diff --git a/.github/workflows/pages.yml b/.github/workflows/pages.yml
index c88ed14..648969a 100644
--- a/.github/workflows/pages.yml
+++ b/.github/workflows/pages.yml
@@ -34,6 +34,7 @@ jobs:
     timeout-minutes: 30
     outputs:
       trusted-sha: ${{ steps.revision.outputs.sha }}
+      marker: ${{ steps.marker.outputs.path }}
     steps:
       - uses: actions/checkout@v7
         with:
@@ -51,6 +52,13 @@ jobs:
       - run: node scripts/site-ci.ts assemble
         env:
           GH_TOKEN: ${{ github.token }}
+      # The CDN caches 404s, so each deploy check polls a path nobody requested before.
+      - id: marker
+        run: |
+          marker="deploy-marker/$GITHUB_RUN_ID-$GITHUB_RUN_ATTEMPT"
+          mkdir -p "site/$marker"
+          for n in $(seq 1 30); do echo "$n" > "site/$marker/$n"; done
+          echo "path=$marker" >> "$GITHUB_OUTPUT"
       - uses: actions/upload-pages-artifact@v5
         with:
           path: site/
@@ -92,4 +100,19 @@ jobs:
           GH_TOKEN: ${{ github.token }}
       - name: Deploy current site
         id: deployment
-        uses: actions/deploy-pages@v5
\ No newline at end of file
+        uses: actions/deploy-pages@v5
+      # Pages can report success and keep serving the previous deployment.
+      - name: Require Pages to serve this deployment
+        env:
+          SITE_URL: ${{ steps.deployment.outputs.page_url }}
+          MARKER: ${{ needs.assemble.outputs.marker }}
+        run: |
+          for n in $(seq 1 30); do
+            if curl -fs --max-time 10 -o /dev/null "${SITE_URL%/}/$MARKER/$n"; then
+              echo "Pages serves this deployment after $n checks"
+              exit 0
+            fi
+            sleep 10
+          done
+          echo "::error::Pages still serves an older deployment. Rerun the deploy job."
+          exit 1
\ No newline at end of file
diff --git a/docs/README.md b/docs/README.md
index 00d3f94..a3c641c 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -115,7 +115,8 @@ Pages skips a preview whose build lacks `index.html`, contains symbolic links, h
 
 Publication calls Pages after a main push or a data change. Pages also runs for
 successful preview builds and preview eligibility changes. Run Pages manually
-after a direct push to `data`.
+after a direct push to `data`. The deploy job fails if the site still serves an
+older deployment after 5 minutes. Rerun the deploy job.
 
 Publication and Pages use serial queues with up to 100 pending jobs or runs each.
 Benchmark checks ignore unrelated labels and title or body edits.