fix(server): prevent stdio transport from closing original stdio handles

When using transport='stdio', the stdio_server context manager wraps
sys.stdin.buffer and sys.stdout.buffer in TextIOWrapper. When these
wrappers are closed (either explicitly or when exiting the context),
they also close the underlying buffers, causing subsequent stdio
operations to fail with ValueError.

This fix duplicates the file descriptors before wrapping them, so that
the original stdin/stdout remain open after the transport is closed.

Fixes #1933

Author: PR-Contributor

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 to avoid closing the original stdio
+    # when the transport is closed (fixes issue #1933).
     if not stdin:
-        stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8", errors="replace"))
+        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"))
     if not stdout:
-        stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))
+        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"))
 
     read_stream_writer, read_stream = create_context_streams[SessionMessage | Exception](0)
     write_stream, write_stream_reader = create_context_streams[SessionMessage](0)