XML Sitemap Validator

An XML sitemap is the list of pages you want search engines to crawl, with optional hints about when each one last changed. Google reads it to find new pages faster and to decide what to recrawl, but only if the file is valid: well-formed XML in the sitemaps.org namespace, absolute URLs on the same host, correctly formatted dates, and at most 50,000 URLs or 50 MB per file. A sitemap generated by a plugin is usually fine on day one and drifts afterwards: the site moves to https while the sitemap still lists http, deleted pages stay listed, or a template change produces a stray character that breaks the XML.

This validator fetches the sitemap (plain or gzip-compressed), checks that it parses as XML and reports the exact line of any error, then walks every entry. It counts URLs, flags relative or foreign-host locs, duplicates, invalid lastmod dates, changefreq values outside the allowed list and priorities outside 0.0 to 1.0, and compares the totals with the size limits. For a sitemap index it lists the child sitemaps without fetching each one. For a regular sitemap it also requests the first ten URLs and reports which ones answer 200, redirect or fail, because a sitemap full of redirects and 404s teaches Google to trust it less.

Run it after a migration, after installing or changing an SEO plugin, and whenever Search Console reports "Couldn't fetch" or a sudden drop in discovered pages.