The 431 HTTP status code, also known as “Request Header Fields Too Large,” is a client-side error that indicates the server refuses to process a request because its header fields are too large. This error can be caused by either the total size of the headers or a single header field exceeding the allowed size.
The HTTP 431 status code was first introduced in RFC 6585, published in April 2012. This was part of an effort to standardize additional responses for HTTP/1.1, addressing issues that were not covered by the existing status codes at the time. Introducing this specific error code was crucial for enhancing server security and preventing malicious attacks like buffer overflow attacks, which could exploit large headers.