caller_request_invalid_transition
The requested move is not one the state machine allows: acknowledge applies to open requests only, and resolved is terminal for every edit, assignment included (no reopen in v1 - ADR-0037 records the deferral).
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/caller_request_invalid_transition",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}