431 Request Header Fields Too Large response status code indicates that 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 when the total number of request header fields is too large, or when a single header field is at too large.
This error shouldn't happen on well-tested production systems, but can be found more often while testing a new system.
431 Request Header Fields Too Large
|RFC 6585, section 5: 431 Request Header Fields Too Large||Additional HTTP Status Codes|