diff --git a/.github/workflows/pages.yml b/.github/workflows/pages.yml
index 0fcdfee719..f93130cf15 100644
--- a/.github/workflows/pages.yml
+++ b/.github/workflows/pages.yml
@@ -10,6 +10,15 @@ permissions:
   contents: write
   pages: write
 
+# Serialize deploys to gh-pages. Concurrent master pushes race the
+# JamesIves force-push (cannot lock ref) — run 35056225053 failed at
+# 2026-09-16T04:38Z while the next SHA's deploy succeeded 9s later.
+# Do not cancel an in-progress master deploy; skip intermediate queued
+# runs. Feature-branch builds may cancel superseded in-progress jobs.
+concurrency:
+  group: "${{ github.workflow }}-${{ github.ref }}"
+  cancel-in-progress: ${{ github.ref != 'refs/heads/master' }}
+
 jobs:
   build-and-deploy:
     name: Build and deploy (ruby-${{ matrix.ruby_version }})