subscription_inactive
The subscription is not in a state that permits the request: on campaign launch, the mirrored Stripe subscription is unpaid or canceled, so new launches refuse immediately (past_due is different - it follows the documented seven-day dunning grace before payment_past_due blocks launches; existing conversations are never terminated by this gate); on PUT /v1/subscription/extra-sessions and PUT /v1/subscription/receptionist-connections, any status outside active/trialing refuses the add-on change until billing is resolved. The detail names the status.
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/subscription_inactive",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}