Request Header Fields Too Large — headers exceed the server buffer limit
What 431 Means
The 431 error on the HTTP Status-Codes indicates request header fields too large — headers exceed the server buffer limit. This typically occurs due to accumulated cookie data exceeding server limits.
A 431 Request Header Fields Too Large response occurs when the total size of the HTTP headers (or a single header field) exceeds the server's configured buffer limit. This is often caused by an explosion of tracking cookies or excessive forwarding headers.
Technical Background
Servers allocate fixed memory buffers to parse incoming headers. If these buffers overflow, the server returns a 431 to protect its resources from potential exhaustion or memory corruption attacks.
This error is structurally similar to the Nginx-specific 494 error. It acts as a gatekeeper at the very beginning of request processing, often before any application-level routing or logic takes place.
The 431 status can specify which header was too large in the response body, helping developers identify whether the issue is a specific token, a massive cookie set, or a general header bloat.
Common Causes
- Accumulated cookie data exceeding server limits
- Excessive metadata added by proxy servers or security appliances
- Malformed or unusually long custom headers in the request stream
Typical Scenarios
- A user visits a site so frequently that the accumulated cookies for the domain outgrow the web server's buffer size
- A complex enterprise proxy adds multiple layers of tracing headers to a request before it reaches the origin
What to Know
Clear your browser cookies for the specific domain or audit your request headers to ensure they aren't carrying redundant or excessively large metadata. Server administrators may need to increase header buffer sizes if the data is legitimate.
Frequently Asked Questions
Common questions about HTTP 431 error
It means the hidden data sent by your browser (headers and cookies) is too big for the server to read. Usually, clearing cookies for that site fixes it.