431 Request Header Fields Too Large

Client error: the problem lies with the request. Repeating it unchanged will normally fail again.

When a server should send it. When the request headers are too large, all together or one of them. Usually it is the Cookie header grown too big; nginx returns 400 with "Request Header Or Cookie Too Large" in the same case.

Common causes. Too many or oversized cookies for the domain.

What to do. Clear the site's cookies. Developers should keep cookies small.

Browsers and HTTP clients. Clearing the site's cookies in the browser usually fixes it. The site should stop setting so many or such large cookies.

Google Search. Like every 4xx except 429, it tells Google the content does not exist: an indexed URL is removed from the index, and a new one is not processed. Crawl frequency for the URL gradually drops. Source: Google Search Central, How HTTP status codes, and network and DNS errors affect Google Search.

Caching. Not cacheable by default (RFC 9110 section 15.1 does not list it). A cache stores it only when the response says so with Cache-Control or Expires.

HTTP/1.1 431 Request Header Fields Too Large
Content-Type: text/plain; charset=utf-8

Request Header Fields Too Large

location /example {
    return 431;
}

RewriteEngine On
RewriteRule ^example$ - [R=431,L]

http_response_code(431);
echo 'Request Header Fields Too Large';
exit;

res.writeHead(431, { 'Content-Type': 'text/plain' });
res.end('Request Header Fields Too Large');

# in a view function
return 'Request Header Fields Too Large', 431