431 Request Header Fields Too Large

What it means

HTTP 431 Request Header Fields Too Large means the combined size of the request headers, or a single header field, exceeds what the server allows. It can point at one oversized field or at the headers as a whole, and a frequent culprit is a cookie that has grown too big.

When it happens

It happens when cookies pile up until they overflow the header limit, when too many or overly long custom headers are sent, or when an authentication token in a header is unusually large.

How to fix it

• Clear oversized or stale cookies for the site, which is the most common fix.
• Reduce the number and size of custom headers the client sends.
• Raise the server's header-size limit (such as large_client_header_buffers in Nginx) if the large headers are legitimate.
• Move large data out of headers and into the request body where it belongs.

SEO impact

Negative if it hits real pages. Bloated cookies or headers that trigger 431 can block crawlers, so keep header size under control on pages you want indexed.