How can you tell if your site has truly switched to mobile-first indexing?

Why does Google notify about the switch to mobile-first indexing with a time delay?

Google migrates sites to mobile-first indexing progressively and automatically. When the algorithm detects that a site meets the technical criteria (content parity between mobile/desktop, identical structured data tags, acceptable loading times), the switch occurs on the infrastructure side.

The Search Console notification is generated afterward, sometimes with several days or even weeks of delay. This lag is explained by the volume of affected sites and Google's internal auditing processes before official confirmation. As a result: you may have been indexed mobile-first for two weeks without knowing it.

What does it really mean when "the user-agent is set for mobile"?

Googlebot uses two distinct user agents: Googlebot Desktop and Googlebot Smartphone. Before mobile-first indexing, it was the desktop version that crawled your site to build the main index. After the switch, the smartphone version takes over.

The URL Inspection Tool explicitly shows which agent was used during the last successful crawl. If you see "Googlebot Smartphone" as the main user-agent, your site is indexed mobile-first. If it is still "Googlebot Desktop", the migration has not occurred—or Google has detected a blocking issue.

What criteria trigger the migration to mobile-first indexing?

Google evaluates several signals before switching a site: content parity between mobile and desktop (text, images, videos, internal links), presence and consistency of structured data, accessibility of resources (CSS, JS), and absence of critical rendering errors in mobile.

If your mobile version hides content in non-crawlable accordions, omits entire sections present on desktop, or blocks essential scripts via robots.txt, Google will delay the migration. It’s not enough to have a responsive site: you need a strict functional equivalence.

• Check the user agent via the URL Inspection Tool to know your actual status
• Don’t rely on the Search Console notification as a reliable timing signal
• Compare mobile/desktop content to identify disparities delaying the switch
• Monitor server logs: a spike in Googlebot Smartphone crawling often indicates a transition in progress
• Test mobile rendering with the Mobile-Friendly Test and mobile optimization testing tool

Is this statement consistent with real-world observations?

Yes, and it's even an understatement. In the field, there are regularly observed delays of 10 to 30 days between the actual switch and the Search Console notification. Some sites never receive a notification despite a migration confirmed by server logs and URL inspection.

Google has always been transparent about the fact that mobile-first indexing is a gradual, site-by-site process. What is less clear is the lack of guarantee on the reliability of the notification system. For an SEO managing technical migrations, this uncertainty necessitates the implementation of active monitoring rather than relying on automatic alerts.

What nuances should be added to this official recommendation?

The URL Inspection Tool only shows the last successful crawl. If Google attempted to crawl your mobile page three days ago and encountered a 500 error, and then came back in desktop yesterday, you will see "Googlebot Desktop" while the migration has technically occurred. This information needs to be cross-referenced with server logs to have a complete picture.

Another rarely mentioned point: certain hybrid sites (distinct mobile and desktop versions with separate URLs, m. configuration, or AMP) may exhibit mixed indexing behaviors during a transitional period. The primary agent may be mobile, but certain specific URLs may still be crawled in desktop if Google detects inconsistencies in alternate/canonical annotations. [To be verified] in your logs if you observe this kind of dual behavior.

In which cases is this active verification rule critical?

For e-commerce sites with differentiated mobile/desktop content, failing to quickly detect the switch can result in a drop in rankings if the mobile version is underwhelming (truncated descriptions, non-crawlable overlay filters, lazy-loaded images without fallback). You must know immediately which content Google is indexing.

For technical migrations (redesign, switch to responsive), the URL inspection becomes your validation dashboard. If you deploy a new mobile version meant to be equivalent to the desktop, but Google hasn’t switched to mobile-first three weeks later, that’s a red flag: there’s a structural issue that Search Console may not have yet flagged.

How can I concretely check the mobile-first indexing status of my site?

Log in to Google Search Console, select your property, then use the URL Inspection Tool on a strategic page (homepage, key product page, reference article). In the "Coverage" report, under the "Crawl" section, read the line "Crawled user-agent".

If you see "Smartphone", you are in mobile-first indexing. If it’s "Desktop", the migration hasn’t occurred. Repeat the operation on various types of pages (categories, landing pages, blog articles) to detect any possible heterogeneous behaviors—a rare case but observed on sites with subdomains or directories treated differently.

What mistakes should be avoided during the transition to mobile-first indexing?

The first classic mistake: hiding content on mobile via tabs or accordions closed by default, without schema.org markup allowing Google to understand the structure. Googlebot mobile indexes what it sees at first render—if your text is hidden in CSS and requires user interaction, it may not be taken into account.

The second mistake: blocking critical CSS or JavaScript resources in robots.txt to "lighten" the mobile version. Google needs a complete rendering to evaluate the page. If you block scripts that load dynamic content, indexing will be incomplete. The third trap: forgetting to check structured data on mobile—if they are only present on the desktop side, you lose rich snippets after the switch.

What should I do if the notification is delayed or if the site does not migrate?

If your site is responsive, technically compliant, and the migration still hasn’t occurred after several months, inspect the mobile Core Web Vitals and the specific crawling errors for Googlebot Smartphone in Search Console. A high LCP above 4 seconds or catastrophic CLS can delay the switch.

Also check the canonical and alternate annotations if you have separate mobile/desktop URLs. A configuration error (desktop canonical pointing to desktop from the mobile page, or missing alternate) can block the migration. Correct this, force a re-crawl via the URL Inspection Tool, and monitor the logs to confirm that Googlebot Smartphone is back crawling the site.

• Use the URL Inspection Tool on 5-10 representative pages to confirm the user agent
• Analyze server logs to identify the proportion of Googlebot Smartphone vs Desktop crawl
• Compare text content, images, and links between mobile and desktop versions with an HTML diff tool
• Check that structured data (JSON-LD, microdata) is identical on both versions
• Test mobile rendering with the mobile optimization testing tool and correct JavaScript errors
• Monitor mobile Core Web Vitals in Search Console and PageSpeed Insights