4×× Client Error

431 Request Header Fields Too Large

The server is unwilling to process the request because its header fields are too large. The request MAY be resubmitted after reducing the size of the request header fields.

It can be used both when the set of request header fields in total is too large, and when a single header field is at fault. In the latter case, the response representation SHOULD specify which header field was too large.

For example:

HTTP/1.1 431 Request Header Fields Too Large
Content-Type: text/html

<html>
  <head>
    <title>Request Header Fields Too Large</title>
  </head>
  <body>
    <h1>Request Header Fields Too Large</h1>
    <p>The "Example" header was too large.</p>
  </body>
</html>

Responses with the 431 status code MUST NOT be stored by a cache.

Source: RFC6585 Section 5

431 Code References

.NET HTTP Status Enum HttpStatusCode.RequestHeaderFieldsTooLarge

Rust HTTP Status Constant http::StatusCode::REQUEST_HEADER_FIELDS_TOO_LARGE

Go HTTP Status Constant http.StatusRequestHeaderFieldsTooLarge

Symfony HTTP Status Constant Response::HTTP_REQUEST_HEADER_FIELDS_TOO_LARGE

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