Fixes #1933

Problem

When using transport="stdio", the server closes sys.stdin.buffer and sys.stdout.buffer when exiting, causing subsequent stdio operations to fail with:

ValueError: I/O operation on closed file.

Root Cause

The stdio_server() function wraps sys.stdin.buffer and sys.stdout.buffer directly. When these wrappers are closed (when the context manager exits), they also close the original system streams.

Solution

Use os.dup() to create duplicate file descriptors before wrapping:

• Duplicate sys.stdin.fileno() and sys.stdout.fileno()
• Open new file handles from the duplicated descriptors
• Wrap these duplicated handles instead of the originals

This ensures the original stdin/stdout remain open when the wrapper streams are closed.

Testing

Tested with the reproduction case from the issue:

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("Demo")
mcp.run(transport="stdio")
print("?")  # This now works without error

Pressing Ctrl+D to exit the server no longer causes "I/O operation on closed file" error.

This fix implements the proposed solution from the original issue.