fix(router): annotate fallback as Final[ModelMapInfo] and add unit coverage (#47922)

litellm/router.py

@@ -12207,7 +12207,7 @@ class Router:
                     model_info_name,
                     e,
                 )
-                fallback: ModelMapInfo = {
+                fallback: Final[ModelMapInfo] = {
                     "key": model_info_name,
                     "max_tokens": None,
                     "max_input_tokens": None,

tests/unit/test_router_model_not_mapped_fallback.py

@@ -0,0 +1,48 @@
+from typing import Final
+from unittest.mock import patch
+
+import pytest
+
+import litellm
+from litellm.exceptions import ModelNotMappedError
+from litellm.router import Router
+
+
+def test_get_router_model_info_returns_fallback_on_model_not_mapped_error() -> None:
+    router: Final = Router(
+        model_list=[
+            {
+                "model_name": "mcp-stdio-group",
+                "litellm_params": {
+                    "model": "openai/unmapped-mcp-stdio-model",
+                    "api_key": "test-key",
+                },
+            }
+        ]
+    )
+
+    with patch.object(
+        litellm,
+        "get_model_info",
+        side_effect=ModelNotMappedError(message="Model 'unmapped-mcp-stdio-model' is not mapped", model="unmapped-mcp-stdio-model"),
+    ):
+        info: Final = router.get_router_model_info(model="mcp-stdio-group")
+
+    assert info is not None
+    assert info["max_tokens"] is None
+    assert info["max_input_tokens"] is None
+    assert info["max_output_tokens"] is None
+    assert info["input_cost_per_token"] == 0
+    assert info["output_cost_per_token"] == 0
+    assert info["litellm_provider"] == "openai"
+    assert info["mode"] == "chat"
+
+
+def test_get_router_model_info_reraises_unrelated_exceptions() -> None:
+    router: Final = Router(model_list=[])
+
+    with (
+        patch.object(litellm, "get_model_info", side_effect=RuntimeError("unexpected failure")),
+        pytest.raises(RuntimeError, match="unexpected failure"),
+    ):
+        router.get_router_model_info(model="any-model")