fix: skip structured output for list[Any]

Author: mplemay

src/mcp/server/mcpserver/utilities/func_metadata.py

@@ -349,6 +349,9 @@ def _try_create_model_and_schema(
     elif isinstance(type_expr, GenericAlias):
         origin = get_origin(type_expr)
 
+        if origin in (list, tuple, set, frozenset, Sequence) and _annotation_contains_any(type_expr):
+            return None, None, False
+
         # Special case: dict with string keys can use RootModel
         if origin is dict:
             args = get_args(type_expr)
@@ -474,6 +477,18 @@ def _create_wrapped_model(func_name: str, annotation: Any) -> type[BaseModel]:
     return create_model(model_name, result=annotation)
 
 
+def _annotation_contains_any(annotation: Any) -> bool:
+    """Return True if a type annotation contains `Any` anywhere within it."""
+    if annotation is Any:
+        return True
+
+    origin = get_origin(annotation)
+    if origin is None:
+        return False
+
+    return any(_annotation_contains_any(arg) for arg in get_args(annotation) if arg is not Ellipsis)
+
+
 def _create_dict_model(func_name: str, dict_annotation: Any) -> type[BaseModel]:
     """Create a RootModel for dict[str, T] types."""
     # TODO(Marcelo): We should not rely on RootModel for this.

tests/server/mcpserver/test_func_metadata.py

@@ -674,6 +674,9 @@ def func_list_str() -> list[str]:  # pragma: no cover
     def func_dict_str_int() -> dict[str, int]:  # pragma: no cover
         return {"a": 1, "b": 2}
 
+    def func_list_any() -> list[Any]:  # pragma: no cover
+        return ["a", "b", "c"]
+
     def func_union() -> str | int:  # pragma: no cover
         return "hello"
 
@@ -689,6 +692,10 @@ def func_optional() -> str | None:  # pragma: no cover
         "title": "func_list_strOutput",
     }
 
+    # Test list[Any] - should stay unstructured because it can contain arbitrary non-serializable values
+    meta = func_metadata(func_list_any)
+    assert meta.output_schema is None
+
     # Test dict[str, int] - should NOT be wrapped
     meta = func_metadata(func_dict_str_int)
     assert meta.output_schema == {