What I've learned in last week is that a harness is basically a while loop.

In each iteration you make an LLM call, perform some work (e.g. tool call), augment the prompt (append or compact etc.)- not necessarily in that other- and continue.

Until an end condition is satisfied. Then you break out.

It's two, actually:

https://minimal-agent.com/

This example has one, but if you add a second one around their example code, and take user input, and there's a basic harness already.

That's the most basic version of a harness. But a harness is really about automating context delivery to the LLM based on your use-case.

A pluggable loop with events and hooks.