URL: /rules/core/canonical-form-drift

---
title: "Canonical Form Drift"
description: "Finds canonical URLs that disagree in form (www, scheme, trailing slash, tracking params) across the site"
---

Finds canonical URLs that disagree in form (www, scheme, trailing slash, tracking params) across the site

| | |
|---|---|
| **Rule ID** | `core/canonical-form-drift` |
| **Category** | [Core SEO](/rules/core) |
| **Scope** | Site-wide |
| **Severity** | warning |
| **Weight** | 3/10 |

## How it works

Every canonical this rule looks at is individually valid. What is wrong is
collective: half your site canonicalises to `https://www.example.com/thing/` and
half to `https://example.com/thing`, so the two halves consolidate onto different
URLs and the ranking signal splits between them. No per-page rule can see this,
because a page only knows its own canonical.

The rule compares four axes of URL form across every canonical in the crawl:

- **Scheme**: `https` versus `http`.
- **Host**: `www` versus apex.
- **Trailing slash**: present versus absent.
- **Tracking parameters**: retained versus stripped (`utm_*`, `gclid`, `fbclid`,
  `ref`, and friends).

For each axis it works out the site's dominant form and reports only the pages that
break it.

It stays quiet unless it has grounds to speak:

- Fewer than 10 crawled pages: no site norm exists, so the rule is skipped.
- Fewer than 10 comparable canonicals on an axis: that axis is dropped, not guessed
  at.
- No form reaching 70% on an axis: the site has no convention there, so nobody on
  that axis is a deviant. A site that genuinely mixes forms is a judgement call for
  a human, not an accusation from a rule.

Pages built from one template emit canonicals in one form, so they define the norm
rather than deviate from it. A consistently param-bearing canonical is a form like
any other: if every page does it, the site is not contradicting itself and nothing
is reported.

Two refinements keep the trailing-slash axis honest, because it is the one most
likely to accuse a healthy site:

- It is compared **within a section** (the first path segment of the canonical),
  not site-wide. Using a slash in `/blog/how-to/` and none in `/product` is a normal
  shape, so each section is judged against itself and needs its own 10 pages and 70%
  agreement.
- The site root (`/`) and file-like paths (`/feed.xml`) never carry a trailing slash,
  so they are left out of that comparison entirely.

## Validity is a different rule

This rule reports **form only**, so one root cause is never reported twice:

- A missing, relative or unparseable canonical is [Canonical URL](/rules/core/canonical).
- A `Link: rel="canonical"` header that disagrees with the tag is
  [Canonical Header](/rules/core/canonical-header).
- A canonical pointing at another host is [Canonical URL](/rules/core/canonical)
  as well, and its form follows the other site's conventions, so it is excluded here.

Pages in any of those states are dropped before the comparison runs.

[Canonical URL](/rules/core/canonical) also notes, as `info`, that a page's canonical
points somewhere other than itself. That is a per-page observation with no claim
about the rest of the site, and it is exactly the input this rule reads: the finding
here is the site-wide pattern those individual notes add up to.

## Solution

Each of these canonicals is valid on its own, but the site does not agree with
itself: some pages canonicalise to one URL form and some to another. Search engines
treat the two forms as different URLs, so links, crawl budget and ranking signals
split between them.

Pick one form, the one your redirects already send traffic to, and emit it
everywhere: one scheme (`https`), one host (`www` or apex, not both), one
trailing-slash convention, and never any tracking parameters in the canonical.
Canonicals are usually generated in one template or middleware, so this is normally
a single fix in the place that builds the URL rather than a per-page edit.

## Enable / Disable

### Disable this rule

```toml squirrel.toml
[rules]
disable = ["core/canonical-form-drift"]
```

### Disable all Core SEO rules

```toml squirrel.toml
[rules]
disable = ["core/*"]
```

### Enable only this rule

```toml squirrel.toml
[rules]
enable = ["core/canonical-form-drift"]
disable = ["*"]
```