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fix(server): duplicate stdio fds to prevent closing real stdio by superShen0916 · Pull Request #2866 · modelcontextprotocol/python-sdk · GitHub

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fix(server): duplicate stdio fds to prevent closing real stdio #2866

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fix(server): duplicate stdio fds to prevent closing real stdio #2866

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25 changes: 18 additions & 7 deletions src/mcp/server/stdio.py
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Original file line number Diff line number Diff line change
Expand Up @@ -17,9 +17,10 @@ async def run_server():
```
"""

import os
import sys
from contextlib import asynccontextmanager
from io import TextIOWrapper
from io import TextIOWrapper, UnsupportedOperation

import anyio
import anyio.lowlevel
Expand All @@ -34,14 +35,24 @@ async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.
"""Server transport for stdio: this communicates with an MCP client by reading
from the current process' stdin and writing to stdout.
"""
# Purposely not using context managers for these, as we don't want to close
# standard process handles. Encoding of stdin/stdout as text streams on
# python is platform-dependent (Windows is particularly problematic), so we
# re-wrap the underlying binary stream to ensure UTF-8.
# Duplicate the underlying file descriptors so that closing the async
# wrappers does not close the real stdio, which would break subsequent
# operations (e.g. print) after the server exits. When the stream does
# not have a real fd (e.g. in tests), fall back to the original stream.
if not stdin:
stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8", errors="replace"))
try:
stdin_fd = os.dup(sys.stdin.fileno())
stdin_bin = os.fdopen(stdin_fd, "rb", closefd=True)
stdin = anyio.wrap_file(TextIOWrapper(stdin_bin, encoding="utf-8", errors="replace"))
except (OSError, UnsupportedOperation):
stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8", errors="replace"))
if not stdout:
stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))
try:
stdout_fd = os.dup(sys.stdout.fileno())
stdout_bin = os.fdopen(stdout_fd, "wb", closefd=True)
stdout = anyio.wrap_file(TextIOWrapper(stdout_bin, encoding="utf-8"))
except (OSError, UnsupportedOperation):
stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))

read_stream_writer, read_stream = create_context_streams[SessionMessage | Exception](0)
write_stream, write_stream_reader = create_context_streams[SessionMessage](0)
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