Bing’s Updated Sitemap Guidance: Why ‘lastmod’ Matters More Than Ever
Bing now puts high emphasis on the lastmod tag in XML sitemaps. If you are running a site, this field is a major cue to Bing’s AI on whether your page is new, has changed, or can be skipped for now. So, if you are not using lastmod, or if you are using it in a slapdash way, there is a good chance you are hurting your visibility in Bing Search. That’s not hype. That’s what Bing’s own guidance says.
How Bing Uses lastmod in Sitemaps
Bing’s crawlers are quick, but still face limits. They want to spend their time wisely, not re-checking pages that didn’t really change. Bing has said that accurate, page-level lastmod fields tell its AI when to crawl, so this tag shapes how often your content appears in results.
If you want to get straight to the technical part: Bing expects lastmod to use the ISO 8601 format with both date and time (for example: 2023-06-11T13:40:33+00:00). Not just the date, and not just “June 2023.” That extra precision matters.
If you set lastmod to the moment your sitemap was generated, but the page itself didn’t change, Bing’s algorithm can tell. Over time, this could make crawling less effective, especially on large or dynamic sites.
Is lastmod All That Important (or Is Bing Overstating It)?
I know what you might be thinking. Does one attribute on a sitemap really matter that much? I get the skepticism; I had it too. I have seen plenty of sites with “lazy” sitemaps rank just fine, on Google, at least. But Bing’s own documentation and commentary from their engineers go out of their way to move lastmod to the top of the priority list for recrawling.
The old tags, like changefreq and priority, once played a part, but Bing has confirmed those are ignored now. Only lastmod is left in the driver’s seat.
So, yes, it’s worth changing the habit of treating sitemaps as a checkbox.
Sitemaps are not just files you hand over to search engines and forget. Used right, they can give strong signals to Bing’s AI, especially as search keeps moving in that direction.
How To Fill lastmod Correctly (And What Not To Do)
Using lastmod the right way comes down to this:
- Set it to the exact time the content on the page last changed, not when you generated the sitemap.
- Use the ISO 8601 format. Example: 2025-07-28T20:21:00+00:00
- Only update lastmod when real changes happen. Fixing a typo? Maybe. Flipping an image? Maybe not. Updating a price? Yes.
Sometimes, CMS plugins or site builders “auto-update” lastmod for all URLs each time you touch the sitemap. That is the fastest way to confuse Bing. If you are not sure if your tool does this, test it. Change a page, see if the lastmod on just that URL updates. That’s the behavior you want.
Sitemap Submission: How, Where, and How Often?
Getting a sitemap to Bing is straightforward, but you have two main choices:
- Reference your sitemap in your robots.txt. (Just add “Sitemap: https://example.com/sitemap.xml”)
- Upload the sitemap directly in Bing Webmaster Tools.
Once submitted, Bing fetches it right away and checks it daily.
But how do you know it’s working? In Bing Webmaster Tools, open the Sitemaps section. You will see when the sitemap was last read and if there were any errors. If errors show up a lot, or if the last read date is old for no reason, that points to either a formatting issue or connectivity problem.
Can You Use IndexNow At the Same Time?
Yes, actually, and I’d recommend it for anyone who keeps adding or changing URLs more than once a week.
Sitemaps tell Bing the “big picture” each day: here is everything, here’s what changed. IndexNow is more like a push notification for Bing: “Hey, this one specific URL changed right now!” Together, they create a two-layer system.
Think of sitemaps as a snapshot for Bing’s AI, while IndexNow is the instant ping. They do not replace each other, but they work together to make your latest content appear faster in Bing and other search engines that use IndexNow (like Yandex, for example).
Bing’s Sitemap Limits: Can They Keep Up with Huge Sites?
If you run a big ecommerce shop, huge forum, digital publisher, or SaaS site, you might wonder if Bing can handle your scale. They can!
Here is what Bing has confirmed:
| Sitemap Element | Limit |
|---|---|
| Max URLs per sitemap | 50,000 |
| Max sitemaps per index file | 50,000 |
| Max URLs per index | 2.5 billion |
| Support for multiple index files? | Yes, up to 2.5 trillion URLs per site |
With that much room, only the biggest sites on the planet will run into walls. For everyone else, these limits mean you don’t need to worry about leaving content behind.
What About Google? Should You Use lastmod for Them Too?
It’s a fair question. Google is still the main source of organic traffic for lots of websites, and they do support lastmod as well. But their approach isn’t as transparent. Sometimes Google crawls even when lastmod didn’t change; sometimes, new lastmod data seems to prompt a quicker crawl, but there’s never a guarantee.
At the same time, using lastmod the way Bing asks doesn’t harm your performance on Google. If anything, it keeps your technical setup clean for every search engine you care about. But here’s something I noticed: sites that keep a tightly managed sitemap with correct lastmod values often see fresher content pulled into Google Discover and Top Stories much sooner.
So, best to just do it the right way for both.
Why Accurate Sitemaps Matter for AI Search
The trend in search is clear. More engines, including Bing and Google, use AI signals to help decide what to crawl, what to index, and what users actually see. That means the small technical habits, like setting lastmod right, have broader effects than before.
It’s easy to think of sitemaps as “set and forget” files, but if you do, you might be missing out on more timely crawls. Bing’s engineers have said directly: “Keep your sitemap clean, accurate, and current.” It sounds simple, but it’s not always done well, even by SEO professionals. I suppose some think search engines will “figure it out.” Maybe they will. But when you are competing for every click, why risk leaving something to chance?
Signs Your Sitemap Needs Fixing
If you are unsure about the state of your sitemap, check for these issues:
- lastmod updates every time the sitemap is generated, even if the page did not change
- Sitemap includes “dead” links or URLs that no longer work
- lastmod fields use date only, without time
- Sitemap throws errors in Bing Webmaster Tools
- You have pages that were excluded from search, and there is no apparent reason
Address these, and you usually see pages get indexed and updated noticeably faster.
Recommended Workflow for Sitemaps and Bing
Here’s a practical way to handle this all, especially if your site changes a lot:
- Automate your sitemap. Use a tool or plugin that lets you control lastmod on a per-page basis, not just when the file is generated.
- Review your process. If editing a blog post results in a new lastmod on that post (but not others), you’re set.
- Monitor Webmaster Tools regularly. Look for errors or odd crawl patterns.
- Set up IndexNow for your CMS. Submit URLs immediately when you add, remove, or significantly update them.
- Periodically validate your sitemap with an online tester. Confirm correct formatting, valid URLs, and date+time precision.
Small effort, big impact over time.
Why Bing Ignores Other Sitemap Fields Now
Maybe this is the least surprising update, but Bing has stopped caring about changefreq and priority fields. If your sitemap plugin still spits them out, it is fine, but they are ignored, and you can even remove them to clean up extra noise.
This move simplifies things. Instead, Bing wants to see signals that directly reflect what’s changed and when. That’s what lastmod gives.
Can Bad lastmod Data Hurt You?
It’s tempting to just say: “Bing will never punish you directly,” but I have seen some odd real-world results. Once, on a recipe site, someone let their CMS default to updating all pages’ lastmod daily. New posts got indexed, but everything else was crawled constantly, with no change. Result: orphaned new recipes sometimes disappeared for a couple weeks before coming back.
This is not an official penalty, but more like Bing spending crawl budget inefficiently. For big sites, with lots of products or articles, this is a problem. Bing’s rules are soft, but the reality is, bad lastmod data makes it much less likely your actual changed content is seen fast.
What About Manual Sitemaps?
If you prefer to build your XML sitemaps by hand, there’s nothing stopping you. Just treat lastmod carefully. Look up the exact UTC time you changed a file. Insert it. If you batch-update, update just those URLs. Manual sitemaps are less common now, but for small, static sites, it works.
Common lastmod Mistakes
Here are some things to look out for:
- Setting lastmod to the same time for every page, every time
- Skipping the time, only giving the date
- Letting a plugin control everything without checking its logic first
- Not updating lastmod when you do real content edits, like adding sections or rewriting paragraphs
- Forgetting to update lastmod after a major redesign or navigation change
The solution? Pick one small thing to fix each month. Over time, your sitemaps will be a lot smarter.
What Can You Expect if You Do This Well?
It might seem minor, but accurate sitemaps, especially with good lastmod fields, mean:
- Faster inclusion of fresh content
- Less wasted crawl on unchanged pages
- Better sync between your live site and what Bing sees
- A stronger case for your site in Bing’s AI “freshness” calculations
For some, traffic actually climbs as old, forgotten pages fall away and new ones rise into view. For others, it just means updates show up in search a few hours earlier.
In the world of AI-powered search engines, small technical tweaks begin to have bigger organic reach. It’s not a guarantee, but it’s one of the few signals you can actually control on your end.
Finishing Thoughts
If you care about visibility in Bing or want to stay ahead for any search platform using AI, it is time to revisit some basics. Keep your sitemap clean. Track lastmod precisely with date and time. Avoid the urge to set and forget. Mix in IndexNow for faster real-time updates at scale.
Is it the only thing that helps? No. High-quality content, internal linking, and useful experiences will always matter more. But when there is a signal that is both controllable and one of the clearest ways to get attention from new AI-powered crawlers, why not take it seriously?
If you ever feel lost on sitemap strategy, pause, look at your own files, and ask: does this truly reflect what changed on my site? If not, start adjusting. You will probably notice the effects much sooner than you think.
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