TF-Base

TF-Base, also referred to as TF-Port, is public GitHub repository created by Nicknine, that contains leaked Team Fortress 2 source code from late January 2008 that was ported to work with Source SDK Base 2013 Multiplayer. Due to no official means of being able to create Team Fortress 2 Sourcemods, the community used this as an unofficial "SDK". Every mod created Pre-TF2 SDK release were using TF-Base in someway or form even if the projects were forked from another mod.