431
4xx Client ErrorRequest Header Fields Too Large
The server won't process the request because its headers are too large.
431 Request Header Fields Too Large means the headers, one of them or all of them together, exceed the server's limit. Nine times out of ten the culprit is cookies: large auth tokens and tracking cookies pile up on a domain until every request tips over the cap. That's why 'clear your cookies' genuinely fixes this one.
What 431 looks like on the wire
GET / HTTP/1.1 Host: example.com Cookie: session=...; theme=...; (16 KB of accumulated cookies) HTTP/1.1 431 Request Header Fields Too Large Connection: close
About 4xx Client Error
The request has a problem the client needs to fix, like a bad URL, missing auth, or invalid input.
How to fix 431 Request Header Fields Too Large
- Clear cookies for the site, and stop storing large tokens in cookies if it's your app.
- Audit what gets set - JWTs and analytics cookies grow quietly over time.
- If big headers are genuinely needed, raise the server's header size limit.
Where it's defined
431 Request Header Fields Too Large is defined in RFC 6585 §5. MDN's 431 reference covers browser behavior and examples.