← Back to status codes
HTTP Status Code
431 Request Header Fields Too Large
Client ErrorThe server refused the request because one or more request headers are too large.
HTTP status code reference, response example, common causes, fixes, and related status codes.
What does HTTP 431 Request Header Fields Too Large mean?
HTTP 431 Request Header Fields Too Large is a status code sent by a server to indicate the result of an HTTP request.
Status codes help browsers, APIs, apps, and backend systems understand whether a request succeeded, failed, was redirected, or needs additional action.
In practice, HTTP 431 Request Header Fields Too Large usually appears when a server responds under specific request, validation, permission, or infrastructure conditions.
Response example
HTTP/1.1 431 Request Header Fields Too Large
HTTP example
HTTP/1.1 431 Request Header Fields Too Large
Common causes
- Large cookies
- Oversized authorization header
- Too many custom headers
How to fix it
- Reduce header size
- Clear large cookies
- Avoid unnecessary custom headers
Common mistakes
- Assuming the status code alone explains the full backend issue
- Ignoring related response headers that add important context
- Treating temporary errors as permanent failures
- Retrying too aggressively without checking the cause
- Debugging the frontend only when the problem is server-side
How browsers and APIs use it
Browsers, APIs, and backend services use HTTP status codes to understand the outcome of a request. Depending on the status code, an application may render content, retry a request, redirect the user, show an error, or trigger a different flow in the client or server.
Developer note
HTTP 431 is often caused by cookie bloat in browser-based apps. If you see it, inspect request headers in devtools.