4xx Client error
409 Conflict
The request conflicts with the current state of the resource.
Typical cases: creating something that already exists (duplicate username) or an edit conflict when two clients update the same resource.
Common causes and fixes
- Return a clear message saying what conflicts, so the client can fix it or ask the user.
- For concurrent edits, use version numbers or ETags with
If-Match(which returns 412 on mismatch).
Example response
HTTP/1.1 409 Conflict
Date: Thu, 22 Oct 2026 08:00:00 GMT Check a URL’s status with curl -I https://example.com, which prints the status line and headers.