Canonical URL — agent readiness

Why an agent cares

An agent that cannot tell which of several addresses is authoritative may cite, cache or link the wrong one. RFC 6596 permits far more than search engines honour, so most findings here are about what will be ignored rather than what is invalid.

Adoption

Web baseline. Standard equipment on the web. Its absence is the exception rather than its presence.

Direction of travel: ► flat, as at 2026-07-28.

RFC 6596, and two decades of CMS and SEO defaults. Absence is the exception, not the norm.

What we check

Absence is never a finding. Every check below runs only once canonicalPresent is true, so a site that has not adopted this is not assessed on it and appears in no report. 4 of these 6 can fail it. In a Lumar crawl these land on the page's own row.

Check	Severity	Raises	On whose authority
`canonicalOutsideHead`	high	usability	Google Search profile
`canonicalConflicting`	high	usability	Google Search profile
`canonicalEmptyHref`	medium	usability	WHATWG HTML / Google Search profile
`canonicalInvalidUrl`	medium	usability	Lumar readiness profile
`canonicalRelative`	informational	nothing	specification (RFC 6596)
`canonicalCrossOrigin`	informational	nothing	specification (RFC 6596)

Examples

Both of these are fixtures the test suite runs through the real collector, not snippets written for a document: the first is a shape we score as valid, the second is one we flag.

correcthtmltest/e2e/test-pages/discoverability/correct.html (excerpt)

<!-- canonicalPresent + correct: single, absolute, self-referencing URL. -->
    <link rel="canonical" href="https://example.com/discoverability/correct.html" />

present but wronghtmltest/e2e/test-pages/discoverability/issues.html (excerpt)

<!-- canonicalRelative (informational, NOT a validity failure): a relative href — RFC 6596 permits it;
         Google just recommends an absolute URL. -->
    <link rel="canonical" href="/discoverability/some-other-page.html" />

<!-- canonicalConflicting: a second in-head declaration with a different target, so no single canonical wins. -->
    <link rel="canonical" href="https://example.com/discoverability/issues.html?ref=dup" />

Specifications

Document	Revision	Kind
RFC 6596 — The Canonical Link Relation	`RFC 6596`	specification
Google Search: consolidate duplicate URLs	`2026-07-28`	vendor profile

Last re-read against the published documents: 2026-08-12.