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Avoiding to parse string arguments containing valid JSON if the argument annotation is str #1113

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Merged
merged 1 commit into from
Jul 10, 2025
Merged

Avoiding to parse string arguments containing valid JSON if the argument annotation is str #1113

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2 changes: 1 addition & 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 @@ -130,7 +130,7 @@ def pre_parse_json(self, data: dict[str, Any]) -> dict[str, Any]:
for field_name in self.arg_model.model_fields.keys():
if field_name not in data.keys():
continue
if isinstance(data[field_name], str):
if isinstance(data[field_name], str) and self.arg_model.model_fields[field_name].annotation is not str:
try:
pre_parsed = json.loads(data[field_name])
except json.JSONDecodeError:
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