HTTP 409 Conflict

What HTTP 409 Conflict means

409 Conflict means the request couldn't be completed because it conflicts with the current state of the resource — a classic example is a version/edit conflict or trying to create something that already exists.

Which category is it?

409 is a 4xx code — Client error. 4xx means the request has a problem the client must fix — bad syntax, missing authentication, or a resource that isn't there. It's the caller's responsibility.

Common causes

• Editing a resource that changed since you fetched it
• Creating a resource with a duplicate unique key
• Concurrent updates to the same record

How to fix it

• Re-fetch the latest state and reapply the change
• Use optimistic concurrency (ETags/versions)
• Handle duplicates before creating

Example 409 response

PUT /api/docs/12 HTTP/1.1
If-Match: "v3"

HTTP/1.1 409 Conflict
Content-Type: application/json

{"error":"document was modified, current version is v4"}

How to diagnose a 409

1. Fetch the resource again to see its current state and version.
2. Compare it with your change to see what conflicts: a newer version, a duplicate key, or a state that does not allow the action.
3. Retry with the up-to-date version, or merge the changes and resubmit.

409 compared with similar codes

• 412: 412 Precondition Failed is returned when an If-Match or similar header condition fails. 409 is a broader conflict with the current state.
• 422: 422 means the data is invalid. 409 means valid data collides with existing state.

409 Conflict: common questions

What causes a 409 Conflict on create?

Usually a unique constraint: the resource you are creating already exists, for example an email address or username already in use.

Other client error (4xx) codes