subscription_unmanaged
PUT /v1/subscription/extra-sessions or PUT /v1/subscription/receptionist-connections on a subscription that carries no Stripe reference (a dev-seeded or externally provisioned row), so there is no subscription item to change. Same family as stripe_subscription_missing: the API refuses rather than guessing which Stripe subscription to mutate.
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_unmanaged",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}