fix(server): duplicate stdio fds to prevent closing real stdio

When using transport="stdio", the async wrappers around sys.stdin/sys.stdout
would close the underlying file descriptors on server exit, causing
subsequent operations like print() to fail with ValueError.

Use os.dup() to duplicate the file descriptors before wrapping, so
closing the async wrappers does not affect the original stdio streams.
Fall back to the original behavior when the stream lacks a real fd
(e.g. BytesIO in tests).

Fixes #1933

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: shenpeng.sp0916

src/mcp/server/stdio.py

@@ -17,9 +17,10 @@ async def run_server():
     ```
 """
 
+import os
 import sys
 from contextlib import asynccontextmanager
-from io import TextIOWrapper
+from io import TextIOWrapper, UnsupportedOperation
 
 import anyio
 import anyio.lowlevel
@@ -34,14 +35,24 @@ async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.
     """Server transport for stdio: this communicates with an MCP client by reading
     from the current process' stdin and writing to stdout.
     """
-    # Purposely not using context managers for these, as we don't want to close
-    # 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.
+    # Duplicate the underlying file descriptors so that closing the async
+    # wrappers does not close the real stdio, which would break subsequent
+    # operations (e.g. print) after the server exits. When the stream does
+    # not have a real fd (e.g. in tests), fall back to the original stream.
     if not stdin:
-        stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8", errors="replace"))
+        try:
+            stdin_fd = os.dup(sys.stdin.fileno())
+            stdin_bin = os.fdopen(stdin_fd, "rb", closefd=True)
+            stdin = anyio.wrap_file(TextIOWrapper(stdin_bin, encoding="utf-8", errors="replace"))
+        except (OSError, UnsupportedOperation):
+            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"))
+        try:
+            stdout_fd = os.dup(sys.stdout.fileno())
+            stdout_bin = os.fdopen(stdout_fd, "wb", closefd=True)
+            stdout = anyio.wrap_file(TextIOWrapper(stdout_bin, encoding="utf-8"))
+        except (OSError, UnsupportedOperation):
+            stdout = anyio.wrap_file(TextIOWrapper(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)