handoff_state_invalid
Not possible at this stage of the handoff (responding to one that is not awaiting you, reporting progress before acceptance, completing a blocked or completed delivery, reopening one that is not complete). status says where it is.
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/handoff_state_invalid",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}