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Agent LighthouseAgent Lighthouse

    Searches the text of every published page. The evidence sources themselves are not in this index — search all of them on the trusted sources page.

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    access-crawl-control/canonical

    Canonical URLs point at the right page

    What it checks

    The canonical URL each page declares, resolved and compared against the page itself — not merely that a tag is present.

    <link rel="canonical"> is read from <head> with rel matched token-wise and case-insensitively, so rel="Canonical", rel=" canonical " and rel="shortlink canonical" all count. Each href is resolved against the page URL (new URL(href, page.url)), so relative canonicals are unambiguous and a malformed value cannot pass as absolute. Comparison runs on one key — host without www., lower-cased path, no trailing slash, query or fragment.

    State Result
    ≥ 2 non-root pages declare the site root as their canonical fail, priority high — homepage collapse
    a canonical does not resolve to an http(s) URL fail, priority high
    most declaring pages collapse onto one non-root URL warn, priority high
    a page carries two conflicting canonicals warn, priority medium
    a canonical names another domain warn, priority medium
    no page declares a canonical warn, priority medium
    some pages do not declare one warn, priority low
    every page resolves to itself pass
    no pages scanned na

    Why it matters

    Googlebot reads <link rel="canonical">, and uses it as a strong signal when choosing which of a set of duplicate URLs to index and consolidate signals onto. The URL it selects is the one eligible to be shown in Search — and so, in turn, as a supporting link in AI Overviews and AI Mode.

    Evidence

    • Google describes rel="canonical" as “a strong signal that the specified URL should become canonical”. It explains the effect: “It helps search engines to be able to consolidate the signals they have for the individual URLs (such as links to them) into a single, preferred URL.” It also recommends a self-referential canonical on the canonical page — developers.google.com/…/consolidate-duplicate-urls (verified 2026-08-21)
    • RFC 6596, “The Canonical Link Relation” (Informational, April 2012), defines the relation type. It designates “an Internationalized Resource Identifier (IRI) as preferred over resources with duplicative content”. That is a published specification behind the attribute, with known consumers — rfc-editor.org/…/rfc6596.html (verified 2026-08-21)
    • Google’s crawl-budget guidance corroborates the crawling-side effect: “Consolidate duplicate content” to “focus crawling on unique content rather than unique URLs”, because otherwise duplicate URLs waste “a lot of Google crawling time on your site” — developers.google.com/…/large-site-managing-crawl-b… (verified 2026-08-21)
    • The link from indexing to AI surfaces is documented: “To be eligible to be shown as a supporting link in AI Overviews or AI Mode, a page must be indexed and eligible to be shown in Google Search with a snippet” — developers.google.com/…/ai-features (verified 2026-08-21)

    Limits

    Google explicitly states the tag is not required: “While we encourage you to use these methods, none of them are required; your site will likely do just fine without specifying a canonical preference” (developers.google.com/…/consolidate-duplicate-urls, verified 2026-08-21). The signal is a hint, not a directive — Google may select a different canonical than the one declared — so absence of the tag is not a documented defect, which undercuts this audit’s presence-only FAIL.

    RFC 6596 is Informational, not Standards Track (rfc-editor.org/…/rfc6596.html, verified 2026-08-21). No AI vendor outside Google names the signal: OpenAI’s (developers.openai.com/…/bots) and Perplexity’s (docs.perplexity.ai/…/perplexity-crawlers) crawler documentation mention robots.txt only (both verified 2026-08-21), and Google states no special markup is needed for its generative features (developers.google.com/…/ai-optimization-guide, verified 2026-08-21). Note also that the graded mechanism concerns the canonical’s value; as recorded above, this audit measures only presence, so a site-wide canonical pointing at / — the failure mode this mechanism actually warns about — passes.

    How it scores

    A named crawler’s use of the signal is stated in vendor documentation, and the link relation itself is a registered, published standard.

    Sources