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fix: override input with json.loads only when output value is valid #350

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fix: override input with json.loads only when output value is valid #350

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40adc38
func metadata: override input with json.loads only when value is valid
omerc7 Mar 23, 2025
528afdd
keep original code structure
omerc7 Mar 23, 2025
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16 changes: 15 additions & 1 deletion src/mcp/server/fastmcp/utilities/func_metadata.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down Expand Up @@ -93,6 +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
try:
# Validate parsed value
TypeAdapter(_field_info.annotation).validate_python(pre_parsed)
except ValidationError:
continue # Parsed value is invalid - skip

new_data[field_name] = pre_parsed
assert new_data.keys() == data.keys()
return new_data
Expand Down
8 changes: 8 additions & 0 deletions tests/server/fastmcp/test_func_metadata.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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,
Expand All @@ -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!"

Expand All @@ -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,
)
Expand Down Expand Up @@ -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",
Expand Down