AFAIK the two most popular open source harnesses right now are OpenCode and Pi. They take a pretty different approach, OpenCode includes a lot of features while Pi is very minimal by design and focused on extensibility, to the point where many people are just asking Pi to write a plugin for itself whenever they want it to have a new feature. I personally like Pi's philosophy more and I think its developer justified the choices really well in his blog post:

https://mariozechner.at/posts/2025-11-30-pi-coding-agent/#to... (the pi-coding-agent section)

The common term for a tool that wraps an LLM with a workflow is “harness”.