The server is unwilling to process the request because either an individual header field, or all the header fields collectively, are too large.
What does HTTP 431 mean?
The 431 Request Header Fields Too Large code is part of the 4XX class. This is a client error code — the request was faulty, unauthorized, or cannot be fulfilled. The problem is on the request side.
When is it used?
A server returns 431 Request Header Fields Too Large in situations matching the definition above. You will encounter it inspecting network requests, reading logs, or testing an API. Whether it signals a problem depends on its class: 2xx and 3xx are normal, while 4xx and 5xx usually need attention on the client or server respectively.