S&box is coded in C#. Under the hood, it uses the Source 2 engine (CS2, HL:Alyx, DOTA2) and some of its systems: rendering, resources, physics, and audio.
When you create games and addons in s&box, you will be creating them in C#.
We have developed a hotload system which is capable of compiling & hotloading your changes to code within a few milliseconds, which negates the need for a scripting language.
We use a scene system, similar to Godot and Unity. This allows faster iteration, without everything being code-based. The scene system aims to make how everything works more transparent, by being easily visible, and easily accessible.
Issues and feature suggestions should be posted in the sbox-issues repo.
Please see Reporting Errors.