HTTP 431 — Request Header Fields Too Large
4xx Client Error
What it means
The headers exceed the server's limit, usually because of oversized cookies.
What to do about it
Clear cookies for the domain, or stop storing large blobs in them. A JWT in a cookie is a common cause.
Where it sits
431 belongs to the 4xx family: The request contains something the server will not or cannot process. The fix is normally on the client side.
HTTP/1.1 431 Request Header Fields Too LargeChecking it yourself
# see the status code and headers only
curl -sI https://example.com/path
# follow redirects and print each hop
curl -sIL -o /dev/null -w "%{http_code} %{url_effective}\n" https://example.com/path
# JavaScript
const r = await fetch(url);
console.log(r.status, r.statusText);