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fix: stdio_server emits LF only on Windows stdout (fixes #2433) #2908

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fix: stdio_server emits LF only on Windows stdout (fixes #2433) #2908

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4 changes: 3 additions & 1 deletion src/mcp/server/stdio.py
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Original file line number Diff line number Diff line change
Expand Up @@ -41,7 +41,9 @@ async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.
if not stdin:
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"))
# newline="" prevents \n → \r\n translation on Windows, which would
# corrupt newline-delimited JSON messages (the MCP protocol uses \n).
stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8", newline=""))

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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21 changes: 21 additions & 0 deletions tests/server/test_stdio.py
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Expand Up @@ -169,3 +169,24 @@ async def lifespan(server: MCPServer) -> AsyncIterator[None]:
assert events == ["setup", "cleanup"]
response = jsonrpc_message_adapter.validate_json(captured.getvalue().decode().strip())
assert response == JSONRPCResponse(jsonrpc="2.0", id=1, result={})


def test_mcpserver_run_stdio_emits_lf_not_crlf(monkeypatch: pytest.MonkeyPatch) -> None:
"""`MCPServer.run("stdio")` emits LF only, not CRLF, on Windows.

The default stdout path in stdio_server() wraps sys.stdout.buffer with
newline="" so JSON lines end with \\n regardless of platform.
"""
ping = JSONRPCRequest(jsonrpc="2.0", id=1, method="ping")
stdin_bytes = io.BytesIO(ping.model_dump_json(by_alias=True, exclude_none=True).encode() + b"\n")
captured = _KeepOpenBytesIO()
# Simulate a "default" stdout WITHOUT newline="" — the fix in stdio_server
# must still produce LF-only output by wrapping sys.stdout.buffer itself.
monkeypatch.setattr(sys, "stdin", TextIOWrapper(stdin_bytes, encoding="utf-8"))
monkeypatch.setattr(sys, "stdout", TextIOWrapper(captured, encoding="utf-8"))

_run_stdio_bounded(MCPServer(name="LfStdioServer"))

raw = captured.getvalue()
assert b"\r\n" not in raw, f"stdout should not contain CRLF: {raw!r}"
assert raw.endswith(b"\n"), f"stdout should end with LF: {raw!r}"