Summary: The Canonical Tag Checker finds every canonical declared for a URL, in the HTML and in the HTTP Link header, then checks where it points and whether that target answers 200 and can be indexed. It flags the mistakes that make Google ignore a canonical: several different values, a relative URL, a tag outside the head, a target that redirects or returns an error.
The canonical tag tells Google which version of a page to index when several addresses show the same content: tracking parameters, sorting options, http and https, with and without www. Set well, it merges those variants into one page. Set badly, it can quietly hand your ranking to another URL.
What the checker verifies
- Every declared canonical: link tags in the HTML and the Link HTTP header. Two different values cancel each other out.
- Self-reference: the recommended setup for a page that should rank on its own.
- Parameter variants: a URL with ?utm_source pointing to its clean version is correct, and shown as such.
- The target: it must answer 200, not redirect, and not carry a noindex.
Common mistakes and their fix
- Canonical to the home page on every page: often left by a theme. Each page should point to itself.
- Canonical to a redirecting URL: point directly to the final address, the one that answers 200.
- noindex and canonical to another page: two contradictory signals. Keep the canonical for duplicates, the noindex otherwise.
- Canonical in the body: Google only reads it in the head.
To check all the indexing signals at once, use the indexability checker. To see the Link header and the other response headers, use the HTTP headers checker. Duplicates also come from content that targets the same query: see keyword cannibalization.
