how toi access the new source engine beta:

create steam desktop shortcut
look at shortcut properties, it should be something like

"C:Program FilesValveSteamSteam.exe"

which is what it is in my case
whatever it says, add -beta engine to the end so now mine looks like this:

"C:Program FilesValveSteamSteam.exe" -beta engine

then quit steam and launch it from the shortcut

if u have steam start with windows then open regedit, press f3 and search for the original contents of the target box wihouth the quotes, so for me that would be

C:Program FilesValveSteamSteam.exe

it should have a -silent parameter on the end. after the -silent add -beta engine

so mine now looks like

"C:Program FilesValveSteamSteam.exe" -silent -beta engine


when you restart steam if you have done it correctly source will update and install the new engine. to rever back to the old engine simpl remove the -beta engine tags that you added to the shortcut and the registry then restart steam and it will download the old engine

have fun with it