4xx Client Error
431 Request Header Fields Too Large
The header block, or one individual header, is too big.
What causes it
- Cookie bloat — a domain accumulating dozens of tracking cookies until every request exceeds the limit.
- A very long `Referer` or a huge JWT in `Authorization`.
How to fix it
- Clear cookies for the domain to confirm the cause, then trim what you set.
- Keep JWTs small: put a session ID in the cookie and the claims in your own store.
At a glance
- Class
- 4xx Client Error
- Reference
- RFC 6585 §5
- Standardised
- Yes
- Retry the request?
- No — fix the request
Other 4xx client error codes
- 400 Bad Request
- 401 Unauthorized
- 402 Payment Required
- 403 Forbidden
- 404 Not Found
- 405 Method Not Allowed
- 406 Not Acceptable
- 407 Proxy Authentication Required
- 408 Request Timeout
- 409 Conflict
- 410 Gone
- 411 Length Required
- 412 Precondition Failed
- 413 Content Too Large
- 414 URI Too Long
- 415 Unsupported Media Type
- 416 Range Not Satisfiable
- 417 Expectation Failed
- 418 I'm a teapot
- 421 Misdirected Request
- 422 Unprocessable Content
- 423 Locked
- 424 Failed Dependency
- 425 Too Early
- 426 Upgrade Required
- 428 Precondition Required
- 429 Too Many Requests
- 451 Unavailable For Legal Reasons
- 499 Client Closed Request