409 4xx RFC 9110 §15.5.10

Conflict

Request conflicts with current state of the resource. Common with concurrent updates and unique constraints.

What HTTP 409 means

RFC 9110 §15.5.10. Used for write conflicts (concurrent updates with stale ETags), unique constraint violations, or state machine errors. APIs sometimes return 409 instead of 422 for business rule violations.

Typical causes

Optimistic concurrency conflict (stale ETag)
Unique constraint violation
State machine violation
Duplicate creation