With a main provider selected, an unusable main route (expired xAI/Codex
OAuth token, 401/402/429 mid-session) fell through the built-in discovery
chain (OpenRouter -> Nous -> custom -> api-key) and quietly ran every
compression, title and memory-flush call on whichever OTHER account was
still logged in. Reported as "using Grok on my Premium+ sub, my Nous
Portal balance kept draining" — the chat visibly stayed on Grok while the
side tasks were billed elsewhere, and re-logging into X did not help
because the aux side never consulted the selected provider.
The discovery chain is now reserved for installs with no selected main
provider (`model.provider: auto` / unset). Otherwise the ladder is
main -> auxiliary.<task>.fallback_chain -> fallback_providers -> refuse
with a warning naming the dead provider and the fix. Both entry points
gate on the same predicate: the resolve-time route and the mid-request
payment/auth hop (_try_payment_fallback).
Existing chain tests that asserted the hop now pin `provider=auto`, the
one case where discovery is still the contract.
7e0b5cd235