Source 2 is a video game engine developed by Valve as the successor to the original Source engine. The engine was announced in 2015, with the first game to use it, Dota 2, being ported from the original engine that same year. Since then, Valve's Artifact, Dota Underlords, and Half-Life: Alyx have all been made with the engine.
Plans for a successor to the original Source engine began following the release of Half-Life 2: Episode Two in 2007.. After a few years in development, the first engine tech demo was created in summer of 2010 as a remake of the final map of Left 4 Dead 2's campaign Swamp Fever, titled The Plantation. Images of this tech demo were leaked onto the internet in early 2014.  For GDC 2014, Valve employee Sergiy Migdalskiy showed off a Source 2 physics debugging tool being used in Left 4 Dead 2 to teach developers on how to develop physics debugging software for games.
Source 2 was first made available to the public via Steam Workshop tools for Dota 2 in 2014 before it was officially announced at the Game Developers Conference in March 2015, with Valve stating that their intent for it was to allow for content to be created more efficiently. At the time of announcement, Valve stated that it would support Vulkan graphical API and use a new in-house physics engine called Rubikon, which would replace the need for Havok. Gabe Newell, president of Valve, said that the company were prioritizing the development of their own games before they would release the engine and its software development kit to the public as a means of ensuring the highest quality for developers; adding that they were intending to make the engine free to use for game developers as long as the game is published on their Steam service.
In June 2015, Valve announced that the entirety of Dota 2 would be ported over to Source 2 in an update called Dota 2 Reborn. Reborn was first released to the public as an opt-in beta update that same month before officially replacing the original client in September 2015, making it the first game to use the engine. Source 2 has also been used for Valve's Artifact and Dota Underlords, with the engine being ported to support Android and iOS for the latter. The engine also supports the creation of games in virtual reality, being used in SteamVR Home, the Robot Repair tech demo within The Lab, and Half-Life: Alyx. The first public beta of Source 2 tools, made specifically for creating custom content for Half-Life: Alyx, was released in May 2020.
- Keighley, Geoff (2020). Half-Life: Alyx - Final Hours