Repository: modelcontextprotocol/python-sdk

Issue:
Input should be a valid string [type=string_type, input_value=123, input_type=int]
MCP Server tools code:
`@mcp.tool(description="查询物流信息")
async def query_logistics(order_id: str) -> str:
  """查询物流信息。当用户需要根据订单号查询物流信息时，调用此工具

  Args:
      order_id: 订单号
  Returns:
      物流信息的字符串描述
  """
  # 统一的物流信息数据
  tracking_info = [
    {"time": "2024-01-20 10:00:00", "status": "包裹已揽收",
     "location": "深圳转运中心"},
    {"time": "2024-01-20 15:30:00", "status": "运输中", "location": "深圳市"},
    {"time": "2024-01-21 09:00:00", "status": "到达目的地",
     "location": "北京市"},
    {"time": "2024-01-21 14:00:00", "status": "派送中",
     "location": "北京市朝阳区"},
    {"time": "2024-01-21 16:30:00", "status": "已签收",
     "location": "北京市朝阳区三里屯"}
  ]

  # 格式化物流信息
  result = f"物流单号：{order_id}\n\n物流轨迹：\n"
  for item in tracking_info:
    result += f"[{item['time']}] {item['status']} - {item['location']}\n"

  return result
`
MCP client execute code：
`  self._streams_context = sse_client(url=server_url)
        streams = await self._streams_context.__aenter__()

        self._session_context = ClientSession(*streams)
        self.session: ClientSession = await self._session_context.__aenter__()

        # Initialize
        await self.session.initialize()

        # List available tools to verify connection
        print("Initialized SSE client...")
        print("Listing tools...")
        response = await self.session.list_tools()
        tools = response.tools
        print("\nConnected to server with tools:", [tool.name for tool in tools])

        result = await self.session.call_tool("query_logistics", {"order_id":"123"})
        print("Result of query_logistics tool:", result)`
error info：
Connected to server with tools: ['query_logistics']
Result of add tool: meta=None content=[TextContent(type='text', text='Error executing tool query_logistics: 1 validation error for query_logisticsArguments\norder_id\n  Input should be a valid string [type=string_type, input_value=123, input_type=int]\n    For further information visit https://errors.pydantic.dev/2.10/v/string_type', annotations=None)] isError=True


Discussion:

Fix (patch):
diff --git a/src/mcp/server/fastmcp/utilities/func_metadata.py b/src/mcp/server/fastmcp/utilities/func_metadata.py
index cf93049..629580e 100644
--- a/src/mcp/server/fastmcp/utilities/func_metadata.py
+++ b/src/mcp/server/fastmcp/utilities/func_metadata.py
@@ -88,7 +88,7 @@ class FuncMetadata(BaseModel):
                     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.


Test changes:
diff --git a/tests/server/fastmcp/test_func_metadata.py b/tests/server/fastmcp/test_func_metadata.py
index 6461648..b1828ff 100644
--- a/tests/server/fastmcp/test_func_metadata.py
+++ b/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