Mobile-first indexing is Google's policy of using the mobile version of a site as the primary source for indexing and ranking. Announced in 2016 and rolled out gradually, it became the default for all new sites in 2019 and was completed for all sites in October 2023.
What "primary source" means in practice: when Googlebot decides what content exists on your site, what links you have, what structured data you serve, and what topical signals to attribute to each URL, it uses what is rendered to the mobile crawler. The desktop version is essentially ignored for indexing purposes — even though it is still served to desktop users.
The implications cascade. Content hidden on mobile (collapsed accordions, "read more" toggles, mobile-only navigation simplifications) is still seen by Google but may be weighted differently if it requires interaction to access. Content that exists only on desktop — extended footers, deeper category navigation, comprehensive sidebars — is invisible to indexing. Structured data must be present on the mobile version. Schema Markup only on desktop does not count.
Common issues operators discovered during the mobile-first rollout: responsive sites where mobile breakpoints accidentally hid critical content; separate-URL mobile sites (m.example.com pattern) where mobile content was a thinner subset of desktop; image alt attributes missing on mobile templates; and structured data scoped to a desktop-only header partial.
The right architecture today is responsive design serving the same HTML to all devices, with CSS handling layout differences. Avoid separate mobile URLs; they create maintenance overhead and indexing ambiguity. Serve the same Title Tag, Meta Description, H1 Tag, structured data, internal links, and content across viewports.
Mobile-first indexing interacts with Core Web Vitals: CWV thresholds for ranking are applied per-page and per-device, with mobile data typically scrutinised more heavily because most queries now originate on mobile. A site with great desktop CWV and poor mobile CWV is treated as having poor CWV for ranking purposes.
Audit by inspecting your URLs through Search Console's URL Inspection with the "Mobile" rendering option. Compare against the desktop version. Anything missing on mobile is, for indexing purposes, missing from your site.
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