What does HTTP status code 409 mean?
The 409 Conflict status code indicates that the request cannot be completed because it conflicts with the current state of a resource (e.g. database constraints).
When to use it in APIs
Use 409 for optimistic concurrency conflicts (stale version tags), duplicate unique keys (like email address collisions), or idempotency key mismatches.
How it compares to other codes
- 422 Unprocessable Entity: Used for general field-level business validation. 409 is reserved for state-driven conflicts.
