fix(mcpserver): preserve Annotated/Field metadata for dict[str, T] return types

When a tool returns `Annotated[dict[str, T], Field(description="...")]`, the
`_try_create_model_and_schema` dict branch was passing the unwrapped `type_expr`
(i.e. `dict[str, T]`) to `_create_dict_model` instead of `original_annotation`,
so any `Field` description or other Pydantic metadata was dropped from the output
schema. Fix by passing `original_annotation` so the `RootModel` picks it up.

Fixes #2935

Author: anneheartrecord

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

@@ -349,9 +349,9 @@ def _try_create_model_and_schema(
         if origin is dict:
             args = get_args(type_expr)
             if len(args) == 2 and args[0] is str:
-                # TODO: should we use the original annotation? We are losing any potential `Annotated`
-                # metadata for Pydantic here:
-                model = _create_dict_model(func_name, type_expr)
+                # Use the original annotation (which may be `Annotated[dict[str, T], Field(...)]`)
+                # so that any Annotated metadata (e.g. Field description) is preserved in the schema.
+                model = _create_dict_model(func_name, original_annotation)
             else:
                 # dict with non-str keys needs wrapping
                 model = _create_wrapped_model(func_name, original_annotation)

tests/server/mcpserver/test_func_metadata.py

@@ -245,6 +245,31 @@ def func_dict_int_key() -> dict[int, str]:  # pragma: no cover
     assert "result" in meta.output_schema["properties"]
 
 
+def test_structured_output_dict_str_preserves_annotated_metadata():
+    """dict[str, T] return types should preserve Annotated/Field metadata in output schema.
+
+    Regression test for https://github.com/modelcontextprotocol/python-sdk/issues/2935
+    """
+
+    def get_config() -> Annotated[dict[str, int], Field(description="Configuration values")]:  # pragma: no cover
+        return {"timeout": 30}
+
+    meta = func_metadata(get_config)
+    assert meta.output_schema is not None
+    assert meta.output_schema.get("description") == "Configuration values", (
+        f"Expected 'description' in schema, got: {meta.output_schema}"
+    )
+
+    # Additional metadata (title, max_length, etc.) should also be preserved
+    def get_headers() -> Annotated[dict[str, str], Field(description="HTTP headers", title="Headers")]:  # pragma: no cover
+        return {"Content-Type": "application/json"}
+
+    meta2 = func_metadata(get_headers)
+    assert meta2.output_schema is not None
+    assert meta2.output_schema.get("description") == "HTTP headers"
+    assert meta2.output_schema.get("title") == "Headers"
+
+
 @pytest.mark.anyio
 async def test_lambda_function():
     """Test lambda function schema and validation"""