64-bit mode with Source SDK

Deprecated:The info below is most likely outdated as Valve no longer provides a 64-bit build of and Half-Life 2 and the Source SDK. This page will remain for archival purposes only.
Third-party Source games however, may have 64-bit support, or Source 2 which fully support 64-bit, but it is unknown if this issue also applies to them.

The current Source SDK release has issues starting up on systems running the Source engine in 64-bit mode with the 64-bit version of Windows XP.

To correct the problem:

1. In the Steam Games window, right-click on Half-Life 2 (or any installed Valve game you want to run) and select the Properties option.
2. Then select Set launch options... and add the line -32bit to the edit box.
3. Close the dialog and launch the game you just edited (ie. Half-Life 2). When it has successfully started up, close the game.

You should now be able to run the SDK on your 64-bit OS. Note, however, that if you run any of your Valve games without the -32bit option, your SDK will go back to being incompatible and you'll need to repeat this process.

This means that if you run a game through Hammer, you'll also need to launch that game with the -32bit command line parameter as well. The same is true running a game from a batch file, command line, etc.