HTTP 431 Request Header Fields Too Large

What does HTTP 431 mean?

The request headers, or a single header, are too large. Almost always caused by too many or too large cookies on a domain — for example tracking cookies piling up, or a large session stored in a cookie.

4xx Client error: The request has a problem the client can fix: wrong URL, missing credentials, bad data or too many requests.

Common causes of a 431 error

• Oversized cookies.
• Very long Referer or custom headers.

How to fix 431 Request Header Fields Too Large

• Clear cookies for the site.
• Store sessions server-side and keep cookies small.
• Raise large_client_header_buffers (Nginx) if headers are legitimately large.

431 and SEO

Crawlers send few cookies, so they rarely see 431.

Example response

HTTP/1.1 431 Request Header Fields Too Large

How to return a 431 response

Node.js (Express)

res.status(431).json({ error: 'Request Header Fields Too Large' });

PHP

http_response_code(431);

Python (Flask)

return {'error': 'Request Header Fields Too Large'}, 431

Go (net/http)

w.WriteHeader(431)

nginx

return 431;

Related status codes

• 400 Bad Request — The server cannot process the request because it is malformed.
• 413 Content Too Large — The request body is larger than the server allows.
• 414 URI Too Long — The URL is longer than the server is willing to process.

Check which status code a URL returns with the HTTP header checker.