agent_chat_unavailable
POST /v1/agents/{id}/chat: no configured LLM provider can serve a chat turn - the provider catalog (VOCAPABLE_LLM_PROVIDERS_JSON) is absent, or it resolves none of the draft's route and carries no reachable default. A deployment fault, not the caller's.
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/agent_chat_unavailable",
"title": "…",
"status": 503,
"detail": "…",
"instance": "/v1/…"
}