diff --git a/manage_runner.sh b/manage_runner.sh
index 310bf80..140af9b 100755
--- a/manage_runner.sh
+++ b/manage_runner.sh
@@ -217,9 +217,17 @@ build_runner_labels() {
 # Resolve registration token based on repos field.
 # repos=all → instance-level token from .env
 # repos=<name> → fetch repo-level token from Gitea API
+# Comma-separated repos are NOT supported here — configure_runners.sh
+# expands them into separate INI sections before they reach this point.
 # Sets RUNNER_REG_TOKEN.
 # ---------------------------------------------------------------------------
 resolve_registration_token() {
+  if [[ "$RUNNER_REPOS" == *,* ]]; then
+    log_error "Runner '$RUNNER_NAME': repos='$RUNNER_REPOS' contains commas."
+    log_error "  Each act_runner process can only register to one repo."
+    log_error "  Run ./setup/configure_runners.sh to expand into separate runner sections."
+    return 1
+  fi
   if [[ "$RUNNER_REPOS" == "all" ]] || [[ -z "$RUNNER_REPOS" ]]; then
     RUNNER_REG_TOKEN="${GITEA_RUNNER_REGISTRATION_TOKEN:-}"
   else