With a recent update to the Halo: The Master Chief Collection, Xbox Game Studios included the needed Easy Anti-Cheat file for Linux / Steam Deck to work online but they broke it in the process for some.
It's a very good sign that official support may come, as I pointed out on Twitter but I found that it stopped the game working entirely on both my Fedora Linux desktop and the Steam Deck. Looking around, I'm not alone in this issue. Even when trying to use the mode they included with the game to play without anti-cheat (so you can at least play offline), that doesn't work now either.
Thankfully, there is a temporary fix so you can continue playing offline until they sort it properly. Here's what you need to do (for Steam Deck, be in Desktop Mode):
- Right click on the game in your Steam Library.
- Go to Properties.
- Local files on the left tab.
- Click Browse.
- Find the "easyanticheat" folder, and inside remove "easyanticheat_x64.so" (or the whole folder, both ways work)
Now you will have it back as before, with the option to load without anti-cheat working once again.
Is this safe? Yes, as you'll be using the built-in option to play without it anyway. If you want to restore it, you can just Verify the files inside of Steam and it will grab them again.