sandbox_session_in_progress
POST /v1/agents/{id}/voice-session: this account already holds its allowed number of open in-browser sessions. Retry-After carries the session ceiling in seconds, which is when the one in progress frees at the latest. A concurrency fact about the caller, so 409 - nothing is broken and no quota is spent.
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/sandbox_session_in_progress",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}