Loving the customer testimonials :D ..

If someone feels like an eli5 - What are the use-cases for something like this?

I think that this is a plugin library for teams that want to offer a platform for the public (or an LLM-AI) to submit code to. If your team writes some code, you don't generally sandbox it from yourself, you just amend your program: you don't need a sandbox. But, if you want to run code that you don't trust, you should run it in a way that prevents it from causing problems if it is actually dangerous (like a virus or accidentally overwrites your files with blank files). That's what a sandbox like kyushu promises to do.

So, with a sandbox library like this, you could - say - write a website that hosts games (like itch.io or newgrounds) that hosts games on the world wide web. The sandbox part can give you confidence that, if a villain's programmer henchmen uploads a virus instead of a game, it can't infect your platform or other games on the website. Or, if a LLM-AI written game is accidentally tries to take up all the memory of the computer, it can't ask the operating system for more than is in the sandbox.

it is also perfect for running untrusted user code safely, if you want to buld a plugin system or your own edge functions this can be really helpful

Haha, glad someone noticed those testimonials ;)

Others mentioned better use cases than I could probably come with. Not sure it's a strong use case but, one thing I could maybe mention too is the fact that it ships as a standalone artifact. It's portable and, if reproducible, can provide some sort of guarantee on what's effectively running for those who care.

ELI5: Imagine you want to run a heavy, powerful 3D video game engine inside a standard web browser or a lightweight desktop app, without making it slow or unsafe.

JavaScript alone can't handle that kind of heavy lifting efficiently. That’s where Wasm comes in. It lets you run high-performance native code (like C++) at near-native speed safely in the sandbox.

For example, I'm currently using Wasm to run a complex 3D geometry engine (Manifold) inside a lightweight CAD app (Nasscad). It gives you web flexibility with desktop power.