Great taxonomy. There's one category I reckon is missing though: in-process simulation. Vercel's CTO built just-bash which reimplements 80+ bash commands as TypeScript functions. No child processes, no kernel interaction, virtual filesystem in memory. It sits below your Level 1 in terms of overhead (millisecond startup, zero infrastructure) but trades off real binary execution. For the 95% of agent workflows that just need text processing and file analysis, it's more than enough. I wrote about it in depth here: reading.sh/vercels-cto-…