HTTP 409 Conflict
The request conflicts with the current state of the resource — e.g. a duplicate or an edit based on an outdated version.
409 Conflict Meaning
APIs return 409 for duplicates (“username already taken”), optimistic-locking failures (someone else changed the record first) and invalid state transitions (cancelling an already-shipped order).
Common causes
- Creating a resource that already exists (unique constraint)
- Version/ETag mismatch — the resource changed since you read it
- Invalid state transition
- Concurrent writes to the same resource
⚡ Quick fix
- Read the response body for the conflicting field or state
- Re-fetch the latest version, re-apply the change and retry
- Use idempotency keys for create operations that may be retried
Detailed fix by platform
REST API
- Send
If-Match: <etag>on updates and handle 409/412 by refreshing and merging.
Git
- Git hosting APIs return 409 for e.g. creating a branch that exists or merging with conflicts.
How to diagnose
- State — What is the current server state of the resource?
- Concurrency — Did another client modify it?
- Uniqueness — Does an equivalent resource already exist?
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Last updated 15 Oct 2026