# 431 Request Header Fields Too Large

> The server refuses to process the request because header fields are too large.

Source: https://apiguide.dev/status-codes/431
Class: client-error
Defined in: RFC 6585 <https://www.rfc-editor.org/rfc/rfc6585.html>
Updated: 2026-07-09

## What does HTTP status code 431 mean?

The 431 Request Header Fields Too Large status code indicates that the server rejects the request because the headers (or a single header field) are too large.

## When to use it in APIs

Usually triggered when a client sends extremely large cookies, authorization tokens (like bloated JWTs), or request parameters.

## How it compares to other codes

* **400 Bad Request**: A generic request failure. 431 explicitly points to header size violations.