sandbox_capacity_unavailable
No sandbox capacity right now. In-browser sessions run on a fixed pool deliberately smaller than the platform's live-call capacity, so a test never takes the capacity a live call needs. 503 and not 429: nothing the caller did causes it and nothing they change fixes it. Retry-After is the session ceiling - an upper bound on when a slot could free, never a promise that one will.
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_capacity_unavailable",
"title": "…",
"status": 503,
"detail": "…",
"instance": "/v1/…"
}