Welcome to the Valve Developer Community. Whether you're building a "Triple-A" commercial project or a mod for Half-Life 2, you'll want to sign up as a Valve developer today. It's free to join, and open to anyone working with the Source Engine and the Source SDK.
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Source and the Source SDK
Our knowledge base of technical information about using the engine and all of the related tools is posted on a community-driven web site called a wiki. Sign up as a developer to contribute.
GoldSrc and the Half-Life SDK
Dota 2 Workshop Tools
Half-Life: Alyx Workshop Tools
SteamVR Workshop Tools
For Mod Teams