Fix stdio_server closing process stdio

src/mcp/server/stdio.py

@@ -17,9 +17,11 @@ async def run_server():
     ```
 """
 
+import os
 import sys
 from contextlib import asynccontextmanager
-from io import TextIOWrapper
+from io import TextIOWrapper, UnsupportedOperation
+from typing import TextIO
 
 import anyio
 import anyio.lowlevel
@@ -29,6 +31,20 @@ async def run_server():
 from mcp.shared.message import SessionMessage
 
 
+def _wrap_stdio_text_stream(stream: TextIO, mode: str, errors: str = "strict") -> anyio.AsyncFile[str]:
+    """Wrap a stdio text stream without closing the original handle on teardown."""
+    try:
+        wrapped_stream = TextIOWrapper(
+            os.fdopen(os.dup(stream.fileno()), mode, closefd=True),
+            encoding="utf-8",
+            errors=errors,
+        )
+    except (AttributeError, UnsupportedOperation):
+        wrapped_stream = TextIOWrapper(stream.buffer, encoding="utf-8", errors=errors)
+
+    return anyio.wrap_file(wrapped_stream)
+
+
 @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
@@ -38,10 +54,13 @@ async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.
     # 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.
+    close_stdin = stdin is None
+    close_stdout = stdout is None
+
     if not stdin:
-        stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8", errors="replace"))
+        stdin = _wrap_stdio_text_stream(sys.stdin, "rb", errors="replace")
     if not stdout:
-        stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))
+        stdout = _wrap_stdio_text_stream(sys.stdout, "wb")
 
     read_stream_writer, read_stream = create_context_streams[SessionMessage | Exception](0)
     write_stream, write_stream_reader = create_context_streams[SessionMessage](0)
@@ -71,7 +90,13 @@ async def stdout_writer():
         except anyio.ClosedResourceError:  # pragma: no cover
             await anyio.lowlevel.checkpoint()
 
-    async with anyio.create_task_group() as tg:
-        tg.start_soon(stdin_reader)
-        tg.start_soon(stdout_writer)
-        yield read_stream, write_stream
+    try:
+        async with anyio.create_task_group() as tg:
+            tg.start_soon(stdin_reader)
+            tg.start_soon(stdout_writer)
+            yield read_stream, write_stream
+    finally:
+        if close_stdin:
+            await stdin.aclose()
+        if close_stdout:
+            await stdout.aclose()

tests/server/test_stdio.py

@@ -1,5 +1,6 @@
 import io
 import sys
+import tempfile
 from io import TextIOWrapper
 
 import anyio
@@ -73,12 +74,15 @@ async def test_stdio_server_invalid_utf8(monkeypatch: pytest.MonkeyPatch):
     """
     # \xff\xfe are invalid UTF-8 start bytes.
     valid = JSONRPCRequest(jsonrpc="2.0", id=1, method="ping")
-    raw_stdin = io.BytesIO(b"\xff\xfe\n" + valid.model_dump_json(by_alias=True, exclude_none=True).encode() + b"\n")
+    raw_stdin = tempfile.TemporaryFile("w+b")
+    raw_stdin.write(b"\xff\xfe\n" + valid.model_dump_json(by_alias=True, exclude_none=True).encode() + b"\n")
+    raw_stdin.seek(0)
+    raw_stdout = tempfile.TemporaryFile("w+b")
 
     # Replace sys.stdin with a wrapper whose .buffer is our raw bytes, so that
     # stdio_server()'s default path wraps it with errors='replace'.
     monkeypatch.setattr(sys, "stdin", TextIOWrapper(raw_stdin, encoding="utf-8"))
-    monkeypatch.setattr(sys, "stdout", TextIOWrapper(io.BytesIO(), encoding="utf-8"))
+    monkeypatch.setattr(sys, "stdout", TextIOWrapper(raw_stdout, encoding="utf-8"))
 
     with anyio.fail_after(5):
         async with stdio_server() as (read_stream, write_stream):
@@ -92,3 +96,60 @@ async def test_stdio_server_invalid_utf8(monkeypatch: pytest.MonkeyPatch):
                 second = await read_stream.receive()
                 assert isinstance(second, SessionMessage)
                 assert second.message == valid
+
+    sys.stdin.close()
+    sys.stdout.close()
+
+
+@pytest.mark.anyio
+async def test_stdio_server_does_not_close_process_stdio(monkeypatch: pytest.MonkeyPatch):
+    """Default stdio_server() teardown must not close the caller's stdio handles."""
+    valid = JSONRPCRequest(jsonrpc="2.0", id=1, method="ping")
+    raw_stdin = tempfile.TemporaryFile("w+b")
+    raw_stdin.write(valid.model_dump_json(by_alias=True, exclude_none=True).encode() + b"\n")
+    raw_stdin.seek(0)
+    raw_stdout = tempfile.TemporaryFile("w+b")
+
+    monkeypatch.setattr(sys, "stdin", TextIOWrapper(raw_stdin, encoding="utf-8"))
+    monkeypatch.setattr(sys, "stdout", TextIOWrapper(raw_stdout, encoding="utf-8"))
+
+    with anyio.fail_after(5):
+        async with stdio_server() as (read_stream, write_stream):
+            await write_stream.aclose()
+            async with read_stream:  # pragma: no branch
+                received = await read_stream.receive()
+                assert isinstance(received, SessionMessage)
+                assert received.message == valid
+
+    assert not sys.stdin.closed
+    assert not sys.stdout.closed
+
+    sys.stdout.write("still-open")
+    sys.stdout.flush()
+    raw_stdout.seek(0)
+    assert raw_stdout.read() == b"still-open"
+
+    sys.stdin.close()
+    sys.stdout.close()
+
+
+@pytest.mark.anyio
+async def test_stdio_server_falls_back_when_stream_has_no_fileno(monkeypatch: pytest.MonkeyPatch):
+    """Streams without a real fd (e.g. pytest capture, in-memory buffers) must
+    fall back to wrapping the underlying ``.buffer`` instead of crashing."""
+    valid = JSONRPCRequest(jsonrpc="2.0", id=1, method="ping")
+    stdin_buf = io.BytesIO(valid.model_dump_json(by_alias=True, exclude_none=True).encode() + b"\n")
+    stdout_buf = io.BytesIO()
+
+    # io.BytesIO raises UnsupportedOperation from .fileno(), forcing the
+    # buffer-wrapping fallback in _wrap_stdio_text_stream.
+    monkeypatch.setattr(sys, "stdin", TextIOWrapper(stdin_buf, encoding="utf-8"))
+    monkeypatch.setattr(sys, "stdout", TextIOWrapper(stdout_buf, encoding="utf-8"))
+
+    with anyio.fail_after(5):
+        async with stdio_server() as (read_stream, write_stream):
+            await write_stream.aclose()
+            async with read_stream:  # pragma: no branch
+                received = await read_stream.receive()
+                assert isinstance(received, SessionMessage)
+                assert received.message == valid