func metadata: override input with json.loads only when value is valid

Author: omerc7

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

@@ -7,7 +7,15 @@
     ForwardRef,
 )
 
-from pydantic import BaseModel, ConfigDict, Field, WithJsonSchema, create_model
+from pydantic import (
+    BaseModel,
+    ConfigDict,
+    Field,
+    TypeAdapter,
+    ValidationError,
+    WithJsonSchema,
+    create_model,
+)
 from pydantic._internal._typing_extra import eval_type_backport
 from pydantic.fields import FieldInfo
 from pydantic_core import PydanticUndefined
@@ -93,7 +101,12 @@ def pre_parse_json(self, data: dict[str, Any]) -> dict[str, Any]:
                     # Should really be parsed as '"hello"' in Python - but if we parse
                     # it as JSON it'll turn into just 'hello'. So we skip it.
                     continue
-                new_data[field_name] = pre_parsed
+                try:
+                    # Validate parsed value
+                    TypeAdapter(_field_info.annotation).validate_python(pre_parsed)
+                    new_data[field_name] = pre_parsed
+                except ValidationError:
+                    continue  # Parsed value is invalid - skip
         assert new_data.keys() == data.keys()
         return new_data
 

tests/server/fastmcp/test_func_metadata.py

@@ -59,6 +59,7 @@ def complex_arguments_fn(
     must_be_none_with_default: None = None,
     an_int_with_equals_field: int = Field(1, ge=0),
     int_annotated_with_default: Annotated[int, Field(description="hey")] = 5,
+    num_annotated_with_str: str = "",
 ) -> str:
     _ = (
         an_int,
@@ -81,6 +82,7 @@ def complex_arguments_fn(
         must_be_none_with_default,
         an_int_with_equals_field,
         int_annotated_with_default,
+        num_annotated_with_str,
     )
     return "ok!"
 
@@ -107,6 +109,7 @@ async def test_complex_function_runtime_arg_validation_non_json():
             "my_model_a": {},
             "my_model_a_forward_ref": {},
             "my_model_b": {"how_many_shrimp": 5, "ok": {"x": 1}, "y": None},
+            "num_annotated_with_str": "5",
         },
         arguments_to_pass_directly=None,
     )
@@ -381,6 +384,11 @@ def test_complex_function_json_schema():
                 "title": "Int Annotated With Default",
                 "type": "integer",
             },
+            "num_annotated_with_str": {
+                "default": "",
+                "title": "Num Annotated With Str",
+                "type": "string",
+            },
         },
         "required": [
             "an_int",