Windows: TextIOWrapper in stdio_server() emits CRLF instead of LF, corrupting newline-delimited JSON messages

Summary

mcp/server/stdio.py creates TextIOWrapper(sys.stdout.buffer, encoding="utf-8") without specifying newline="". On Windows, the default newline=None causes \n → \r\n translation, so every JSON-RPC message written to stdout ends with \r\n instead of \n.

The MCP spec uses newline-delimited JSON with \n as the delimiter. Emitting \r\n is a protocol-level impurity.

Affected file

mcp/server/stdio.py lines 46–49

# Current (buggy on Windows)
if not stdin:
    stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8"))
if not stdout:
    stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))

Reproduction

On Windows, spawn a Python subprocess that uses this code and read the raw bytes:

import subprocess, sys

script = r'''
import sys
from io import TextIOWrapper
stdout = TextIOWrapper(sys.stdout.buffer, encoding="utf-8")
stdout.write('{"jsonrpc":"2.0","result":"ok"}\n')
stdout.flush()
'''

proc = subprocess.Popen([sys.executable, "-c", script], stdout=subprocess.PIPE)
out, _ = proc.communicate(timeout=5)
print(repr(out))
# Output on Windows: b'{"jsonrpc":"2.0","result":"ok"}\r\n'
# Output on Linux:   b'{"jsonrpc":"2.0","result":"ok"}\n'

Verified on:

• OS: Windows 11 Pro (10.0.26200)
• Python: 3.11
• mcp: 1.26.0

Why this matters

While the current JS MCP SDK client (StdioClientTransport) strips trailing \r via .replace(/\r$/, "") before parsing, this is a server-side bug that:

1. Violates the NDJSON wire format (which specifies LF-only line endings)
2. Creates an asymmetry: the Python stdio_client sends bare \n, but the Python stdio_server responds with \r\n
3. Could break any MCP client that does a strict split("\n") and then fails to JSON.parse the line with a trailing \r

The comment on line 43 even acknowledges: "Encoding of stdin/stdout as text streams on python is platform-dependent (Windows is particularly problematic)" — but the fix applied (re-wrap to ensure UTF-8) doesn't also fix the newline translation mode.

Fix

Add newline="" to both TextIOWrapper calls. newline="" disables translation while still operating in text mode:

if not stdin:
    stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8", newline=""))
if not stdout:
    stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8", newline=""))

With this fix:

# newline="" result:
buf = io.BytesIO()
wrapper = TextIOWrapper(buf, encoding="utf-8", newline="")
wrapper.write('{"jsonrpc":"2.0","result":"ok"}\n')
wrapper.flush()
repr(buf.getvalue())
# b'{"jsonrpc":"2.0","result":"ok"}\n'  ← correct on all platforms

The same fix should be applied to the stdin wrapper so that incoming messages with bare \n are not translated either (avoiding any future issues if a client sends strict LF).

Context

This was discovered while debugging Windows MCP tool timeouts with mem0-mcp-selfhosted. The eager-init approach fixed the actual timeout, but this CRLF emission was identified as a secondary protocol-level issue during investigation.

Bắt đầu trong src/mcp/server/stdio.py tại các lệnh gọi TextIOWrapper quanh dòng 46–49, sau đó tái hiện đầu ra thô của subprocess trên Windows. Hoàn tất khi cả stdin và stdout đều giữ nguyên các byte LF thuần cho JSON được phân cách bằng dòng mới trên mọi nền tảng.