Bing Pushes lastmod Tags as Key Signal for AI-Driven Search Indexing

As search continues its evolution into the AI era, Microsoft’s Bing is doubling down on a simple but powerful signal: the lastmod tag in XML sitemaps.

In a recent update to its official sitemap guidance, Bing emphasized that this tag plays a critical role in AI-powered indexing, helping its systems decide what to recrawl and when. For site owners and SEOs alike, this is a strong reminder that technical SEO fundamentals still matter—especially in the age of AI.

Why lastmod Now Matters More Than Ever

At its core, the lastmod tag tells search engines when a page was last meaningfully updated. For years, it’s been a quiet player in the sitemap spec, often underutilized or misused. But Bing now says that lastmod is a “top signal” for its AI-powered crawling system.

The guidance makes it clear: if you want Bing to prioritize crawling the right pages, especially on large or frequently updated sites, your sitemaps should contain precise and accurate lastmod values.

✅ Format tip: Use the ISO 8601 standard with both date and time, e.g. 2004-10-01T18:23:17+00:00.

Critically, don’t just set lastmod to the time your sitemap was generated—that’s a common mistake that can lead to unnecessary crawling or indexing inefficiencies. Bing is urging site owners to only update lastmod when content has actually changed.

What Doesn’t Matter Anymore: changefreq and priority

In the same update, Bing confirmed what many SEOs already suspected: the changefreq and priority fields in sitemaps are now ignored.

These fields were intended to indicate how often a page is likely to change and how important it is compared to other pages. But in practice, they’ve been inconsistent and unreliable across sites—so Bing’s AI-driven systems no longer use them.

How to Submit and Monitor Your Sitemap

Bing recommends two standard ways to submit sitemaps:

• Reference it in your robots.txt file.
• Submit directly via Bing Webmaster Tools.

Once submitted, Bing fetches the sitemap immediately and continues to check it daily for updates. Within Webmaster Tools, you can view:

• Submission status.
• Last read date.
• Any processing or parsing errors.

This allows you to verify if your sitemap is being properly interpreted and contributing to better indexation.

Pairing Sitemaps with IndexNow for Speed

While sitemaps offer broad visibility into your site structure, IndexNow—Microsoft’s real-time indexing protocol—lets you instantly ping Bing when a specific page is added, updated, or deleted.

Bing recommends using both to get the best of both worlds:

• Sitemaps for full site structure.
• IndexNow for fast, URL-specific updates.

“By combining sitemaps for comprehensive site coverage with IndexNow for fast, URL-level submission, you provide the strongest foundation for keeping your content fresh, discoverable, and visible,” Bing’s team explained.

Editorial Insight: For SEOs and developers already juggling crawl budget strategies for Google, IndexNow offers a compelling advantage for Bing, Yandex, and other adopters—especially when you’re dealing with high-volume content or time-sensitive updates.

Designed to Scale: Trillions of URLs Supported

For enterprise websites, publishers, and ecommerce giants, scale matters. Fortunately, Bing’s sitemap protocol is built to handle trillions of URLs, with support for:

• 50,000 URLs per sitemap.
• 50,000 sitemaps per index file.
• 2.5 billion URLs per index.
• Multiple index files for up to 2.5 trillion URLs.

That level of capacity makes it possible for massive platforms to maintain comprehensive, optimized sitemap structures without compromise.

According to Fabrice Canel and Krishna Madhavan of Microsoft AI and Bing, using these limits to their full extent helps keep large-scale content discoverable and competitive in AI-powered search.

Why This Matters: SEO in the AI Age

Bing’s renewed emphasis on lastmod is more than a technical update—it’s a strategic signal about where search is heading.

In a world where AI systems are constantly evaluating which content is current, helpful, and worth surfacing, accurate metadata becomes a key input for smart crawling and ranking decisions.

For marketers and SEO professionals, the message is clear:

• Don’t neglect your sitemaps.
• Keep lastmod precise and truthful.
• Embrace real-time tools like IndexNow.
• Avoid outdated tags like priority and changefreq.

Whether you’re running a small blog or a global e-commerce platform, this level of clarity gives your content the best shot at being indexed quickly and accurately in the AI search landscape.