Are new subdomains automatically eligible for mobile-first indexing?

Why is there a clarification about subdomains and MFI?

Since the widespread shift to mobile-first indexing, one might think that all new content automatically inherits the indexing logic of the main domain. This is not true. Google treats each subdomain as a distinct entity, with its own eligibility for MFI.

If your main domain is under MFI but you create a new subdomain — let’s say blog.yoursite.com or shop.yoursite.com — the latter will first be crawled and indexed based on its desktop version. It will only switch to MFI when Google determines its mobile version is sufficiently mature, stable, and equivalent to the desktop version.

What does this change for the indexing of a new subdomain?

The newly created subdomain enters a classic validation cycle. Googlebot will explore it in desktop mode first, assess the consistency between desktop and mobile, and then decide whether or not to switch it to MFI. This process can take several weeks, or even months if the mobile version has significant discrepancies.

This is not a bug; it’s a logic of caution. Google does not want to index a shaky mobile version simply because the parent domain is already on MFI. Each subdomain must prove itself. [To verify]: Google has never provided a precise timeline for this switch by subdomain.

What are the concrete cases where this causes problems?

Imagine you launch a new blog on a subdomain to isolate your editorial content from the main site. If the mobile version of this blog is treated as an afterthought — poorly designed responsive layout, unoptimized images, a shaky mobile UI — Google will continue to crawl in desktop. The result: your mobile content, despite being the majority in terms of traffic, will not be properly evaluated by the algorithm.

Another case: you deploy an international version on fr.yoursite.com, but the mobile version is still under construction. Google will crawl the desktop version, index the content, and the MFI switch will wait. In the meantime, your mobile users experience a degraded experience, and your mobile ranking may suffer.

• Each subdomain is evaluated independently for MFI, even if the main domain is already switched.
• The transition to MFI of a new subdomain can take several weeks, depending on the maturity of the mobile version.
• A subdomain with an incomplete or shaky mobile version will remain desktop-first, negatively impacting the mobile ranking.
• Google will not automatically inherit the MFI logic from the parent domain — it must prove desktop-mobile parity.
• This process also applies to existing subdomains that were not yet on MFI at the time of the widespread transition.

Is this statement consistent with real-world observations?

Yes, and it confirms what several SEOs have observed: Google treats subdomains as almost independent entities, nearly as distinct sites. This is not new — it has been known for years that subdomains have their own backlink profile, their own crawl budget, and sometimes even their own differentiated indexing.

What is less clear is the switching timeline. Google speaks of 'validation' but provides no measurable criteria. [To verify]: it is unclear if this validation is automatic (through progressive crawling) or if it requires a mobile traffic threshold, Core Web Vitals, or anything else. It’s vague and leaves room for interpretation.

In what cases does this rule not apply?

If your subdomain is already online and crawled before the main domain switches to MFI, it may have already transitioned on its own. In other words, this is not an absolute rule concerning all subdomains: it primarily targets new subdomains created after the widespread transition to MFI.

Another case: if you have a very old subdomain, with a solid crawl history, and the mobile version has been the same as the desktop version for a long time, Google can expedite the switch. But again, nothing is guaranteed. Google’s statement remains cautious and generic, without providing any concrete action lever.

What nuances need to be added to this statement?

The problem is that Google speaks of 'will not automatically transition' without specifying when or how they will transition. Does it take 2 weeks? 3 months? Can you force the issue with a request in Search Console? No answers.

Let’s be honest: this statement mainly serves to protect Google in case of a bug or delay. If a subdomain remains stuck in desktop-first for 6 months, Google can always say 'we warned you, it’s not automatic.' Convenient. [To verify]: no official tool allows for tracking the MFI status of a specific subdomain in Search Console.

What should you do when launching a new subdomain?

First, treat the subdomain as a standalone site. Never assume it will automatically inherit the behavior of the main domain. Ensure that the mobile version is strictly equivalent to the desktop version: same content, same tags, same structured data, same internal links.

Next, declare the subdomain in Google Search Console as a separate property. Activate the 'Mobile-first indexing' report if available, and monitor Googlebot smartphone crawls. If you see that Google is still heavily crawling in desktop mode after 2-3 weeks, it’s a signal: your mobile version is not yet deemed mature.

What mistakes should you avoid to not block the MFI switch?

The classic mistake: deploying a subdomain with a simplified mobile version, less rich in content than the desktop. Google detects the gap and refuses to switch. Result: your subdomain remains desktop-first, and your mobile ranking stagnates. If you have content hidden in accordions, poorly configured lazy-loaded images, or entire blocks missing on mobile, you are creating a barrier to the switch.

Another pitfall: launching a subdomain without a dedicated XML sitemap, without a suitable robots.txt, and without monitoring server logs. You’re flying blind. If Googlebot smartphone cannot find your pages, or if they are mistakenly blocked, the MFI transition will never happen. And that’s where it gets stuck.

How can I check if my subdomain is under MFI?

Check the Search Console, section 'Settings' > 'Indexing'. Google usually indicates if the site (or subdomain) is under MFI. If not, look at the server logs: identify the user-agent Googlebot smartphone vs desktop. If the smartphone represents less than 70% of the crawls, you are probably not yet under MFI.

You can also use the URL inspection tool in Search Console: submit a URL from the subdomain and see which user-agent Google uses for rendering. If it’s still desktop, you have your answer. Also monitor the Core Web Vitals specifically for this subdomain: a subdomain under desktop-first may have potentially skewed mobile metrics.

• Declare the subdomain as a separate property in Search Console upon launch.
• Check for strict parity between desktop and mobile versions (content, tags, links, structured data).
• Analyze the server logs to identify the proportion of crawls by Googlebot smartphone vs desktop.
• Use the URL inspection tool to verify which user-agent Google uses for rendering.
• Submit a dedicated XML sitemap for the subdomain and monitor mobile indexing in coverage reports.
• Avoid any hidden or differentiated content between desktop and mobile that would slow down MFI validation.