All HTTP status codes
4xx Client Error
RFC 6585
431 Request Header Fields Too Large
The server is unwilling to process the request because its header fields are too large.
What it means
HTTP 431 indicates the server refuses to process the request because the headers are too large. May be the entire header set or a single field. Common with oversized cookies or auth tokens.
When it's used
Oversized cookies, very long JWT tokens in Authorization header, accumulated tracking cookies.
Common causes
- Too many cookies set on the domain
- Single cookie exceeds 4KB
- JWT token grew too large with claims
- Accidentally duplicated headers
How to fix it
- Audit cookies on the domain — remove unused ones
- Move large state from cookies to server-side sessions
- Increase server header buffer size if legitimate
- Use shorter JWT claims or move data server-side
Example response
HTTP/1.1 431 Request Header Fields Too Large Connection: close
SEO impact
Rare for crawlers. If hit, Googlebot may skip the page.