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Problem types

idempotency_key_reuse

HTTP 409

The same Idempotency-Key was reused for a DIFFERENT request body. Use a fresh UUID per logical request; reusing one for an identical retry is the supported case and returns the original result.

Response shape

{
  "type": "https://cogdepot.com/problems/idempotency_key_reuse",
  "title": "<short human-readable summary>",
  "status": 409,
  "detail": "<what happened on this occurrence>",
  "reason": "idempotency_key_reuse"
}

Branch on reason. The title is prose and may be reworded; reason is the stable contract.