To mod the game, you’ll need to turn off anti-cheat functionality, then mode the game and play. You’ll need to revert files and restore anti-cheat to play official multiplayer playlists or use matchmaking.
Developer 343 made the confirmation via Reddit this morning.
“We’re still working to improve this down the road, but for now you will have an option when you launch to bypass anti-cheat,” a spokesperson confirmed. “This will allow you to play around with campaign and customs but not allow you to play any match made games.”