In long-lived interactive sessions, _try_activate_fallback() advances
_fallback_index before attempting client resolution. When resolution
fails (provider not configured, etc.) the function returns False without
ever setting _fallback_activated=True. _restore_primary_runtime() then
skips its reset block entirely (guarded by `if not _fallback_activated`),
leaving _fallback_index >= len(_fallback_chain) for all subsequent turns.
The eager-fallback guard at the top of the retry loop checks
`_fallback_index < len(_fallback_chain)`, so the condition fails silently
and no fallback is ever attempted again for that session.
Cron jobs spawn a fresh AIAgent per run and never hit this path, which is
why the same fallback chain works reliably for cron but not interactive.
Fix: reset _fallback_index=0 in the `not _fallback_activated` early-return
branch so every new turn starts with the full chain available.
Fixes#20465
33528b428d