409
4xx Client ErrorConflict
The request conflicts with the current state of the resource.
409 Conflict means the request can't be applied because it clashes with the resource's current state. Typical cases are an edit based on a stale version (optimistic-locking failure) or trying to create something that already exists. The client usually needs to re-fetch the latest state and try again.
What 409 looks like on the wire
PUT /api/documents/33 HTTP/1.1
Host: api.example.com
Content-Type: application/json
{"version": 41, "title": "Q3 plan"}
HTTP/1.1 409 Conflict
Content-Type: application/json
{"error": "Document is already at version 42. Reload and retry."}About 4xx Client Error
The request has a problem the client needs to fix, like a bad URL, missing auth, or invalid input.
How to fix 409 Conflict
- Re-fetch the current resource and reapply your change.
- For version conflicts, send the latest ETag or version number.
- On create conflicts, check whether the resource already exists.
Where it's defined
409 Conflict is defined in RFC 9110 §15.5.10. MDN's 409 reference covers browser behavior and examples.