switch_model() unconditionally set agent.provider but only set
agent.base_url when the resolved value was truthy. When a real
provider change resolved an empty base_url (e.g. minimax after
copilot), the agent ended up with provider="minimax" but
base_url still pointing at api.githubcopilot.com. That incoherent
pair then got snapshotted into agent._primary_runtime, so it kept
re-applying on every subsequent turn via restore_primary_runtime()
until the process restarted.
try_activate_fallback() and _swap_credential() were audited and
confirmed unaffected: both always derive base_url from an actually
constructed client, never from a possibly-empty resolver hint.
Fix: when base_url is empty AND the provider is genuinely changing,
raise ValueError instead of silently keeping the old provider's URL.
This routes through switch_model()'s existing snapshot/rollback
path, and callers (tui_gateway/server.py's _apply_model_switch)
already catch and surface a clean "switch failed, staying on X"
message. Re-selecting the SAME provider with an empty base_url
(credential-only refresh) still keeps the current URL, unchanged.
Fixes#47828
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
joaomarcos