What it means
The most common cause is large or numerous cookies for the domain, and sometimes a long Referer or token in a header. Some servers return a 400 with “Request Header Or Cookie Too Large” instead of a 431.
Most common causes
- Accumulated cookies (tracking, A/B tests, sessions) for the domain.
- An oversized JWT in the Authorization header.
- Local development (localhost), where cookies from different projects pile up.
- A header size limit set too low on the server.
What you can do as a visitor
- Clear cookies for that domain only.
- Try a private window.
What the site owner or developer should do
- Reduce the number and size of cookies; don't store data in cookies.
- Store large tokens server-side and put only a session ID in the cookie.
- If needed, raise the header limit (Node.js --max-http-header-size, Nginx large_client_header_buffers).
Impact on SEO and Google
Googlebot doesn't send cookies, so a 431 rarely affects indexing.
Frequently asked questions
Why do I get a 431 on localhost?
Cookies from all your local projects are sent to the same localhost. Clear them.
Is 431 the same as a 400 “Cookie Too Large”?
Yes, it's the same problem; some servers use 400 instead.