Run a free 7-point canonical audit on any URL. Catches loops, broken targets, duplicate tags, and conflicting signals in a single check — no sign-up.Results in a few seconds. Nothing is stored.
A canonical tag is a one-line instruction — and a one-line instruction can go wrong in a surprising number of ways.
Without a declared canonical, Google decides on its own which variant of the URL is the “real” one. Tracking parameters, http/https, and www/non-www versions then compete against each other. A self-referencing canonical settles the question.
A canonical that points somewhere else declares this page a duplicate of the target — the target gets indexed instead. That’s exactly right for consolidation, and exactly wrong when a template accidentally stamps the same canonical on every page.
When the canonical target answers with an error status, the instruction is dead: Google can’t consolidate onto a page that doesn’t exist, drops the hint entirely, and picks a version itself — often not the one you wanted.
Page A canonicalizes to page B, and B canonicalizes back to A. Both instructions cancel out, Google ignores them, and indexing becomes a coin flip. Loops are nearly invisible in the source of a single page — you only see them by following the chain, which is what this tool does.
A canonical pointing at another domain hands your indexing to that domain. Legitimate for syndicated articles — devastating when a staging hostname or an old migration target is left behind in the template and silently deindexes the live site.
Two canonical tags with different targets is a contradiction, and Google’s response to contradictions is to trust neither. The usual culprit: an SEO plugin and a theme each injecting their own tag without knowing about the other.
Canonicals can be declared twice: by the server in an HTTP Link header and by the page in a <link> tag. When CDN or server config disagrees with the CMS, the two signals collide — and a collided signal is a discarded signal.
Beyond the 7 checks, every audit reads robots.txt, meta robots, and the X-Robots-Tag header, and records page status and the full redirect chain — the signals that decide whether Google sees your canonical at all.
One request from you, a handful from us — no crawler queue, no account, no quota.
<link rel="canonical"> tags, the HTTP Link header, meta robots, and robots.txt are read and compared.The same 7-point audit, as JSON over one GET request. Integrations sign in with an API key.
cc_ and are issued by email. Request a key and tell us what you’ll use it for. Treat it like a password.X-API-Key header (or as Authorization: Bearer). GET /api/me confirms a key works. An invalid key returns 401. Every endpoint, field and error is in the API reference.# Run a check
curl -G https://canonicalchecker.com/api/check \
-H "X-API-Key: cc_…" \
-d url=example.com
# Response (trimmed)
{
"verdict": "pass",
"canonical": {
"effective": "https://example.com/",
"source": "html"
},
"checks": [
{ "id": "declared", "status": "pass" },
…
]
}
Everything you need to know about canonical tags and duplicate content.
A canonical tag (<link rel="canonical" href="…">) tells search engines which URL is the preferred, “master” version of a page when several URLs show the same or very similar content. Search engines consolidate ranking signals onto the canonical URL and typically index that one.
A self-referencing canonical points at the page’s own URL. It looks redundant but isn’t: it neutralizes duplicate variants created by tracking parameters, session IDs, and protocol or www differences before they become a problem. It’s the recommended default for every indexable page.
A loop means the canonical chain circles back on itself — A points to B and B points back to A. Google ignores looped canonicals and chooses a version on its own. Fix it by deciding which URL should be indexed, giving that page a self-referencing canonical, and pointing every other variant directly at it (no chains).
The directive is simply discarded — Google can’t consolidate signals onto a page that doesn’t resolve. The page with the broken canonical may still get indexed, but you’ve lost control over which variant wins. Point the canonical at a live, 200-status URL.
Not inherently. It’s the standard mechanism for syndicated content: the republishing site canonicalizes to the original so the original ranks. It becomes dangerous when it’s unintentional — a staging domain or migration leftover in your template can deindex whole sections of the live site.
If they conflict, quite possibly none of them: Google has stated that conflicting canonicals may all be ignored. In practice the first tag in the <head> often wins, but you shouldn’t rely on it. Find the plugin, theme, or template that injects the extra tag and remove it.
A 301 redirect is the stronger signal: users and crawlers both land on the target, and there’s no ambiguity. Use a canonical when both URLs must stay accessible (filtered views, print versions, UTM-tagged URLs, syndication). Use a 301 when the duplicate URL has no reason to exist.
No — it’s a strong hint, not a directive. Google can override it when other signals (internal links, sitemaps, redirects, near-identical content elsewhere) disagree. That’s why consistency matters: canonical, sitemap URLs, and internal links should all point at the same variant.
Completely free, no account, no quota. Each check fetches your page live, computes the result, and returns it — nothing is logged or stored.