431 Request Header Fields Too Large

431 Request Header Fields Too Large (RFC 6585) indicates that the server is unwilling to process the request because either an individual header field or all header fields collectively are too large. This commonly occurs when cookies have grown too large due to excessive data storage, or when JWT tokens become very long.

• Cookie header too large due to accumulated session/tracking cookies
• Very long JWT or session token in Authorization header
• Custom headers with large data payloads
• Many large cookies set by analytics or advertising scripts

• Clear unnecessary cookies and reduce cookie size
• Move large payloads from headers to request body
• Use short-lived, opaque session tokens instead of large JWTs
• Audit third-party scripts setting large cookies
• Increase server's LimitRequestFieldSize (Apache) or large_client_header_buffers (nginx) if appropriate