrace
+      );
+
+      expect(result.context.errors.getErrors()).toHaveLength(0);
+
+      const workflowRoot = result.value!.assertMapping("root")!;
+      const jobs = workflowRoot.get(1).value.assertMapping("jobs");
+      const build = jobs.get(0).value.assertMapping("job");
+      const env = build.get(1).value.assertMapping("env");
+      const testToken = env.get(0).value;
+
+      if (!isBasicExpression(testToken)) {
+        throw new Error("expected TEST to be a basic expression");
+      }
+
+      expect(testToken.blockScalarHeader).toBe("|");
+    });
+
+    it("preserves strip chomping (|-) for literal block scalar", () => {
+      const result = parseWorkflow(
+        {
+          name: "test.yaml",
+          content: `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      TEST: |-
+        \${{ github.event_name }}
+    steps:
+      - run: echo hi`
+        },
+        nullTrace
+      );
+
+      expect(result.context.errors.getErrors()).toHaveLength(0);
+
+      const workflowRoot = result.value!.assertMapping("root")!;
+      const jobs = workflowRoot.get(1).value.assertMapping("jobs");
+      const build = jobs.get(0).value.assertMapping("job");
+      const env = build.get(1).value.assertMapping("env");
+      const testToken = env.get(0).value;
+
+      if (!isBasicExpression(testToken)) {
+        throw new Error("expected TEST to be a basic expression");
+      }
+
+      expect(testToken.blockScalarHeader).toBe("|-");
+    });
+
+    it("preserves keep chomping (|+) for literal block scalar", () => {
+      const result = parseWorkflow(
+        {
+          name: "test.yaml",
+          content: `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      TEST: |+
+        \${{ github.event_name }}
+    steps:
+      - run: echo hi`
+        },
+        nullTrace
+      );
+
+      expect(result.context.errors.getErrors()).toHaveLength(0);
+
+      const workflowRoot = result.value!.assertMapping("root")!;
+      const jobs = workflowRoot.get(1).value.assertMapping("jobs");
+      const build = jobs.get(0).value.assertMapping("job");
+      const env = build.get(1).value.assertMapping("env");
+      const testToken = env.get(0).value;
+
+      if (!isBasicExpression(testToken)) {
+        throw new Error("expected TEST to be a basic expression");
+      }
+
+      expect(testToken.blockScalarHeader).toBe("|+");
+    });
+
+    it("preserves folded clip (>) chomping", () => {
+      const result = parseWorkflow(
+        {
+          name: "test.yaml",
+          content: `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      TEST: >
+        \${{ github.event_name }}
+    steps:
+      - run: echo hi`
+        },
+        nullTrace
+      );
+
+      expect(result.context.errors.getErrors()).toHaveLength(0);
+
+      const workflowRoot = result.value!.assertMapping("root")!;
+      const jobs = workflowRoot.get(1).value.assertMapping("jobs");
+      const build = jobs.get(0).value.assertMapping("job");
+      const env = build.get(1).value.assertMapping("env");
+      const testToken = env.get(0).value;
+
+      if (!isBasicExpression(testToken)) {
+        throw new Error("expected TEST to be a basic expression");
+      }
+
+      expect(testToken.blockScalarHeader).toBe(">");
+    });
+
+    it("preserves folded strip (>-) chomping", () => {
+      const result = parseWorkflow(
+        {
+          name: "test.yaml",
+          content: `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      TEST: >-
+        \${{ github.event_name }}
+    steps:
+      - run: echo hi`
+        },
+        nullTrace
+      );
+
+      expect(result.context.errors.getErrors()).toHaveLength(0);
+
+      const workflowRoot = result.value!.assertMapping("root")!;
+      const jobs = workflowRoot.get(1).value.assertMapping("jobs");
+      const build = jobs.get(0).value.assertMapping("job");
+      const env = build.get(1).value.assertMapping("env");
+      const testToken = env.get(0).value;
+
+      if (!isBasicExpression(testToken)) {
+        throw new Error("expected TEST to be a basic expression");
+      }
+
+      expect(testToken.blockScalarHeader).toBe(">-");
+    });
+
+    it("preserves with explicit indent (|2)", () => {
+      const result = parseWorkflow(
+        {
+          name: "test.yaml",
+          content: `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      TEST: |2
+        \${{ github.event_name }}
+    steps:
+      - run: echo hi`
+        },
+        nullTrace
+      );
+
+      expect(result.context.errors.getErrors()).toHaveLength(0);
+
+      const workflowRoot = result.value!.assertMapping("root")!;
+      const jobs = workflowRoot.get(1).value.assertMapping("jobs");
+      const build = jobs.get(0).value.assertMapping("job");
+      const env = build.get(1).value.assertMapping("env");
+      const testToken = env.get(0).value;
+
+      if (!isBasicExpression(testToken)) {
+        throw new Error("expected TEST to be a basic expression");
+      }
+
+      expect(testToken.blockScalarHeader).toBe("|2");
+    });
+
+    it("preserves with explicit indent and strip (|-2)", () => {
+      const result = parseWorkflow(
+        {
+          name: "test.yaml",
+          content: `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      TEST: |-2
+        \${{ github.event_name }}
+    steps:
+      - run: echo hi`
+        },
+        nullTrace
+      );
+
+      expect(result.context.errors.getErrors()).toHaveLength(0);
+
+      const workflowRoot = result.value!.assertMapping("root")!;
+      const jobs = workflowRoot.get(1).value.assertMapping("jobs");
+      const build = jobs.get(0).value.assertMapping("job");
+      const env = build.get(1).value.assertMapping("env");
+      const testToken = env.get(0).value;
+
+      if (!isBasicExpression(testToken)) {
+        throw new Error("expected TEST to be a basic expression");
+      }
+
+      expect(testToken.blockScalarHeader).toBe("|-2");
+    });
+
+    it("handles flow scalars (no chomp info for inline)", () => {
+      const result = parseWorkflow(
+        {
+          name: "test.yaml",
+          content: `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      TEST: \${{ github.event_name }}
+    steps:
+      - run: echo hi`
+        },
+        nullTrace
+      );
+
+      expect(result.context.errors.getErrors()).toHaveLength(0);
+
+      const workflowRoot = result.value!.assertMapping("root")!;
+      const jobs = workflowRoot.get(1).value.assertMapping("jobs");
+      const build = jobs.get(0).value.assertMapping("job");
+      const env = build.get(1).value.assertMapping("env");
+      const testToken = env.get(0).value;
+
+      if (!isBasicExpression(testToken)) {
+        throw new Error("expected TEST to be a basic expression");
+      }
+
+      expect(testToken.blockScalarHeader).toBeUndefined();
+    });
+
+    it("preserves block scalar info for format expressions with multiple sub-expressions", () => {
+      const result = parseWorkflow(
+        {
+          name: "test.yaml",
+          content: `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      TEST: |
+        Hello \${{ github.event_name }} World \${{ github.ref }}
+    steps:
+      - run: echo hi`
+        },
+        nullTrace
+      );
+
+      expect(result.context.errors.getErrors()).toHaveLength(0);
+
+      const workflowRoot = result.value!.assertMapping("root")!;
+      const jobs = workflowRoot.get(1).value.assertMapping("jobs");
+      const build = jobs.get(0).value.assertMapping("job");
+      const env = build.get(1).value.assertMapping("env");
+      const testToken = env.get(0).value;
+
+      if (!isBasicExpression(testToken)) {
+        throw new Error("expected TEST to be a basic expression");
+      }
+
+      // The format expression should preserve the block scalar info
+      expect(testToken.blockScalarHeader).toBe("|");
+    });
+
+    it("preserves block scalar info on StringToken for isExpression fields", () => {
+      const result = parseWorkflow(
+        {
+          name: "test.yaml",
+          content: `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    if: |
+      github.event_name == 'push'
+    steps:
+      - run: echo hi`
+        },
+        nullTrace
+      );
+
+      expect(result.context.errors.getErrors()).toHaveLength(0);
+
+      const workflowRoot = result.value!.assertMapping("root")!;
+      const jobs = workflowRoot.get(1).value.assertMapping("jobs");
+      const build = jobs.get(0).value.assertMapping("job");
+      const ifToken = build.get(1).value;
+
+      // For isExpression fields without ${{ }}, the token is a StringToken
+      if (!isString(ifToken)) {
+        throw new Error("expected if to be a string");
+      }
+
+      expect(ifToken.blockScalarHeader).toBe("|");
+    });
+
+    it("preserves block scalar info on StringToken for isExpression fields with strip", () => {
+      const result = parseWorkflow(
+        {
+          name: "test.yaml",
+          content: `on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    if: |-
+      github.event_name == 'push'
+    steps:
+      - run: echo hi`
+        },
+        nullTrace
+      );
+
+      expect(result.context.errors.getErrors()).toHaveLength(0);
+
+      const workflowRoot = result.value!.assertMapping("root")!;
+      const jobs = workflowRoot.get(1).value.assertMapping("jobs");
+      const build = jobs.get(0).value.assertMapping("job");
+      const ifToken = build.get(1).value;
+
+      if (!isString(ifToken)) {
+        throw new Error("expected if to be a string");
+      }
+
+      expect(ifToken.blockScalarHeader).toBe("|-");
+    });
+  });
 });

workflow-parser/src/model/convert.test.ts

@@ -1,4 +1,5 @@
 /* eslint-disable @typescript-eslint/no-non-null-assertion */
+import {FeatureFlags} from "@actions/expressions/features";
 import {nullTrace} from "../test-utils/null-trace.js";
 import {parseWorkflow} from "../workflows/workflow-parser.js";
 import {convertWorkflowTemplate, ErrorPolicy} from "./convert.js";
@@ -578,4 +579,140 @@ jobs:
       }
     });
   });
+
+  describe("schedule timezone with feature flags", () => {
+    it("allows timezone when allowCronTimezone is enabled", async () => {
+      const result = parseWorkflow(
+        {
+          name: "wf.yaml",
+          content: `on:
+  schedule:
+    - cron: '0 0 * * *'
+      timezone: America/New_York
+jobs:
+  build:
+    runs-on: ubuntu-latest`
+        },
+        nullTrace
+      );
+
+      const template = await convertWorkflowTemplate(result.context, result.value!, undefined, {
+        errorPolicy: ErrorPolicy.TryConversion,
+        featureFlags: new FeatureFlags({allowCronTimezone: true})
+      });
+
+      expect(result.context.errors.getErrors()).toHaveLength(0);
+      expect(template.events?.schedule).toHaveLength(1);
+      expect(template.events?.schedule?.[0]).toEqual({
+        cron: "0 0 * * *",
+        timezone: "America/New_York"
+      });
+    });
+
+    it("reports error when timezone is present but allowCronTimezone is disabled", async () => {
+      const result = parseWorkflow(
+        {
+          name: "wf.yaml",
+          content: `on:
+  schedule:
+    - cron: '0 0 * * *'
+      timezone: America/New_York
+jobs:
+  build:
+    runs-on: ubuntu-latest`
+        },
+        nullTrace
+      );
+
+      const template = await convertWorkflowTemplate(result.context, result.value!, undefined, {
+        errorPolicy: ErrorPolicy.TryConversion,
+        featureFlags: new FeatureFlags({allowCronTimezone: false})
+      });
+
+      // When timezone feature is disabled, error points at the timezone key
+      expect(result.context.errors.getErrors()).toHaveLength(1);
+      expect(result.context.errors.getErrors()[0].message).toContain("Key 'timezone' is not supported");
+      // Schedule entry is dropped due to unsupported key
+      expect(template.events?.schedule).toHaveLength(0);
+    });
+
+    it("reports error when timezone is present with no feature flags provided", async () => {
+      const result = parseWorkflow(
+        {
+          name: "wf.yaml",
+          content: `on:
+  schedule:
+    - cron: '0 0 * * *'
+      timezone: America/New_York
+jobs:
+  build:
+    runs-on: ubuntu-latest`
+        },
+        nullTrace
+      );
+
+      await convertWorkflowTemplate(result.context, result.value!, undefined, {
+        errorPolicy: ErrorPolicy.TryConversion
+      });
+
+      // Default is timezone disabled, so error points at the timezone key
+      expect(result.context.errors.getErrors()).toHaveLength(1);
+      expect(result.context.errors.getErrors()[0].message).toContain("Key 'timezone' is not supported");
+    });
+
+    it("reports error when cron is missing from schedule entry", async () => {
+      const result = parseWorkflow(
+        {
+          name: "wf.yaml",
+          content: `on:
+  schedule:
+    - timezone: America/New_York
+jobs:
+  build:
+    runs-on: ubuntu-latest`
+        },
+        nullTrace
+      );
+
+      const template = await convertWorkflowTemplate(result.context, result.value!, undefined, {
+        errorPolicy: ErrorPolicy.TryConversion,
+        featureFlags: new FeatureFlags({allowCronTimezone: true})
+      });
+
+      // Both schema validation and converter report the missing cron
+      expect(result.context.errors.getErrors().length).toBeGreaterThanOrEqual(1);
+      const errorMessages = result.context.errors
+        .getErrors()
+        .map(e => e.message)
+        .join(", ");
+      expect(errorMessages).toMatch(/Required property is missing: cron|Missing required key 'cron'/);
+      expect(template.events?.schedule).toHaveLength(0);
+    });
+
+    it("converts schedule without timezone when allowCronTimezone is enabled", async () => {
+      const result = parseWorkflow(
+        {
+          name: "wf.yaml",
+          content: `on:
+  schedule:
+    - cron: '0 0 * * *'
+jobs:
+  build:
+    runs-on: ubuntu-latest`
+        },
+        nullTrace
+      );
+
+      const template = await convertWorkflowTemplate(result.context, result.value!, undefined, {
+        errorPolicy: ErrorPolicy.TryConversion,
+        featureFlags: new FeatureFlags({allowCronTimezone: true})
+      });
+
+      expect(result.context.errors.getErrors()).toHaveLength(0);
+      expect(template.events?.schedule).toHaveLength(1);
+      expect(template.events?.schedule?.[0]).toEqual({
+        cron: "0 0 * * *"
+      });
+    });
+  });
 });

workflow-parser/src/model/convert.ts

@@ -1,3 +1,4 @@
+import {FeatureFlags} from "@actions/expressions/features";
 import {TemplateContext} from "../templates/template-context.js";
 import {TemplateToken, TemplateTokenError} from "../templates/tokens/template-token.js";
 import {FileProvider} from "../workflows/file-provider.js";
@@ -37,12 +38,18 @@ export type WorkflowTemplateConverterOptions = {
    * By default, conversion will be skipped if there are errors in the {@link TemplateContext}.
    */
   errorPolicy?: ErrorPolicy;
+
+  /**
+   * Optional feature flags to control which experimental features are enabled.
+   */
+  featureFlags?: FeatureFlags;
 };
 
 const defaultOptions: Required<WorkflowTemplateConverterOptions> = {
   maxReusableWorkflowDepth: 4,
   fetchReusableWorkflowDepth: 0,
-  errorPolicy: ErrorPolicy.ReturnErrorsOnly
+  errorPolicy: ErrorPolicy.ReturnErrorsOnly,
+  featureFlags: new FeatureFlags()
 };
 
 export async function convertWorkflowTemplate(
@@ -54,6 +61,11 @@ export async function convertWorkflowTemplate(
   const result = {} as WorkflowTemplate;
   const opts = getOptionsWithDefaults(options);
 
+  // Store feature flags in context state so converters can access them
+  if (opts.featureFlags) {
+    context.state["featureFlags"] = opts.featureFlags;
+  }
+
   if (context.errors.getErrors().length > 0 && opts.errorPolicy === ErrorPolicy.ReturnErrorsOnly) {
     result.errors = context.errors.getErrors().map(x => ({
       Message: x.message
@@ -142,6 +154,7 @@ function getOptionsWithDefaults(options: WorkflowTemplateConverterOptions): Requ
       options.fetchReusableWorkflowDepth !== undefined
         ? options.fetchReusableWorkflowDepth
         : defaultOptions.fetchReusableWorkflowDepth,
-    errorPolicy: options.errorPolicy !== undefined ? options.errorPolicy : defaultOptions.errorPolicy
+    errorPolicy: options.errorPolicy !== undefined ? options.errorPolicy : defaultOptions.errorPolicy,
+    featureFlags: options.featureFlags ?? defaultOptions.featureFlags
   };
 }

workflow-parser/src/model/converter/events.ts

@@ -1,3 +1,4 @@
+import {FeatureFlags} from "@actions/expressions/features";
 import {TemplateContext} from "../../templates/template-context.js";
 import {MappingToken} from "../../templates/tokens/mapping-token.js";
 import {SequenceToken} from "../../templates/tokens/sequence-token.js";
@@ -55,7 +56,8 @@ export function convertOn(context: TemplateContext, token: TemplateToken): Event
       // Schedule is the only event that can be a sequence, handle that separately
       if (eventName === "schedule") {
         const scheduleToken = item.value.assertSequence(`event ${eventName}`);
-        result.schedule = convertSchedule(context, scheduleToken);
+        const featureFlags = context.state["featureFlags"] as FeatureFlags | undefined;
+        result.schedule = convertSchedule(context, scheduleToken, featureFlags);
         continue;
       }
 
@@ -147,25 +149,47 @@ function convertFilter<T extends TypesFilterConfig & WorkflowFilterConfig & Vers
   return result;
 }
 
-function convertSchedule(context: TemplateContext, token: SequenceToken): ScheduleConfig[] | undefined {
+function convertSchedule(
+  context: TemplateContext,
+  token: SequenceToken,
+  featureFlags?: FeatureFlags
+): ScheduleConfig[] | undefined {
+  const flags = featureFlags ?? new FeatureFlags();
+  const allowTimezone = flags.isEnabled("allowCronTimezone");
   const result = [] as ScheduleConfig[];
+
   for (const item of token) {
     const mappingToken = item.assertMapping(`event schedule`);
-    if (mappingToken.count == 1) {
-      const schedule = mappingToken.get(0);
-      const scheduleKey = schedule.key.assertString(`schedule key`);
-      if (scheduleKey.value == "cron") {
-        const cron = schedule.value.assertString(`schedule cron`);
-        // Validate the cron string
+    const config: ScheduleConfig = {cron: ""};
+    let valid = true;
+
+    for (const entry of mappingToken) {
+      const key = entry.key.assertString(`schedule key`);
+
+      if (key.value === "cron") {
+        const cron = entry.value.assertString(`schedule cron`);
         if (!isValidCron(cron.value)) {
           context.error(cron, "Invalid cron expression. Expected format: '* * * * *' (minute hour day month weekday)");
         }
-        result.push({cron: cron.value});
+        config.cron = cron.value;
+      } else if (key.value === "timezone") {
+        if (allowTimezone) {
+          const timezone = entry.value.assertString(`schedule timezone`);
+          config.timezone = timezone.value;
+        } else {
+          context.error(key, `Key 'timezone' is not supported`);
+          valid = false;
+        }
       } else {
-        context.error(scheduleKey, `Invalid schedule key`);
+        context.error(key, `Invalid schedule key`);
+        valid = false;
       }
-    } else {
-      context.error(mappingToken, "Invalid format for 'schedule'");
+    }
+
+    if (valid && config.cron) {
+      result.push(config);
+    } else if (valid && !config.cron) {
+      context.error(mappingToken, "Missing required key 'cron' in schedule entry");
     }
   }
 

workflow-parser/src/model/converter/if-condition.ts

@@ -136,3 +136,32 @@ function walkTreeToFindStatusFunctionCalls(tree: Expr | undefined): boolean {
 
   return false;
 }
+
+/**
+ * Validates a pre-if or post-if condition string.
+ * Unlike step if conditions, pre-if and post-if are evaluated as-is by the runner
+ * (they default to always() only when the field is missing entirely).
+ * This function validates the expression and reports errors through the context.
+ *
+ * @param context The template context for error reporting
+ * @param token The token containing the condition
+ * @param condition The condition string to validate
+ * @returns The validated condition string, or undefined on error
+ */
+export function validateRunsIfCondition(
+  context: TemplateContext,
+  token: TemplateToken,
+  condition: string
+): string | undefined {
+  const allowedContext = token.definitionInfo?.allowedContext || [];
+
+  // Validate the expression directly - no wrapping needed for pre-if/post-if
+  try {
+    ExpressionToken.validateExpression(condition, allowedContext);
+  } catch (err) {
+    context.error(token, err as Error);
+    return undefined;
+  }
+
+  return condition;
+}

workflow-parser/src/model/workflow-template.ts

@@ -196,6 +196,7 @@ export type SecretConfig = {
 
 export type ScheduleConfig = {
   cron: string;
+  timezone?: string;
 };
 
 export type WorkflowFilterConfig = {

workflow-parser/src/templates/template-reader.ts

@@ -451,7 +451,13 @@ class TemplateReader {
     }
 
     const allowedContext = definitionInfo.allowedContext;
-    const raw = token.source || token.value;
+    const isSingleLine = token.range === undefined || token.range.start.line === token.range.end.line;
+
+    // For single-line strings, use token.value (without YAML quotes) for expression detection,
+    // because token.source includes quote characters that would be incorrectly detected as literal text.
+    // For multi-line block scalars, use token.source directly because it makes position calculation easier
+    // (no quote characters to handle, and token.source preserves the original line/column structure in YAML).
+    const raw = isSingleLine ? token.value : token.source ?? token.value;
 
     let startExpression: number = raw.indexOf(OPEN_EXPRESSION);
     if (startExpression < 0) {
@@ -496,14 +502,17 @@ class TemplateReader {
         );
 
         let tr = token.range!;
-        if (tr.start.line === tr.end.line) {
-          // If it's a single line expression, adjust the range to only cover the sub-expression
+        if (isSingleLine) {
+          // Single-line: Adjust the range to only cover the sub-expression.
+          // Calculate offset to account for YAML quote characters.
+          // For example, `"${{ expr }}"` has source with quotes, value without.
+          const offset = (token.source ?? raw).indexOf(OPEN_EXPRESSION) - raw.indexOf(OPEN_EXPRESSION);
           tr = {
-            start: {line: tr.start.line, column: tr.start.column + startExpression},
-            end: {line: tr.end.line, column: tr.start.column + endExpression + 1}
+            start: {line: tr.start.line, column: tr.start.column + startExpression + offset},
+            end: {line: tr.end.line, column: tr.start.column + endExpression + 1 + offset}
           };
         } else {
-          // Adjust the range to only cover the expression for multi-line strings
+          // Multi-line: Adjust the range to only cover the expression
           const startRaw = raw.substring(0, startExpression);
           const adjustedStartLine = startRaw.split("\n").length;
           const beginningOfLine = startRaw.lastIndexOf("\n");
@@ -604,7 +613,9 @@ class TemplateReader {
       `format('${format.join("")}'${args.join("")})`,
       definitionInfo,
       expressionTokens,
-      raw
+      raw,
+      undefined,
+      token.blockScalarHeader
     );
   }
 
@@ -686,7 +697,8 @@ class TemplateReader {
         definitionInfo,
         undefined,
         token.source,
-        expressionRange
+        expressionRange,
+        token.blockScalarHeader
       ),
       error: undefined
     };

workflow-parser/src/templates/tokens/basic-expression-token.ts

@@ -24,7 +24,19 @@ export class BasicExpressionToken extends ExpressionToken {
   public readonly expressionRange: TokenRange | undefined;
 
   /**
-   * @param originalExpressions If the basic expression was transformed from individual expressions, these will be the original ones
+   * The block scalar header (e.g., "|", "|-", "|+", ">", ">-", ">+") if parsed from a YAML block scalar.
+   */
+  public readonly blockScalarHeader: string | undefined;
+
+  /**
+   * @param file The file ID where this token originated
+   * @param range The range of the entire expression including `${{` and `}}`
+   * @param expression The expression string without `${{` and `}}` markers
+   * @param definitionInfo Schema definition info for this token
+   * @param originalExpressions If transformed from individual expressions (e.g., format()), these are the originals
+   * @param source The original source string from the YAML
+   * @param expressionRange The range of just the expression, excluding `${{` and `}}`
+   * @param blockScalarHeader The block scalar header (e.g., "|", "|-") if parsed from a YAML block scalar
    */
   public constructor(
     file: number | undefined,
@@ -33,13 +45,15 @@ export class BasicExpressionToken extends ExpressionToken {
     definitionInfo: DefinitionInfo | undefined,
     originalExpressions: BasicExpressionToken[] | undefined,
     source: string | undefined,
-    expressionRange?: TokenRange | undefined
+    expressionRange?: TokenRange | undefined,
+    blockScalarHeader?: string | undefined
   ) {
     super(TokenType.BasicExpression, file, range, undefined, definitionInfo);
     this.expr = expression;
     this.source = source;
     this.originalExpressions = originalExpressions;
     this.expressionRange = expressionRange;
+    this.blockScalarHeader = blockScalarHeader;
   }
 
   public get expression(): string {
@@ -55,7 +69,8 @@ export class BasicExpressionToken extends ExpressionToken {
           this.definitionInfo,
           this.originalExpressions,
           this.source,
-          this.expressionRange
+          this.expressionRange,
+          this.blockScalarHeader
         )
       : new BasicExpressionToken(
           this.file,
@@ -64,7 +79,8 @@ export class BasicExpressionToken extends ExpressionToken {
           this.definitionInfo,
           this.originalExpressions,
           this.source,
-          this.expressionRange
+          this.expressionRange,
+          this.blockScalarHeader
         );
   }