I thought it would be bash+fuse but it's actually running in the matrix: simulated bash in Elixir ported from Vercel's just-bash:

> A virtual bash environment with an in-memory filesystem, written in TypeScript and designed for AI agents.

>Broad support for standard unix commands and bash syntax with optional curl, Python, JS/TS, and sqlite support.

https://github.com/vercel-labs/just-bash/tree/main/packages/...

See also (credited by TFA):

https://vercel.com/blog/how-to-build-agents-with-filesystems...