The last entry in the Half-Life series was a very long time ago, so there are more developer tools based on Valve's current Source 2 internal engine.
Along with today's official reveal of their exclusive VR game Half-Life: Alyx, Valve noted that developers will soon be able to use the Source 2 toolset to create their own content, including environments that will work in the upcoming game, although the extent of these creation capabilities is in somewhat dependent on interpretation.
“The Source 2 toolkit will be available for the game, which will allow you to create new levels, as well as allow any player to create and contribute new variables for the community for fun. Hammer, Valve's level design tool, has been updated with all of the VR gameplay tools and components.
Some Valve games, such as Dota 2, were ported from Source to Source 2 during their lifetime. Previously, Dota 2 creation tools were the only, albeit limited, outside developers' access to the engine. Valve has long said it plans to release the engine itself, as developers can potentially use it for free to create their own projects, but any concrete details have been hard to come by since that 2015 announcement.
Half-Life: Alyx and the aforementioned Source 2 tools are scheduled to release in March 2020, although the extent to which these tools are used outside of Half-Life: Alyx's community-made content remains to be seen.
