stdio_server: text a tool prints still reaches the client, because sys.stdout is flushed after fd 1 is restored

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難度
2/5
預估耗時
1-3 小時
新手友好度
76/100
Issue 類型
缺陷
描述清晰度
描述清楚
活躍度
活躍
技術堆疊
python
領域
backend-api-design

研究方向

先閱讀 stdio_server() 的實作，並追蹤 fd 1 在哪裡被重新導向及還原；該 issue 沒有指出檔案或測試。使用一個在 stdout 透過管線連接時呼叫 print() 的工具重現回報的案例，接著檢查待輸出的文字是否在 fd 1 還原之前寫入 stderr，且沒有以 JSON-RPC 訊息的形式出現。

由索引模型根據 Issue 內容生成。

描述

bug v2
Summary

Since #3117, stdio_server() points fd 1 at stderr while it serves, so that stray output from handlers misses the wire. Text written with print() still reaches the client. It goes into sys.stdout's own buffer, which is block-buffered when stdout is a pipe. That buffer is flushed when the interpreter exits, after stdio_server() has given fd 1 back to the client's pipe. The client then reads the text as a JSON-RPC message.

Reproduction

mcp 2.3.0, Python 3.13.16, macOS 26.3.

import asyncio
import sys

from mcp import ClientSession, StdioServerParameters
from mcp.client.stdio import stdio_client
from mcp.server.mcpserver import MCPServer

if sys.argv[1:] == ["server"]:
    server = MCPServer("repro")

    @server.tool()
    def noisy() -> str:
        print("stray output from a tool")  # e.g. a library that prints
        return "done"

    server.run("stdio")
else:
    async def main():
        params = StdioServerParameters(command=sys.executable, args=[__file__, "server"])
        async with stdio_client(params) as (read, write):
            async with ClientSession(read, write) as session:
                await session.initialize()
                result = await session.call_tool("noisy", {})
                print("result:", result.content[0].text)

    asyncio.run(main())

Running python repro.py, the call succeeds, and the client also logs:

pydantic_core._pydantic_core.ValidationError: 1 validation error for union[JSONRPCRequest,JSONRPCNotification,JSONRPCResponse,JSONRPCError]
  Invalid JSON: expected value at line 1 column 1 [type=json_invalid, input_value='stray output from a tool', input_type=str]
Cause

While fd 1 is diverted, the printed text never reaches it: it sits in sys.stdout's buffer. The buffer is written out at interpreter exit, when fd 1 is the protocol pipe again. Adding sys.stdout.flush() right after the print() in the tool removes the error, and the text appears on stderr, which confirms this.

Suggestion

Flush sys.stdout before restoring fd 1, so that pending text goes to the diversion. Making sys.stdout line-buffered while serving (sys.stdout.reconfigure(line_buffering=True)) would also send printed text to stderr when it is printed, rather than at exit.

As a workaround we call sys.stdout.reconfigure(line_buffering=True) before server.run("stdio") and flush after each tool.

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