diff --git a/.github/workflows/governed-pr-check.yml b/.github/workflows/governed-pr-check.yml
index b87c9c0..da75211 100644
--- a/.github/workflows/governed-pr-check.yml
+++ b/.github/workflows/governed-pr-check.yml
@@ -44,6 +44,35 @@ jobs:
         run: |
           gh api "repos/${REPOSITORY}/pulls/${PR_NUMBER}/files?per_page=100" > pr-files.json
 
+      - name: Validate governed PR check tooling source
+        run: |
+          set -euo pipefail
+          PRIMARY_TOOL_PATH="governed-pr-check-tools/.github/scripts/governed_pr_check.py"
+          LEGACY_TOOL_PATH=".github/scripts/governed_pr_check.py"
+
+          if [ -f "$PRIMARY_TOOL_PATH" ]; then
+            echo "[governed-pr-check] tooling path found: $PRIMARY_TOOL_PATH"
+            exit 0
+          fi
+
+          {
+            echo "## Governed PR Check hard-fail"
+            echo "Detected missing required tooling path: $PRIMARY_TOOL_PATH"
+            echo "Reason: wrong workflow revision/tooling source"
+          } >> "$GITHUB_STEP_SUMMARY"
+
+          if [ -f "$LEGACY_TOOL_PATH" ]; then
+            echo "::error::workflow hard-fail: wrong workflow revision/tooling source. Expected $PRIMARY_TOOL_PATH but found $LEGACY_TOOL_PATH."
+            echo "::error::This reusable workflow is pinned to a legacy tooling revision that does not include the tool checkout directory."
+            echo "::error::Pin the caller to a fixed SHA that includes the governed-pr-check-tools checkout path (example: b250900...)."
+          else
+            echo "::error::workflow hard-fail: wrong workflow revision/tooling source. Expected $PRIMARY_TOOL_PATH."
+            echo "::error::No legacy path found at $LEGACY_TOOL_PATH either."
+            echo "::error::The governed PR check script is unavailable in this workflow source. Update reusable workflow source before rerunning."
+          fi
+
+          exit 1
+
       - name: Run governed PR check
         run: |
           python3 governed-pr-check-tools/.github/scripts/governed_pr_check.py \