Fixes incorrect casting into int for numbers in string format (#384)

Author: kafkasl

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

@@ -88,7 +88,7 @@ def pre_parse_json(self, data: dict[str, Any]) -> dict[str, Any]:
                     pre_parsed = json.loads(data[field_name])
                 except json.JSONDecodeError:
                     continue  # Not JSON - skip
-                if isinstance(pre_parsed, str):
+                if isinstance(pre_parsed, (str, int, float)):
                     # 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.

tests/server/fastmcp/test_func_metadata.py

@@ -399,3 +399,18 @@ def test_complex_function_json_schema():
         "title": "complex_arguments_fnArguments",
         "type": "object",
     }
+
+
+def test_str_vs_int():
+    """
+    Test that string values are kept as strings even when they contain numbers,
+    while numbers are parsed correctly.
+    """
+
+    def func_with_str_and_int(a: str, b: int):
+        return a
+
+    meta = func_metadata(func_with_str_and_int)
+    result = meta.pre_parse_json({"a": "123", "b": 123})
+    assert result["a"] == "123"
+    assert result["b"] == 123