431 Request Header Fields Too Large

What it means

One header or the header block as a whole is too big. In browsers the culprit is almost always an accumulation of cookies; in APIs, oversized JWTs or debug headers.

Common causes

• Cookie accumulation on the domain (browsers)
• Oversized JWT in the Authorization header
• Debug headers stuffed with payloads

How to debug it

Clear cookies for the domain / shrink the JWT (drop custom claims) — and check proxy header-size limits if you control the server.

Server-side note: Shrink the token (drop custom claims) or raise proxy header limits (nginx large_client_header_buffers).

The fastest way to pin down a 431 is to reproduce the exact request and inspect what actually went over the wire — status, headers, timing and body, without your app code in the way. That is what an API client is for; ReqPad does it from your phone, with every request saved to history.

Related codes

400 Bad Request · 401 Unauthorized · 402 Payment Required · 403 Forbidden · 404 Not Found · 405 Method Not Allowed — or the full reference.