A 400 Bad Request means the server cannot or will not process a request because something about it looks wrong on the client side. RFC 9110 gives malformed syntax, invalid message framing and deceptive request routing as examples. Google treats a 400 like most other 4xx codes: the content is ignored, the URL is not indexed, and a URL that was already indexed gets removed.
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Common causes
A visitor who hits a 400 has usually done nothing wrong. The request was built badly by a link, a script or a piece of infrastructure.
- Badly encoded characters in a URL or query string
- A URL or set of cookies that is larger than the server accepts
- Tracking or redirect tools that append broken parameters
- A firewall or CDN rule that rejects requests it considers malformed
- Application code that returns 400 for a missing or invalid parameter
How Google treats a 400
Google's documentation says all 4xx codes except 429 are handled in the same way, with the content treated as nonexistent. A previously used URL that now returns a 4xx is dropped over time. Except for 429, these codes do not affect crawl rate.
In the Page indexing report, a 400 may appear under URL blocked due to other 4xx issue, which covers 4xx errors not listed under another status. Google suggests debugging the page with the URL Inspection tool.
When a 400 is the wrong answer
Application code sometimes returns 400 where another code would be more accurate. Use 404 or 410 for a page that does not exist, and a 2xx for a valid page whose optional parameter is simply unknown. Search and filter pages deserve a check, since a harmless unknown parameter should not take a real page out of the index.
Example
A filter parameter that broke the page
Hypothetical HR software Dovepath added validation to its template library, and any unknown parameter now returned a 400. Newsletter links with an extra tracking parameter started failing, and a batch of URLs moved into the URL blocked due to other 4xx issue status.
The team changed the validation to ignore unknown parameters and show the normal page, with a canonical tag pointing to the clean URL.
Common mistakes
Returning 400 for a missing page
A 404 or 410 describes the situation better and is easier to monitor. Keep 400 for requests that are really malformed.
Debugging only with your own browser
A cookie or a firewall rule can cause a 400 for some clients and not others. Compare your browser, curl and the URL Inspection tool.
Frequently Asked Questions
Google applies the handling to the URLs returning the code and says 4xx codes other than 429 do not affect crawl rate. The practical cost is losing the affected pages if they matter.
Oversized or corrupted cookies are one possible cause, since they are sent only by that browser. Clearing cookies for the site is a quick test.
Keep reading
- HTTP Status Codes: HTTP status codes are three-digit server responses that tell browsers and crawlers whether a request succeeded, moved, or failed.
- 404 Error: A 404 error is the HTTP status a server returns when a requested URL does not exist. It is normal and only a problem on pages that should exist.
- 410 Gone: 410 Gone is an HTTP status code saying a page was removed on purpose and will not return. Google treats it much like a 404 and drops the URL from its index.
- URL parameters: URL parameters are key-value pairs after a question mark in a URL. They can filter, sort or track, and can create many duplicate URLs for crawlers.
- Bing Webmaster Tools: Bing Webmaster Tools is Microsoft's free console for site owners. It shows how Bing crawls and indexes a site and offers SEO reports and URL submission.
- Content delivery network: A CDN is a network of servers worldwide that caches and serves a site's files close to each visitor, cutting load times and shielding the origin server.
- Crawl Errors: Crawl errors are failures that happen when a search engine tries to fetch a URL, such as server errors, redirect problems, blocked pages or DNS failures.
- Crawlability: Crawlability is how easily search engine and AI crawlers can discover and fetch the pages on a site.
