fix(router): narrow the optional map before subscripting; cover the warning path

litellm/router.py

@@ -8708,9 +8708,12 @@ class Router:
 
     def _finish_reason_failure_error(self, model: str, reason: str) -> Exception:
         """Build the exception instance configured for a mapped finish reason."""
-        exception_name: Final = self.treat_finish_reason_as_failure[reason]
-        exception_cls: Final = getattr(litellm, exception_name)
         message: Final = f"Response finished with reason '{reason}' (treat_finish_reason_as_failure)."
+        finish_reason_map: Final = self.treat_finish_reason_as_failure
+        if finish_reason_map is None:
+            return litellm.APIError(status_code=500, message=message, llm_provider="", model=model)
+        exception_name: Final = finish_reason_map[reason]
+        exception_cls: Final = getattr(litellm, exception_name)
         if exception_name == "APIError":
             return exception_cls(status_code=500, message=message, llm_provider="", model=model)
         return exception_cls(message=message, llm_provider="", model=model)

tests/router_unit_tests/test_router_finish_reason_failure.py

@@ -175,6 +175,12 @@ def test_unknown_exception_name_raises_at_construction():
         Router(model_list=[], treat_finish_reason_as_failure={"x": "NotAnException"})
 
 
+def test_healthy_terminal_key_warns_at_construction(capsys: pytest.CaptureFixture):
+    Router(model_list=[], treat_finish_reason_as_failure={"stop": "RateLimitError"})
+    logged = capsys.readouterr().err + capsys.readouterr().out
+    assert "healthy terminal reasons" in logged
+
+
 @pytest.mark.asyncio
 async def test_mapped_finish_reason_helpers_direct(monkeypatch: MonkeyPatch):
     """Direct coverage of the knob helpers (the router code-coverage check matches by name)."""