amd_message_end_unsupported
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 asynchronous AMD callback carries a message-end verdict. Since ADR-0023 the platform does request DetectMessageEnd - but only on the voicemail-drop lane, which registers no AsyncAmdStatusCallback at all, so nothing that lane dials can post here. One arriving is evidence that a DetectMessageEnd detection resolved asynchronously, and the route logs telephony.drop_lane_premise before refusing. It stays a refusal - where the synchronous answer webhook deliberately has no verdict refusal at all - because nothing is waiting on this response: TwiML ran long ago on the async lane, so the 422 reaches a carrier error log and no caller's ear, while folding the verdict into machine_start would classify the channel and leave the drop silently never attempted. The answer webhook is holding a live channel, where a 422 is an application-error announcement read to a real person, so it refuses nothing and answers every input with a document - but "no refusal" is not "one behaviour": an AnsweredBy that never arrived degrades to unknown and the ordinary media stream (no detection ran, so a person may be on the line), while a reported AnsweredBy=unknown and a value this platform does not model are answered with <Hangup/> and land the attempt in machine for retry, since detection did run and found no human. There is no answer_verdict_unrecognized: the row that registered one was retired with the refusal it named.
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/amd_message_end_unsupported",
"title": "…",
"status": 422,
"detail": "…",
"instance": "/v1/…"
}