There are no shortage of good harnesses out there, and you can re-use coding ones like pi, opencode, dsh even claude and codex. For most use-cases that is already the loop you need. If you're looking for something less like a coding agent then Vercel's Eve is okay too.
But you can't just plug those into your already existing stack easily, unlike a purely API-based solution no?
To have a really good harness, it seems that you need a separate one for every different project you are working with.