fix: default missing fallback state in switch_model

run_agent.py

@@ -2049,12 +2049,14 @@ def switch_model(self, new_model, new_provider, api_key='', base_url='', api_mod
         # ("switched to anthropic, tui keeps trying openrouter").
         old_norm = (old_provider or "").strip().lower()
         new_norm = (new_provider or "").strip().lower()
+        fallback_chain = list(getattr(self, "_fallback_chain", []) or [])
         if old_norm and new_norm and old_norm != new_norm:
-            self._fallback_chain = [
-                entry for entry in self._fallback_chain
+            fallback_chain = [
+                entry for entry in fallback_chain
                 if (entry.get("provider") or "").strip().lower() not in {old_norm, new_norm}
             ]
-            self._fallback_model = self._fallback_chain[0] if self._fallback_chain else None
+        self._fallback_chain = fallback_chain
+        self._fallback_model = fallback_chain[0] if fallback_chain else None
 
         logging.info(
             "Model switched in-place: %s (%s) -> %s (%s)",

tests/run_agent/test_switch_model_fallback_prune.py

@@ -78,6 +78,17 @@ def test_switch_with_empty_chain_stays_empty():
     assert agent._fallback_model is None
 
 
+def test_switch_initializes_missing_fallback_attrs():
+    agent = _make_agent([])
+    del agent._fallback_chain
+    del agent._fallback_model
+
+    _switch_to_anthropic(agent)
+
+    assert agent._fallback_chain == []
+    assert agent._fallback_model is None
+
+
 def test_switch_within_same_provider_preserves_chain():
     chain = [{"provider": "openrouter", "model": "x-ai/grok-4"}]
     agent = _make_agent(chain)