Agent tooling today is either TypeScript or Python, so I wrote an agent harness for Ruby. Mistri has zero dependencies and stays out of your app’s way: resumable sessions stored as append-only entries, steering a run while it is still running, sub-agents, tool approvals, and one interface for Anthropic, OpenAI and Gemini. It runs a production assistant today. Docs and recipes at https://mistri.sh, source at https://github.com/mcheemaa/mistri. Feedback welcome, especially on the session model.