Posted on 24 April 2014
A fix for the issues brought into Steam’s latest client API which prevented external SDKs and build tools to be called within GameMaker: Studio has now been pushed to Steam’s public client beta prior to being released live.
Anyone can access the Steam client beta, so for instructions on how to opt into the beta, please see here: http://help.yoyogames.com/entries/50884976-Steam-client-AppOverlay-dll-detaching-breaks-many-external-tools-Solved-24-Apr-
A huge thank you to everyone for their patience while we looked into this and to the guys at Valve for responding and fixing this issue so quickly!
-The YoYo Team