What is a non-indexable canonical tag? What should you do about it?
For starters, a canonical tag is meant to answer a very specific question for search engines: which version of this page should be treated as the primary one? When implemented correctly, it helps avoid duplicate content issues when multiple URLs return the same content.
A non-indexable canonical tag is simply a canonical tag that references a URL Google cannot index. If Google can’t access or index the URL that was declared as the canonical URL, Google will typically ignore the canonical entirely. If that happens, Google will make up its own mind about how to reconcile duplicate content.
There are a few common scenarios where a canonical URL becomes non-indexable.
- Redirects. Google does not index redirecting URLs. Instead, it follows the redirect and evaluates the destination. However, Google won’t always assume the redirect destination should be used as the canonical URL instead. (More about redirects.)
- Noindex directive. If the URL referenced in the canonical tag includes a noindex, this creates a contradiction. With the canonical, you are telling Google, “this URL is the preferred version,” while also saying, “don’t include this same URL in the index.” (More about noindex.)
- 404 or other error. A page that returns an error will not be indexed, so it cannot serve as a valid canonical target. This is a more obvious case, but it still shows up surprisingly often in audits, especially on sites with frequent content changes or migrations. (More about 404 errors.)
- Blocked from crawling. If the URL defined in the canonical tag is blocked via robots.txt disallow rules, then Googlebot cannot crawl the page. If it can’t crawl, it can’t evaluate it. And if it can’t evaluate it, it can’t process the canonical tag’s instructions to reconcile the duplication.
In all of these cases, the outcome is similar: Google is likely to ignore the canonical tag. When that happens, Google makes its own decision about which URL to index. Sometimes that works out fine. Other times, it leads to parameterized URLs, duplicate versions, or less optimal pages appearing in search results.
So what should you do about it?
The fix is straightforward in principle: update the canonical tag to point to a URL that is fully and unambiguously indexable. That means the URL should return a 200 status, not redirect, not include a noindex directive, and not be blocked from crawling. It should also represent the version of the page you actually want indexed.
In practice, this can be challenging to fix depending on exactly how canonical tags are defined. The rules are often buried deep within application logic, and that can make fixing this a bigger project. Still, even if it is big, this is usually a high-priority, high-value fix to implement.