Sitemap <lastmod> is a freshness signal. Google uses it to prioritize crawl budget — recent lastmod = crawled sooner. The signal only works if the claim is true.
Plenty of sitemap generators produce lies. A plugin that emits "today" for every URL on every rebuild. A static-site generator that stamps the build timestamp, not the content-change timestamp. A CMS that fires a "touch all" on theme updates. When Google detects the lie — by cross-referencing Last-Modified headers and schema.dateModified — it stops trusting lastmod on the site entirely. Every page pays.
Sitemap lastmod Truthfulness does the cross-check. Fetches your sitemap. Probes each URL's server Last-Modified header and visible schema dateModified. Reports the truthfulness percentage.
The three data sources it compares
- Sitemap
<lastmod>— what your sitemap claims. - HTTP
Last-Modifiedresponse header — what the server actually says. - Schema
dateModified— what the JSON-LD declares is visible on the page.
For a truthful sitemap, all three agree within a day or two. Drift of more than 30 days in the direction "sitemap claims newer than truth" is fake-fresh.
What drives fake-fresh patterns
- Plugin defaults. Yoast SEO, All-in-One SEO, and RankMath for WordPress have had periods where "Update lastmod on every rebuild" was a default.
- Static-site build timestamps. If your Eleventy / Next.js / Astro build emits the build time as lastmod, every deploy touches every URL even when content is unchanged.
- Theme-level "touch all." Some platforms refresh all URLs when a theme or template file changes, even though actual page content didn't.
The fix
The fix depends on the CMS. For WordPress-plus-SEO-plugin: flip the "use post modified date" option; the default should have been that in the first place. For static-site: use the content file's git commit timestamp as lastmod, not the build time. For custom: expose an admin tool that edits lastmod only when actual content fields change.
For the small subset of URLs that are legitimately stale (old blog posts), either remove them from the sitemap or accept that they will show an old lastmod. Old-but-accurate beats fake-new.
Related reading
- Sitemap Audit — XML syntax + URL probe
- Index Coverage Delta — what's actually indexed vs sitemap
Fact-check notes and sources
- Sitemap protocol (lastmod field): sitemaps.org/protocol.html
- Google sitemap guidelines: developers.google.com/search/docs/crawling-indexing/sitemaps/build-sitemap
- Gary Illyes on trust in lastmod: multiple Search Off the Record episodes 2022-2024
The $100 Network covers sitemap hygiene across site networks. The truthfulness check is the monthly audit.
