Back in February, Valve released the source code for Team Fortress 2 (TF2) into the Source SDK, which allows modders to see how it all works and get their standalone mods onto Steam. But also, it allows them to fix up issues directly in Team Fortress 2.
What is it, like an SDK?
Yes, Valve released TF2’s game logic as part of the Source 1 SDK.
Oh, nice, yeah they did that for TFC as well