A single sitemap index file at Bing can reference up to 2.5 billion URLs, and multiple index files can cover as many as 2.5 trillion URLs across one domain — limits Microsoft confirmed this week as it urged site owners to strengthen sitemap signals for AI-powered search. Writing on the Bing Webmaster Blog, Principal Product Managers Fabrice Canel and Krishna Madhavan framed sitemaps as the foundational coverage layer that complements real-time submission via IndexNow.

The guidance comes as Bing Copilot and other AI-generated answer surfaces increasingly rely on fresh crawl data. The authors state plainly that no tool can guarantee when or how content appears in AI results, but that combining sitemaps with IndexNow "gives your content the best chance to be discovered, crawled, and indexed efficiently."

How many URLs can a sitemap contain at Bing?

Bing continues full support of the Sitemaps protocol at sitemaps.org, with these confirmed ceilings:

• Up to 50,000 URLs per individual sitemap file
• Up to 50,000 child sitemap files referenced within one sitemap index file
• Up to 2.5 billion URLs referenced by a single sitemap index file
• Up to 2.5 trillion URLs across a domain using multiple index files

For enterprise-scale properties — large e-commerce platforms and content-rich publishing sites are the examples Microsoft names — using the full limits ensures every relevant URL stays discoverable.

Why does lastmod still matter for AI indexing?

The lastmod field remains a key freshness signal. Bing uses it to prioritize URLs for recrawling and reindexing, or to skip them entirely when content has not changed since the last crawl. This matters most as AI search engines adjust ranking and surfacing "in near real time based on content changes," according to the post.

Microsoft's formatting requirements are specific:

• Use ISO 8601 date formatting, including both date and time (for example, 2004-10-01T18:23:17+00:00)
• The timestamp should reflect the true last modification time of the page content — not the moment the sitemap file was generated
• changefreq and priority tags are ignored by Bing and do not influence crawling or ranking

The second point is a common failure mode: site owners who stamp lastmod with the sitemap build time send false freshness signals and waste crawl budget.

What is Bing's preferred sitemap format?

XML remains the preferred format because it supports structured metadata like lastmod, which helps Bing assess content freshness and relevance. Site owners can also compress sitemap files in gz format (for example, sitemaps.xml.gz) to reduce bandwidth usage and improve submission efficiency.

Bing accepts sitemap submission through two channels:

• robots.txt — add a Sitemap: line pointing to the file location for automatic discovery
• Bing Webmaster Tools — submit directly from a verified account and monitor indexing performance there

How often does Bing read sitemaps?

Microsoft confirms two crawl behaviors. Bing attempts to fetch a sitemap immediately upon submission, whether it arrives via robots.txt reference or Webmaster Tools. After that, it revisits the sitemap on a regular basis — typically at least once per day — to check for updates.

To verify processing, site owners can check the Sitemaps section in Bing Webmaster Tools for three data points: submission status (success or error), the last read date, and any processing errors that may block indexing.

What should site owners do now?

The blog post lays out a three-part playbook, pairing structural coverage with speed:

• Submit a complete XML sitemap with accurate lastmod values so Bing understands site structure and prioritizes crawling across the full URL inventory
• Use IndexNow for real-time URL-level submission, instantly notifying Bing and participating engines when URLs are added, updated, or removed
• Reference the sitemap in robots.txt to support automatic discovery and consistent crawling

The advice carries the most weight for verticals with frequently updated or time-sensitive pages — news, e-commerce inventory, and publishing — where freshness directly affects how quickly changes appear in both traditional results and Copilot answers.

Site owners should monitor their sitemap's last-read dates and error status in Bing Webmaster Tools, and watch the Bing Webmaster Blog for follow-up guidance as Microsoft refines its indexing recommendations for AI-first search.