Source SDK
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The Source SDK (or Source Software Development Kit) is a collection of tools compiled by Valve that's used to create levels or full-on Half-Life 2 Modifications (or mods). Software included in the Source SDK includes:
- Valve Hammer Editor
- Valve Model Viewer
- Valve Face Poser
There's also documentation:
- Release Notes
- SDK Reference Notes
Utilities:
- Create a Mod
- Refresh SDK Content
- Reset Game Configurations
How to get the Source SDK
If you have Half-Life 2, go to the Steam 'Play Games' menu and scroll to the bottom where it says 'Tools.' Double click the Source SDK option and it will download the Source SDK for you.