fix(fastmcp): handle strings containing numbers correctly

This is a port of jlowin/fastmcp#63 to this repo
- I guess the transition was made before this PR was merged into
fastmcp, so it wasn't included here.

See jlowin/fastmcp#62 for the initial bug
report.

Author: sd2k

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

@@ -176,6 +176,21 @@ def func_with_str_types(str_or_list: str | list[str]):
     assert result["str_or_list"] == ["hello", "world"]
 
 
+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
+
+
 def test_skip_names():
     """Test that skipped parameters are not included in the model"""