Steam – How to clear Steam achievements for Portal 2

This is common to all Borderlands players on Steam right now, as achievements are disabled/not working yet. http://forums.steampowered.com/forums/showthread.php?t=1390120

Contrary to all the answers on here, it is in fact possible to reset your progress in co-op. Make sure you are the host (you invited your friend, rather than you were invited by your friend). Start a co-op game. Make sure you are in the hub when doing this.

Warning: This applies to both players. Make sure the other player is fine with you doing this.

Unfortunately, this doesn't work with ss_map. You have to be in a session with another player, and this will set both players' progress.

Enable the developer console in the options, then press ~ to open it. If you are having trouble, see this question. Then, enter the following command in the console:

mp_mark_all_maps_incomplete

You might see a weird message about DLC, but the command will have no other visible effect. Now, enter this command:

changelevel mp_coop_lobby_2

This will reload the lobby, and your progress will be reset at zero.

You can use this command to change your progress manually:

mp_mark_course_complete <course>

Whereas the number is the number of the course you want to unlock (have finished), minus 1. So to unlock course 1, use mp_mark_course_complete 0, to unlock 2, mp_mark_course_complete 1, and so on. This means that mp_mark_course_complete 0 unlocks course 1 fully, and will then open 2.

There is also mp_mark_all_maps_complete if you want to get Professor Portal.

Thanks to fail badp♦ for allowing me to waste his time.