431 Request Header Fields Too Large

The request headers are too large, either one header or all of them together.

431 at a glance

Code: 431
Name: Request Header Fields Too Large
Class: 4xx Client error
Status: IANA-registered standard code
Defined in: RFC 6585, Section 5
Cacheable by default: No by default. Caches store it only when the response carries explicit freshness (Cache-Control max-age, s-maxage or Expires).

What 431 means

431 Request Header Fields Too Large (RFC 6585) means the server refuses to process the request because its headers are over a size limit. The usual culprit is cookies: an app that keeps adding cookies, or many apps sharing a parent domain, until the Cookie header exceeds the limit.

What typically causes a 431

Node.js rejects headers larger than --max-http-header-size (16 KB by default) with 431.
Too many or too large cookies on a shared domain (e.g. many apps on *.example.com), or huge JWTs in cookies.
nginx reports the same problem as "400 Request Header Or Cookie Too Large" instead of 431.
Long Referer or custom headers added by proxies.

Fix it on the server

Reduce cookie size or scope cookies to specific hosts and paths; store sessions server-side.
If you must, raise limits: Node --max-http-header-size, nginx large_client_header_buffers, Apache LimitRequestFieldSize.

Fix it as a client or visitor

Clear cookies for the site (and its parent domain); in development, clear localhost cookies, which collect from many projects.

Headers that go with 431

Header	Role
`Cookie`	Most frequent oversized header.

Example response

HTTP/1.1 431 Request Header Fields Too Large
Connection: close
Content-Length: 0