idempotency_key_reused
This code is raised on a non-customer surface (the /internal voice-runtime audience or telephony provider callbacks). It is excluded from the customer OpenAPI document and is registered here so an operator reading a log line has one place to look.
The same idempotency key was presented with different arguments. A caller bug, not a retry, so it is refused rather than silently answered. Distinct from /v1's idempotency_key_reuse (§9.2).
Problem shape
Every non-2xx response is an RFC 9457 application/problem+json body. The type URI below is stable and machine-matchable, so branch on the final path segment (the code) and treat the status as redundant confirmation. See the error-handling guide.
{
"type": "https://docs.vocapable.com/errors/idempotency_key_reused",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}