Synctera has two distinct idempotency behaviors (docs: "Need to Know" -> Idempotency):

• :standard — the key is optional. A repeated request with the same key returns the original cached response (incl. status code) for 7 days. Safe to auto-generate and auto-retry.

• :ledger — applies to money-movement endpoints (ACH, wires, internal transfers, and anything else that posts to the ledger). The key is REQUIRED — Synctera returns 400 if it's missing. A repeat with the same key is NEVER served from cache: it's rejected with 409 (IDEMPOTENCY_CONCURRENT_USE) or 422 (IDEMPOTENCY_INVALID_REUSE), and keys are retained indefinitely per resource. Because a silent retry here fails loudly by design instead of replaying safely, this client does not auto-generate keys for ledger calls unless the caller opts in via auto_generate_ledger_idempotency_keys: true on the client.