From 73074347952548d3e3476e6954aa34acc3cdb07e Mon Sep 17 00:00:00 2001
From: Christian Sidak <csidak@gmail.com>
Date: Mon, 13 Apr 2026 18:08:38 -0700
Subject: [PATCH] fix: add newline="" to stdio TextIOWrapper to prevent CRLF on
 Windows

Closes #2433
---
 src/mcp/server/stdio.py    |  4 ++--
 tests/server/test_stdio.py | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/src/mcp/server/stdio.py b/src/mcp/server/stdio.py
index 5c1459dff..b66c0a522 100644
--- a/src/mcp/server/stdio.py
+++ b/src/mcp/server/stdio.py
@@ -39,9 +39,9 @@ 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(TextIOWrapper(sys.stdin.buffer, encoding="utf-8", errors="replace", newline=""))
     if not stdout:
-        stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))
+        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)
diff --git a/tests/server/test_stdio.py b/tests/server/test_stdio.py
index 677a99356..713bb6758 100644
--- a/tests/server/test_stdio.py
+++ b/tests/server/test_stdio.py
@@ -63,6 +63,40 @@ async def test_stdio_server():
     assert received_responses[1] == JSONRPCResponse(jsonrpc="2.0", id=4, result={})
 
 
+@pytest.mark.anyio
+async def test_stdio_server_no_crlf(monkeypatch: pytest.MonkeyPatch):
+    """Raw bytes written to stdout must use LF (\\n), never CRLF (\\r\\n).
+
+    On Windows, TextIOWrapper with the default newline=None translates \\n to
+    \\r\\n on write, which corrupts NDJSON framing for JSON-RPC. The fix is to
+    pass newline="" to TextIOWrapper so no translation occurs.
+    """
+    raw_stdout = io.BytesIO()
+    # Wrap with newline="" so we can inspect the exact bytes that
+    # stdio_server writes. The key assertion is that the raw bytes
+    # contain \n and never \r\n.
+    stdout_wrapper = TextIOWrapper(raw_stdout, encoding="utf-8", newline="")
+    stdin_wrapper = TextIOWrapper(io.BytesIO(b""), encoding="utf-8")
+
+    message = JSONRPCRequest(jsonrpc="2.0", id=1, method="ping")
+
+    with anyio.fail_after(5):
+        async with stdio_server(
+            stdin=anyio.AsyncFile(stdin_wrapper),
+            stdout=anyio.AsyncFile(stdout_wrapper),
+        ) as (read_stream, write_stream):
+            async with write_stream:
+                await write_stream.send(SessionMessage(message))
+            async with read_stream:
+                pass
+
+    stdout_wrapper.flush()
+    raw_bytes = raw_stdout.getvalue()
+    assert len(raw_bytes) > 0, "expected output bytes"
+    assert raw_bytes.endswith(b"\n"), "output must end with LF"
+    assert b"\r\n" not in raw_bytes, "output must not contain CRLF"
+
+
 @pytest.mark.anyio
 async def test_stdio_server_invalid_utf8(monkeypatch: pytest.MonkeyPatch):
     """Non-UTF-8 bytes on stdin must not crash the server.