Align output serialization with return types

tests/tools/tool_transform/test_tool_transform.py

@@ -1,6 +1,7 @@
 """Core tool transform functionality."""
 
 import re
+from dataclasses import dataclass
 from typing import Annotated, Any
 
 import pytest
@@ -722,6 +723,27 @@ async def test_transform_fn_wrapped_result_respects_serialize_by_alias():
     assert result.structured_content == {"result": {"id": "42"}}
 
 
+async def test_transform_fn_configured_dataclass_respects_serialize_by_alias():
+    """A transform uses its return annotation for nested dataclass serialization."""
+    from pydantic import ConfigDict, with_config
+
+    @with_config(ConfigDict(serialize_by_alias=True))
+    @dataclass
+    class Item:
+        id: Annotated[str, Field(serialization_alias="itemId")]
+
+    def base() -> None:
+        pass
+
+    async def transform() -> list[Item]:
+        return [Item(id="42")]
+
+    transformed = Tool.from_tool(base, transform_fn=transform)
+    result = await transformed.run({})
+
+    assert result.structured_content == {"result": [{"itemId": "42"}]}
+
+
 class TestProxy:
     @pytest.fixture
     def mcp_server(self) -> FastMCP: