Case: Pre-deploy validator caught 1,100 broken URLs

Case: Pre-deploy validator caught 1,100 broken URLs
A release shipped a sitemap where a path-prefix bug made 1,100 URLs 404. It sat live for 6 days before anyone noticed.
What we did:
— Added a CI step that XSD-validates the sitemap
— Sampled 2% of URLs and checked for HTTP 200
— Failed the build if any sampled URL returned non-200
Measured outcome:
— Next 4 releases: 3 broken-URL batches blocked before deploy
— GSC 'Submitted URL not found (404)' errors: dropped to 0
Gate criteria:
✅ File passes the sitemap XSD
✅ Random 2% URL sample returns 200
✅ Total URL count within 10% of last release
Definition of done: build fails automatically if any criterion is not met.