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fix: use os.dup() to prevent stdio transport from closing real stdin/stdout #2166

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giulio-leone wants to merge 2 commits into from
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fix: use os.dup() to prevent stdio transport from closing real stdin/stdout #2166

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07b2613
fix: use os.dup() to prevent stdio transport from closing real stdin/…
giulio-leone Feb 28, 2026
8a2e856
style: apply ruff format
giulio-leone Feb 28, 2026
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5 changes: 3 additions & 2 deletions src/mcp/server/stdio.py
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,7 @@ async def run_server():
```
"""

import os
import sys
from contextlib import asynccontextmanager
from io import TextIOWrapper
Expand All @@ -39,9 +40,9 @@ async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.
# python is platform-dependent (Windows is particularly problematic), so we
# re-wrap the underlying binary stream to ensure UTF-8.
if not stdin:
stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8"))
stdin = anyio.wrap_file(TextIOWrapper(os.fdopen(os.dup(sys.stdin.fileno()), "rb"), encoding="utf-8"))
if not stdout:
stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))
stdout = anyio.wrap_file(TextIOWrapper(os.fdopen(os.dup(sys.stdout.fileno()), "wb"), encoding="utf-8"))

read_stream: MemoryObjectReceiveStream[SessionMessage | Exception]
read_stream_writer: MemoryObjectSendStream[SessionMessage | Exception]
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