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fix: prevent stdio transport from closing real process stdin/stdout (#1933) #2820

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fix: prevent stdio transport from closing real process stdin/stdout (#1933) #2820

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23 changes: 21 additions & 2 deletions src/mcp/server/stdio.py
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,25 @@ async def run_server():
from mcp.shared.message import SessionMessage


class _NoCloseTextIOWrapper(TextIOWrapper):
"""A TextIOWrapper that does not close the underlying buffer on garbage collection.

Standard TextIOWrapper calls close() in __del__, which closes the underlying
buffer. When wrapping sys.stdin.buffer or sys.stdout.buffer, this causes the
real process stdio to be closed after the server exits, breaking subsequent
print() or input() calls in the parent process.
"""

def close(self) -> None:
# Intentionally not closing the underlying buffer.
# The standard process handles should outlive the server.
pass

def __del__(self) -> None:
# Prevent TextIOWrapper.__del__ from calling close().
pass


@asynccontextmanager
async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.AsyncFile[str] | None = None):
"""Server transport for stdio: this communicates with an MCP client by reading
Expand All @@ -39,9 +58,9 @@ 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", errors="replace"))
stdin = anyio.wrap_file(_NoCloseTextIOWrapper(sys.stdin.buffer, encoding="utf-8", errors="replace"))
if not stdout:
stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))
stdout = anyio.wrap_file(_NoCloseTextIOWrapper(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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