I (somewhat jokingly) wrote one recently too... https://github.com/pnegahdar/nano in under 200 lines. Repl, sessions, non-interactive, approvals, etc

The smarter the models get the less the harnesses matter (outside of devx).

Maybe one day I'll run it through swebech.

So freaking cool..in just 200 (190 actually) lines.

I also wrote one by myself last week (just for fun and learning). It works, including integration with configured mcpServers (like you do in most coding agents). Wrote about the whole step-by-step process and what is needed at what step and why: https://nb1t.sh/building-a-real-agent-step-by-step/

I like it