Canonical tag

A canonical tag is a <link rel="canonical"> element in a page’s <head> that tells Google which URL is the main version when the same content is reachable at several URLs. Google treats it as a strong signal, but it is a hint, not a directive Google must follow.

What is a canonical tag, and where does duplicate content come from?

Google calls the process of picking one representative URL for a piece of content canonicalization. The chosen URL is the canonical URL: Google crawls it most regularly and shows it in search results (Google Search Central, What is URL canonicalization, accessed November 2026).

Duplicates usually appear on their own: HTTP and HTTPS versions, filter parameters such as ?sort=price-asc, a mobile version on a separate URL, a staging site open to crawlers. Google says some duplication is normal and not a spam violation, but Google may show a URL you did not want.

How to declare a canonical URL

Google only accepts the element inside <head> and recommends absolute URLs:

<link rel="canonical" href="https://example.com/ao-thun-trang/">

The main page should also point to itself, called a self-referencing canonical. For a PDF, send the canonical in an HTTP Link header.

Google weighs the canonical tag with other signals: redirects are a strong signal, listing a URL in your XML sitemap is a weak one, and Google generally prefers HTTPS (Google Search Central, How to specify a canonical URL, accessed November 2026).

Hypothetical example: 200 products, each reachable at 3 URLs (the product path, a path through a category, and one with ?color=), for 600 URLs in total. Pointing the canonical to the product path tells Google there are only 200 main URLs.

Common canonical tag mistakes

Google lists these in its post 5 common mistakes with rel=canonical (Google Search Central Blog, 2013, accessed November 2026):

• The tag sits in <body>: Google ignores it.
• Missing https://, as in example.com/page/: Google reads it as a relative path.
• Two canonical tags pointing to different URLs, often because the theme and an SEO plugin both print one.
• Pages 2 and 3 of a category point to page 1, so products on later pages may not get indexed.

The next three follow from Google’s general guidance, because they send conflicting signals:

• A canonical pointing to a redirected URL, a 404 or a noindex page; or a sitemap listing one URL while the canonical names another.
• Every page pointing its canonical to the home page because of a theme misconfiguration.
• Mixing http and https, or www and non-www, across canonicals, internal links and redirects.

Canonical tags and AI search

To appear as a supporting link in AI Overviews and AI Mode, a page must be indexed and eligible to show with a snippet; Google says there are no other technical requirements (Google Search Central, AI features and your website, accessed November 2026). Google shows only the canonical URL from a duplicate cluster, so a wrong canonical can make AI Overviews link to a parameter URL instead of the page you want.

How to fix a missing canonical tag

Adtek’s AI Search Checker looks for a canonical tag in the HTML (2 points). It does not check whether the URL is right, so review the mistakes above.

1. Yoast SEO: the plugin adds canonicals for you. To change one, open the post, go to the Advanced tab, enter the full URL in the Canonical URL field and update (Yoast, Canonical URLs in Yoast SEO, accessed November 2026).
2. Rank Math: by default it uses the post’s current URL as the canonical. To change it, open the Advanced tab in the Rank Math meta box and fill in Canonical URL (Rank Math, Advanced Tab, accessed November 2026).
3. Shopify: Theme Store themes must include a canonical URL in their SEO metadata snippet (Shopify, Theme Store requirements, accessed November 2026). The Dawn theme ships <link rel="canonical" href="{{ canonical_url }}"> in layout/theme.liquid (Shopify Dawn, GitHub, accessed November 2026). If it is missing, add it via Online Store, Edit code.
4. Haravan, Sapo (Vietnamese e-commerce platforms): we found no official documentation on canonicals. View the page source; if the tag is missing, ask their support team.
5. Custom code: print the canonical in the <head> of every page from the server, with an absolute URL and no utm parameters. Google advises against changing the canonical with JavaScript.

To check: press Ctrl+U to view the source, search for “canonical” and make sure there is exactly one tag. In Google Search Console, URL Inspection shows the User-declared canonical and the Google-selected canonical (Search Console Help, URL Inspection tool, accessed November 2026). If they differ, Google has not followed your hint. Then run the AI Search Checker again.

Frequently asked questions

How is a canonical tag different from a 301 redirect?

A redirect sends both users and crawlers to the new URL. A canonical keeps every URL available to users and only hints to Google which one is the main version. Retiring the old URL? Use a redirect.

Can I block duplicate URLs in robots.txt instead?

Google advises against it. URLs disallowed in robots.txt can still be indexed without their content, and Google cannot crawl them to read the canonical.

If you need a review of canonicals and indexing across your site, see Adtek’s SEO solution.