HTTP 431 Request Header Fields Too Large

Client error code, RFC 6585

What it means

The request headers are too big: either one header or all together.

A real response

This is the request and the answer our test server gave when we asked it to return 431.

GET /c/431 HTTP/1.1
Host: example.com

HTTP/1.1 431 Request Header Fields Too Large
Date: Sat, 14 Nov 2026 10:00:00 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 11

<p>demo</p>

If you run the site

RFC 6585 describes 431. The usual cause is cookies that grew too large, for example after many logins to a site on the same domain. Servers have a limit on header size; reduce cookie size, or raise the limit only if you must.

If you are a visitor

Clear the cookies of that site and reload.

A common mistake

Storing large amounts of data in cookies.

Registered in the IANA HTTP Status Code Registry as Request Header Fields Too Large, defined in RFC 6585.

This is an independent guide. It is not connected with the IETF, IANA, any browser maker or any company named here. Meanings were checked against the RFC texts on 14 November 2026. Responses shown were recorded on 14 November 2026 from a small test server on the author's own computer.