Summary

stdio_server() creates TextIOWrapper around sys.stdin.buffer and sys.stdout.buffer without specifying newline="". On Windows, the default newline=None causes \n → \r\n translation on stdout and universal newline mode on stdin, violating the NDJSON wire format used by the MCP spec (which specifies LF-only line endings).

Fix

Add newline="" to both TextIOWrapper calls. newline="" disables translation while still operating in text mode, ensuring consistent \n-only output across all platforms.

Testing

• Existing tests pass (they use StringIO/BytesIO which don't go through the TextIOWrapper default path)
• Verified the fix produces correct b'...\n' output on Windows 11 with Python 3.12

Fixes #2433