diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3154af4..bc40580 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,6 +7,28 @@ on:
     branches: [main]
 
 jobs:
+  # Stale-green guard. A PR can show a green CI run that was executed BEFORE a
+  # breaking commit landed on the base branch — merging it would ship a broken
+  # main. This job FAILS if the PR branch does not contain origin/<base> as an
+  # ancestor, forcing an "Update branch" before the PR can merge.
+  up-to-date-with-base:
+    runs-on: ubuntu-latest
+    if: github.event_name == 'pull_request'
+    steps:
+      - uses: actions/checkout@v5
+        with:
+          fetch-depth: 0
+      - name: Fail if PR branch is behind its base branch
+        run: |
+          BASE="${{ github.event.pull_request.base.ref }}"
+          git fetch origin "${BASE}" --depth=1
+          if git merge-base --is-ancestor "origin/${BASE}" HEAD; then
+            echo "PR branch contains origin/${BASE} — up to date."
+          else
+            echo "::error::PR branch is behind origin/${BASE}. Update the branch (merge/rebase ${BASE}) and re-run CI so it validates against current base."
+            exit 1
+          fi
+
   build-and-test:
     runs-on: ubuntu-latest
     steps:
@@ -18,6 +40,9 @@ jobs:
           cache: 'npm'
 
       - run: npm ci
+      # `npm run build` runs `tsc && vite build && node scripts/prerender.mjs`.
+      # The prerender step is part of CI — a local `vite build` alone is NOT a
+      # valid gate. `npm run gate` (package.json) runs this exact sequence.
       - run: npm run build
       - run: npm test
 
diff --git a/package.json b/package.json
index b31343e..858951b 100644
--- a/package.json
+++ b/package.json
@@ -11,6 +11,7 @@
     "build": "tsc && vite build && node scripts/prerender.mjs",
     "preview": "vite preview",
     "test": "vitest run",
+    "gate": "tsc --noEmit && npm run build && vitest run",
     "test:watch": "vitest",
     "test:e2e": "playwright test",
     "test:e2e:ui": "playwright test --ui",