idempotency_conflict

HTTP 409 · type conflict. Within the 24-hour idempotency window (scoped to the API key), this Idempotency-Key was already used for a request with a different body. The original request stands; this one did nothing and charged nothing. For contrast: same key + same body is a replay — it returns the original response with fn-idempotent-replay: true and never bills twice. The dedupe lives in the ledger, not just the HTTP layer: a replayed spending POST can never create a second reservation.

The shape

{
  "error": {
    "type": "conflict",
    "code": "idempotency_conflict",
    "message": "Idempotency-Key was used 4 minutes ago with a different request body.",
    "doc_url": "https://docs.fiveninelabs.com/errors/idempotency_conflict",
    "request_id": "req_4hh27p",
    "original_request_id": "req_9tt41w"
  }
}

original_request_id identifies the request that claimed the key — findable in the dashboard’s API log.

How to fix

If this is a genuinely new request, give it a fresh Idempotency-Key (UUIDs recommended).
The classic cause is confirming a proposal with the original turn’s key. The confirm re-POSTs the turn with "confirm": true — a different body — so it needs a fresh key. This error firing there is the guardrail working: you got a 409, not a silent spend. See Budgets and proposals.
If you build keys deterministically (e.g. from an order id), make every distinct body produce a distinct key — hash the body in.

Reproduce it

Two calls, same key, different body (works on an fn_test_ key, $0):

curl https://api.fiveninelabs.com/v1/sessions \
  -H "Authorization: Bearer $FIVENINE_TEST_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: demo-conflict-1" \
  -d '{"query": "vinyl records", "max_spend_usd": 1}'

curl https://api.fiveninelabs.com/v1/sessions \
  -H "Authorization: Bearer $FIVENINE_TEST_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: demo-conflict-1" \
  -d '{"query": "espresso machines", "max_spend_usd": 1}'

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idempotency_conflict

The shape

How to fix

Reproduce it

​The shape

​How to fix

​Reproduce it

The shape

How to fix

Reproduce it