Chaos Source is a branch of Valve Software's Source Engine based off Counter‑Strike: Global Offensive.
The main goal of Chaos Source is to provide a shared engine base for many different Source Engine projects, so that those mods can benefit from each other instead of every team having to implement the same features independently.
We're big fans of open source, but since Chaos Source uses licensed source code, the engine code cannot be open sourced.
This is however the only code that cannot be open sourced. All of our other tools like, for example, the asset builder, are all open source, if possible.
No. All of our projects are green-lit, licensed and endorsed by Valve. We do not use any leaked code and will never leak ourselves.
No. Due to the way source is set up and certain conditions, Chaos Source is only accessible in our games and cannot be distributed as standalone SDK.
If you wish to create a single-player game with Chaos Source features, Portal 2: Community Edition would probably suit your project the best, even for non-portal mods.