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

Author: cjg

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

@@ -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: