Sitemap validator: fetches a sitemap or index, checks well-formedness, namespace, type (urlset/sitemapindex), counts entries, validates loc URLs (absolute, same host, no duplicates), lastmod formats, changefreq/priority values, the 50,000-URL and 50 MB limits, gzip, and lists the first errors. Returns valid true/false with error and warning lists. $0.01 per sitemap.