Microsoft updated its Bing sitemap guidelines on 2 July 2025, instructing site owners to provide precise lastmod timestamps so the search engine's AI can better schedule recrawls. The update, published on the Bing says Webmaster Blog, applies worldwide.
Why lastmod matters
Bing now treats lastmod as its primary crawl-scheduling signal. The value must reflect the exact moment a page changed, not when the sitemap was built. Bing prefers the ISO 8601 format with date, time and timezone offset (for example 2004-10-01T18:23:17+00:00).
- Full timestamps let Bing's AI decide whether to fetch or skip a URL.
changefreqandprioritytags are ignored.- A newly submitted sitemap is fetched immediately and rechecked daily.
- List the sitemap in
robots.txtor submit it through Bing Webmaster Tools. - Pairing a sitemap with IndexNow enables near real-time URL updates.
- Protocol limits: 50 000 URLs per sitemap, 50 000 sitemaps per index, 2.5 billion URLs per index and multiple index files up to about 2.5 trillion URLs.
- Timestamps set in the future are rejected and may be logged as errors.
Engineers Fabrice Canel and Krishna Madhavan note that respecting these limits keeps large sites visible while reducing bandwidth for both the crawler and the host server.
Additional recommendations
- Include a timezone offset such as "Z" or "+00:00" to avoid ambiguity.
- Compress sitemaps with gzip to stay under the 50 MB file-size cap.
- Host a single HTTPS version of the sitemap and avoid redirects on its URL.
Bing says accurate data allows its machine-learning models to cut crawl waste and lower the search engine's carbon footprint.
Background
XML sitemaps were introduced by Google and Yahoo in 2005 and adopted by Bing later that year. Although lastmod has always been part of the standard, many publishers leave it empty or reuse the sitemap’s build time.
Microsoft launched the IndexNow API in 2021 to let sites push individual URL changes directly to search engines. The company clarifies that IndexNow complements rather than replaces a full sitemap. Google continues to run a limited IndexNow test and has not expanded the feature.
Source
Bing Webmaster Blog, “Keeping Content Discoverable with Sitemaps in AI Powered Search” (2 July 2025).
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