HTTP 431 Request Header Fields Too Large
What a 431 Request Header Fields Too Large response means, why it happens, and the fastest ways to fix it.
Headers exceed the server's limits — one giant header (usually Cookie) or too many of them.
- Accumulated oversized cookies on a domain.
- Auth tokens stuffed into headers beyond proxy limits.
- Clear cookies for the site; slim down what you store in them.
- Raise header-size limits on the proxy (e.g. large_client_header_buffers in nginx) if legitimately needed.
Notes
HTTP status codes are defined in RFC 9110 (which replaced RFC 7231). The first digit is the class: 1xx informational, 2xx success, 3xx redirection, 4xx client error, 5xx server error.
Same class: 400 · 401 · 403 · 404 · 405 · 408 · 409 · 410 · 412 · 413 · 415 · 418 · 422 · 425 · 429 · 451
All codes: HTTP status code reference
Frequently asked questions
What does HTTP 431 mean?
HTTP 431 Request Header Fields Too Large: Headers exceed the server's limits — one giant header (usually Cookie) or too many of them.
How do I fix a 431 error?
Clear cookies for the site; slim down what you store in them. Raise header-size limits on the proxy (e.g. large_client_header_buffers in nginx) if legitimately needed.
Is a 431 my fault or the server's?
431 is a client error — the request itself needs to change, though server misconfiguration can also trigger it.