431 Request Header Fields Too Large — Causes & Fix
HTTP 431 (Request Header Fields Too Large) means the server refuses the request because its header fields are too large — often a single oversized header like a huge cookie.
- Applies to
- Any website or web server (HTTP standard)
- Time to fix
- 5–30 minutes
What it means
Causes include an oversized or accumulated cookie, or too many/too-large custom headers. HTTP status 431 is defined in RFC 9110 (HTTP Semantics). This is a client-side (4xx) error — the request itself had a problem, so the fix is usually on the requester's side or in how the server validates the request.
Symptoms
- "431 Request Header Fields Too Large"
- Fails on one site while others work
How to fix it
- 1
Clear cookies
1. As a user: clear that site's cookies (a bloated cookie is the usual cause) and retry.
- 2
Trim headers (developers)
1. Reduce header/cookie size, or raise the server's header-size limit.
Frequently asked questions
Why 431 on one site only?
That site set a very large cookie or headers. Clearing its cookies usually fixes it.
Last updated July 20, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.