caller_request_assignee_not_found
The named assignee_user_id is not a user of this tenant. The users.id FK is global, so the tenant-scoped read is the control; pick an assignee from GET /v1/users.
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_assignee_not_found",
"title": "…",
"status": 404,
"detail": "…",
"instance": "/v1/…"
}