Canonical Tag Auditor
Check which URL a page names as canonical, whether that URL holds up, and whether every spelling of the address redirects to it.
What this tool does
It checks your canonical the way Google weighs it: against everything else your site says about which URL is the real one. Having the tag is the easy part. The trouble starts when it disagrees with a redirect, a robots.txt rule or the address the page actually answers at, because Google treats all of those as signals and settles the argument itself.
So we read the tag, then test it. We fetch the URL it names and look for a redirect, an error or a noindex. We try every spelling of the address, with and without www and over http and https, and follow each one to see where it lands. And we read your robots.txt the way Googlebot does, to make sure the page you nominated is one Google is allowed to fetch.
How to use it
- Paste the page you care about, as you’d link to it. The homepage is a fine first run, and a product or article page also shows you what its template does.
- Start with the spellings list. If http, www or a trailing slash serves its own copy of the page, fix the redirects before touching the tag, because Google ranks a redirect above
rel="canonical". - Then work through the problems. Each one names the rule it breaks and what to change.
- Run it again after the fix. Nothing is cached on our side, so the second run sees the change.
How it works
We fetch your page with an ordinary browser’s user agent and read the HTML that comes back, before any script runs. A canonical a script adds after load won’t show up here. Google can pick one up when it renders the page, but its advice is that HTML is the best place to set it, and that a script shouldn’t change it to anything other than what the HTML says.
Some firewalls refuse a browser request that comes from a data centre, which is where we check from. When that happens we ask again as WhatsApp’s link-preview crawler, then as Meta’s, and tell you whose view you’re reading. If all three are turned away, we say so and stop. That is no verdict on what Googlebot gets, and URL Inspection in Search Console is where to find out.
The rules come from Google’s documentation. The tag belongs in the <head> and nowhere else, written as an absolute URL, with no #fragment, and once per page: Google warns that multiple canonical tags “may lead to unexpected results”. Of the ways to declare a canonical, Google lists redirects first and the tag second, so when the two disagree, the redirect wins. And it says outright not to use robots.txt for canonicalization, because a page Googlebot can’t fetch is a page whose signals it never reads.
What this can’t see is your sitemap or your internal links, the other two of the four places that have to agree on one URL. That takes a crawl, and Technical SEO Foundations sets out how to read one.
Standing offer
The hard part is deciding what to do first
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