HTTP 431 Request Header Fields Too Large (RFC 6585) indicates the server refuses to process the request because the header fields are too large — either a single header is too long or the combined headers exceed the limit. Common culprit: cookies. As applications accumulate cookies over time, the total Cookie header can exceed server limits (typically 8KB-16KB). The fix is usually to reduce cookie sizes or increase server limits.
Response includes the status code, standard headers (including Content-Type), and a small diagnostic JSON body describing the request and returned status.
Simulator URL (copy in the app after load — not a normal link):
https://httpstatus.com/api/status/431
Example request:
curl -i "https://httpstatus.com/api/status/431"