the one by https://earendil.com/posts/what-is-a-harness/ is also a good read

and if you want to see what a simple harness implementation looks like you can just dive into the code and ask your favorite agent about it

https://github.com/smol-env/smol

https://github.com/earendil-works/pi

Interesting: I initially confused smol for https://github.com/huggingface/smolagents