4xx Client Error
409 Conflict
The request conflicts with the current state of the resource.
What causes it
- A duplicate create — the unique key already exists.
- An optimistic-concurrency failure: the record changed since you read it.
- A git-style push rejected because the ref moved.
How to fix it
- Include enough detail in the body for the client to resolve it — which field conflicted, and what the current value is.
- For concurrency, pair it with ETags and `If-Match` so the client can re-read and retry.
At a glance
- Class
- 4xx Client Error
- Reference
- RFC 9110 §15.5.10
- Standardised
- Yes
- Retry the request?
- No — fix the request
Other 4xx client error codes
- 400 Bad Request
- 401 Unauthorized
- 402 Payment Required
- 403 Forbidden
- 404 Not Found
- 405 Method Not Allowed
- 406 Not Acceptable
- 407 Proxy Authentication Required
- 408 Request Timeout
- 410 Gone
- 411 Length Required
- 412 Precondition Failed
- 413 Content Too Large
- 414 URI Too Long
- 415 Unsupported Media Type
- 416 Range Not Satisfiable
- 417 Expectation Failed
- 418 I'm a teapot
- 421 Misdirected Request
- 422 Unprocessable Content
- 423 Locked
- 424 Failed Dependency
- 425 Too Early
- 426 Upgrade Required
- 428 Precondition Required
- 429 Too Many Requests
- 431 Request Header Fields Too Large
- 451 Unavailable For Legal Reasons
- 499 Client Closed Request