431 Request Header Fields Too Large

What does 431 Request Header Fields Too Large mean?

The server refuses to process the request because its headers are too large. It belongs to the 4xx Client Error class, which means the request contains bad syntax or cannot be fulfilled. Something about the request needs to change.

When do APIs return 431?

An oversized cookie, or a very long Authorization header carrying a bloated JWT.

How to fix a 431 Request Header Fields Too Large

Trim the headers. If a JWT is the cause, decode it to see what is inflating the payload — tokens carrying large claim sets are a common culprit. Note the limit is often set by a reverse proxy, not the application.

Reproduce a 431 response

The fastest way to understand a status code is to trigger one and read the full response. Open the API tester and send a request to https://httpbin.org/status/431 — you will see the 431 status along with every response header the server sent. Comparing those headers against a working request is often what reveals the cause.

When you are debugging a real endpoint, check the response headers as carefully as the body, and confirm you are using the right HTTP method for the operation.

Related 4xx status codes