# Canonical URL

Tells an agent which address is the real one for this page.

**[Discoverability](/docs/discoverability)** · checked on every URL · 4 checks that can fail it

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** (L4)

Standard equipment on the web. Its absence is the exception rather than its presence. Direction of travel: ► flat, as of 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.

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

## Examples

These are the fixtures the test suite runs through the real collector, not snippets written for a document.

- Correct: `test/e2e/test-pages/discoverability/correct.html`
- Present but wrong: `test/e2e/test-pages/discoverability/issues.html`

## Specifications

| Document | Revision | Kind |
|---|---|---|
| [RFC 6596 — The Canonical Link Relation](https://www.rfc-editor.org/rfc/rfc6596) | RFC 6596 | specification |
| [Google Search: consolidate duplicate URLs](https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls) | 2026-07-28 | vendor profile |

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