Fastmcp tool parameter parsing type error #381
Description
When I define the tool parameter as string, the parameter parsing function of fastmcp parses the numeric string I pass in into a numeric value.
- mcp/server/fastmcp/utilities/func_metadata.py
def pre_parse_json(self, data: dict[str, Any]) -> dict[str, Any]:
"""Pre-parse data from JSON.
Return a dict with same keys as input but with values parsed from JSON
if appropriate.
This is to handle cases like `["a", "b", "c"]` being passed in as JSON inside
a string rather than an actual list. Claude desktop is prone to this - in fact
it seems incapable of NOT doing this. For sub-models, it tends to pass
dicts (JSON objects) as JSON strings, which can be pre-parsed here.
"""
new_data = data.copy() # Shallow copy
for field_name, field_info in self.arg_model.model_fields.items():
import logging
logging.info((field_name, field_info))
logging.info("before parse: ", new_data)
if field_name not in data.keys():
continue
if isinstance(data[field_name], str):
try:
pre_parsed = json.loads(data[field_name])
except json.JSONDecodeError:
continue # Not JSON - skip
if isinstance(pre_parsed, str):
# This is likely that the raw value is e.g. `"hello"` which we
# 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
logging.info("after parse: ", new_data)
assert new_data.keys() == data.keys()
return new_data
test output:
video-1 | ('cid', FieldInfo(annotation=str, required=True, description='xxx.'))
video-1 | before parse: {'cid': '1.2'}
video-1 | after parse: {'cid': 1.2}
video-1 | Error calling tool: Error executing tool getinfo: 1 validation error for getinfoArguments
video-1 | cid
video-1 | Input should be a valid string [type=string_type, input_value=1.2, input_type=float]
video-1 | For further information visit https://errors.pydantic.dev/2.10/v/string_type
Here, json.loads will parse the incoming string "1.2" into the floating point number 1.2
So, when I use pydantic to define parameters in tool, a validation error occurs
from pydantic import validate_call, Field
@server.tool(description="xxx")
@validate_call
async def getinfo(
cid: str = Field(..., description="xxx."),
) -> bool | None:
Is my understanding correct?