400 Bad Request
The server could not understand the request due to malformed syntax.
What it means
HTTP 400 Bad Request means the server rejected the request because something about it was malformed — invalid syntax, a corrupted request, an oversized header, or parameters the server cannot parse.
When it happens
It happens with broken query strings, malformed JSON in an API call, cookies or headers that are too large, or a client sending data the server cannot interpret.
How to fix it
- Check the request syntax — validate JSON bodies and properly encode query parameters.
- Clear oversized or corrupted cookies for the site if browsing triggers the error.
- Reduce header size if a Request Header Too Large variant appears.
- Inspect server logs to see exactly which part of the request was rejected.
SEO impact
Negative if it affects real URLs. A page returning 400 cannot be indexed. If important URLs return 400 from bad internal links or malformed parameters, fix the links so crawlers reach valid 200 pages.
Find out which of your URLs return 400
Paste a list of URLs and Sitewell checks the status code of every one at once — free and without signup.
Related codes
Authentication is required and has failed or not been provided.
402 Payment RequiredA largely reserved code intended for payment-gated access to a resource.
403 ForbiddenThe server understood the request but refuses to authorize it.
404 Not FoundThe server could not find the requested resource.
405 Method Not AllowedThe HTTP method used is not supported for the requested resource.
406 Not AcceptableThe server cannot produce a response matching the formats the client said it would accept.
Related guides
A plain-English primer on HTTP status codes for SEOs: what the 2xx, 3xx, 4xx, and 5xx families mean, which ones affect rankings, and the codes worth knowing.
How to find and fix broken linksA practical walkthrough for finding broken links on your site, working out why each one breaks, and fixing them so visitors and crawlers stop hitting dead ends.