The smarter the models get the less the harnesses matter (outside of devx).
Maybe one day I'll run it through swebech.
I also wrote one by myself last week (just for fun and learning). It works, including integration with configured mcpServers (like you do in most coding agents). Wrote about the whole step-by-step process and what is needed at what step and why: https://nb1t.sh/building-a-real-agent-step-by-step/
For my daily work I like letting different harnesses compete and look over each others work (while subsidized with the subscriptions) so I use OpenADE.