UPDATE: Valve has released the new SDK tools which you can now easily access using the launcher program I've created over here. Here's a simple method to get the current (non-beta) Source SDK working again since Valve updated the TF2 file system. Preparations First make sure you have an unaltered (non-custom, non-beta) version of the Source SDK installed. To insure this is the case select "refresh sdk content" followed by "Reset Game configurations" in the SDK launcher. Also make sure steam cloud support is disabled. Note: All TF2 game files in your user directory are no longer needed or used with the new tf2 setup and can be deleted to save space. However, make sure you transfer any custom files you want to keep (maps, models, materials, etc.) over to the new TF2 location in the common folder before deleting it! Step 1 Go to \steam\steamapps\common\Team Fortress 2\tf and open the file there named “gameinfo.txt”. Add “ToolsAppId 211” to line 22 and "Game |gameinfo_path|." and "Game tf" right above line 77. Save and close. Note: The above step allows the SDK tools to load all needed scheme files and prevents the error message "Failed to load default scheme file. The map views may be missing some visual elements." from appearing when you open or create a new map in Hammer. Step 2 Open the Source SDK and click “Edit Game Configurations” select “Team Fortress 2” and click “Edit...” Under the directory path replace the user folder (e.g. \username\) with “\common\” and click “OK” and close the SDK. Step 3 Relaunch the SDK and Open Hammer, click on "Tools > Options" and change the following: Game Executable Directory = $SteamDir\steamapps\common\team fortress 2 Game Directory = $SteamDir\steamapps\common\team fortress 2\tf Hammer VMF Directory = place where you store your VMFs important! Note: If you don't set the VMF Directory to the location of the current VMF you're working on you may get the error message "Cannot find the file specified" when trying to compile it Step 4 Then go to the "Build Programs" tab and update the following: Game executable = $SteamDir\steamapps\common\team fortress 2\hl2.exe Compiled maps directory = $SteamDir\steamapps\common\team fortress 2\tf\maps I discovered this method after reinstalling Steam and comparing it to my backup. Looks like someone else discovered this method too, but I thought I'd make a more detailed walk through here. Let me know if you have any problems with this method. Hope it helps someone.