Sitemap missing lastmod or does not match the crawl

When the sitemap exists and is valid XML, two independent things are checked under one code: how many URLs in it lack <lastmod>, and how many URLs in it the crawler never found. The report gives the count and the total, so you can tell five out of five hundred from the other case.

The second condition is evaluated only on a full crawl, never on a sample, for the same reason as ACC-06: over a sample, “a URL from the sitemap we did not find” would come out almost every time, because of us rather than your site. A missing <lastmod> is a property of the sitemap itself, so it is always reported.

What the check does not do:

• It does not verify <lastmod> is truthful. Since 15 Sep 2026 separate codes do that: SEO-15 (the same date on nearly all URLs), SEO-16 (invalid format or a date in the future) and SEO-17 (the page gives a later change than the sitemap).
• It ignores <priority> and <changefreq>. Google has said it ignores them too.
• It does not report the opposite direction: live pages missing from the sitemap. That is ACC-06.
• It does not run at all when the sitemap is missing or invalid, also ACC-06.

The finding is site-level and informational: it costs no score at all.

For visibility in AI answers we have no documented effect, same as ACC-06. No operator of a language model declares reading sitemaps at all.

What <lastmod> actually does is in Google's own documentation: it is the value used to decide whether to come back to a page, but it is only taken seriously when it is consistently truthful. A sitemap claiming today for everything becomes information without value and Google stops taking it into account.

Which yields an uncomfortable conclusion worth stating: filling <lastmod> untruthfully is worse than not having it. The finding asks you for a truthful date, not for a populated field. If you would generate it from your build date, leave it out.

The other half of the finding, URLs we never found, is more practical. They are usually old paths left after a rebuild, or pages nothing links to internally (which is also SIT-01). A sitemap sending a crawler at URLs that lead nowhere is a bad hint you are giving it yourself.

Do not write the sitemap by hand. Let it be generated when content is published, from the same source as your navigation; only then will <lastmod> match reality.

<url>
  <loc>https://your-domain/article</loc>
  <lastmod>2026-08-30</lastmod>
</url>

• <lastmod> = the date of the last real content change, not of the build and not of the deployment.
• When you do not know it, leave it out. It is an optional element and a missing value is more honest than an invented one.
• Format YYYY-MM-DD (or a full W3C datetime).
• Drop <priority> and <changefreq> entirely: Google does not use them and they only lengthen the file.

For URLs the crawler never found, go through the list and decide: either drop them from the sitemap (old paths, pages excluded from the index), or add links to them from the site when they belong there. The second option also resolves SIT-01.