What game engine does Valve use?

What game engine does Valve use is primarily Source engine and its successor Source 2, both developed internally by Valve for its games. Valve originally used GoldSrc, which was heavily modified version of Quake engine, for Half-Life and Counter Strike. Then Valve developed Source engine which debuted with Counter Strike Source and Half-Life 2 in 2004, known for advanced physics using Havok, facial animation, and modular design, used for many Valve games including Portal, Portal 2, Team Fortress 2, Left 4 Dead series using Source with modifications. Later Valve developed Source 2, announced in 2015, which is more modern, supports Vulkan, better VR, improved tools like Hammer editor, and is used for Dota 2 after it was ported to Source 2 in 2015, Half-Life Alyx VR game, Counter Strike 2 which replaced CS GO on Source 2 in 2023, and upcoming titles. Source 2 is free for developers and integrated with Steam Workshop. Valve also uses other technologies for specific projects, but Source family is its main proprietary engine. Unlike companies licensing Unreal Engine, Valve uses its own engine to maintain control, optimize for Steam, and support modding community, which has created many popular mods and maps using Source tools.