How it is enforced

Two layers.

1. The compiler checks every call against needs and rejects a script that asks for more.
2. The runtime compiles the program to WebAssembly and runs it on Wasmtime with WASI. A WASI program has no files, network or clock unless the host hands it a handle, so even a bug in the compiler cannot let it touch the machine.

Starting a command goes through a host function that checks the run patterns first, because WASI cannot start processes itself.

Why WebAssembly

Target	Sandbox	Build effort	Startup	Portability
WebAssembly (WASI)	strong, capability-based	medium	fast	one .wasm runs anywhere
JavaScript on Deno	medium, flags	low	medium	needs Deno
Own bytecode VM	whatever you build	high	fast	one runtime to ship
Native (LLVM, Cranelift)	none on its own	high	fastest	per platform
Transpile to Rust or Go	none on its own	medium	slow (compile step)	per platform

First version

A tree-walking interpreter with run-time checks behind a single Host interface, so the WebAssembly backend can reuse the same checks.