I'm sure there are many more harnesses like Pi. Clai[0] (which I've written) is one of them. We also have hax[1] and aichat [2]. I made some additional comparisons here[3].

At the end of the day, it's quite easy to write together a bespoke agentic harness. So I'm sure there are countless additional ones. You don't really need a lib/open source one at all. Just make it write itself.

[0]: https://github.com/baalimago/clai [1]: https://github.com/OleksandrChekhovskyi/hax [2]: https://github.com/sigoden/aichat [3]: https://www.clai.chat/compare/