fix: prevent stdio transport from closing real stdin/stdout

When using transport='stdio', the server wraps sys.stdin.buffer and
sys.stdout.buffer in TextIOWrapper and anyio.AsyncFile. When these
wrappers are garbage collected after the server exits, they close
the underlying sys.stdin/sys.stdout, causing subsequent I/O
operations to fail with ValueError.

Use os.dup() to duplicate the file descriptors before wrapping them,
so closing the wrapped streams only closes the duplicates, leaving
the original stdin/stdout intact.

Fixes #1933

Author: g97iulio1609

src/mcp/server/stdio.py

@@ -17,6 +17,7 @@ async def run_server():
     ```
 """
 
+import os
 import sys
 from contextlib import asynccontextmanager
 from io import TextIOWrapper
@@ -38,10 +39,16 @@ 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.
+    # We duplicate the file descriptors via os.dup() to avoid closing the
+    # real sys.stdin/sys.stdout when the wrapped streams are closed.
     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]