All HTTP status codes

HTTP 409 Conflict

Client Error

The request conflicts with the resource's current state — classic cases are duplicate creation and stale optimistic-locking versions.

Defined in RFC 9110

Common causes

  • Creating a resource that already exists (unique constraint)
  • Updating with an outdated version/ETag (optimistic concurrency)
  • Concurrent edits to the same resource

How to fix it

  1. Fetch the latest state, merge, and retry

  2. Send If-Match with the current ETag for updates

  3. Make creation idempotent (upsert or idempotency keys)

Related status codes

412 Precondition Failed

A conditional header (If-Match, If-Unmodified-Since…) evaluated to false — the resource changed since the client last saw it.

201 Created

The request succeeded and a new resource was created. The Location header usually points at the new resource — the canonical response for a successful POST in REST APIs.

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