fix: preserve optional string tool arguments

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

@@ -3,7 +3,7 @@
 import json
 from collections.abc import Awaitable, Callable, Sequence
 from itertools import chain
-from types import GenericAlias
+from types import GenericAlias, NoneType
 from typing import Annotated, Any, cast, get_args, get_origin, get_type_hints
 
 import anyio
@@ -148,7 +148,7 @@ def pre_parse_json(self, data: dict[str, Any]) -> dict[str, Any]:
                 continue
 
             field_info = key_to_field_info[data_key]
-            if isinstance(data_value, str) and field_info.annotation is not str:
+            if isinstance(data_value, str) and _should_pre_parse_json(field_info.annotation):
                 try:
                     pre_parsed = json.loads(data_value)
                 except json.JSONDecodeError:
@@ -167,6 +167,22 @@ def pre_parse_json(self, data: dict[str, Any]) -> dict[str, Any]:
     )
 
 
+def _is_simple_scalar_annotation(annotation: Any) -> bool:
+    return annotation in {str, int, float, bool, NoneType}
+
+
+def _should_pre_parse_json(annotation: Any) -> bool:
+    """Return whether string input for this annotation should be JSON-decoded."""
+    if annotation is str:
+        return False
+
+    origin = get_origin(annotation)
+    if is_union_origin(origin):
+        return not all(_is_simple_scalar_annotation(arg) for arg in get_args(annotation))
+
+    return True
+
+
 def func_metadata(
     func: Callable[..., Any],
     skip_names: Sequence[str] = (),

tests/server/mcpserver/test_func_metadata.py

@@ -551,6 +551,41 @@ def handle_json_payload(payload: str, strict_mode: bool = False) -> str:
     assert result == f"Handled payload of length {len(json_array_payload)}"
 
 
+@pytest.mark.anyio
+async def test_optional_str_annotation_preserves_json_string():
+    def update_task(task_id: str | None = None) -> str:
+        assert isinstance(task_id, str)
+        return task_id
+
+    meta = func_metadata(update_task)
+
+    uuid = "3400e37e-b251-49d9-91b0-f8dd8602ff7e"
+    json_payload = '{"id": "3400e37e-b251-49d9-91b0-f8dd8602ff7e"}'
+
+    assert meta.pre_parse_json({"task_id": uuid})["task_id"] == uuid
+    assert meta.pre_parse_json({"task_id": json_payload})["task_id"] == json_payload
+    assert meta.pre_parse_json({"task_id": "[1, 2]"})["task_id"] == "[1, 2]"
+
+    result = await meta.call_fn_with_arg_validation(
+        update_task,
+        fn_is_async=False,
+        arguments_to_validate={"task_id": json_payload},
+        arguments_to_pass_directly=None,
+    )
+
+    assert result == json_payload
+
+
+def test_str_or_list_still_pre_parses_lists():
+    def func_with_str_or_list(value: str | list[str]):  # pragma: no cover
+        return value
+
+    meta = func_metadata(func_with_str_or_list)
+
+    assert meta.pre_parse_json({"value": "hello"})["value"] == "hello"
+    assert meta.pre_parse_json({"value": '["hello", "world"]'})["value"] == ["hello", "world"]
+
+
 # Tests for structured output functionality