Valve just released the source code for Team Fortress 2 — on purpose
This update will allow content creators to build entirely new games based on TF2,” the post says ... create an entire ...
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Team Fortress 2 Modders Rejoice as Valve Releases Full Client and Server Game Code
Surprise! Valve's just released a massive update to the Source SDK, adding "all" the Team Fortress 2 client and server game code. More like this Valve Finally Fixes BLU Scout’s Pants ...