Without an explicit `provider`, `setup.runtime_check` called the strict
`resolve_runtime_provider(requested=None)` while `_make_agent` goes through
`_resolve_agent_model_runtime` (startup model + provider pin, then the
configured fallback chain). With the primary blocked and a working fallback
entry the probe answered ok:False (the primary's auth error) and the Desktop
showed onboarding for a backend whose sessions built fine. The probe also
reported `model: null` because the resolver never populates that key.
The default case now runs the session builder's resolver; an explicit
`provider` stays a strict single-provider check (onboarding verifies the
provider just connected — another provider's fallback must not mask a failed
connection) and resolves against the startup model like the session would.
Both branches report the selected model.
Fixes#111775
KoNit-K
6f975b768e