Content about #Harness Engineering

I’ve been working on Kobako for a few months now, and I’ll also be presenting the project at COSCUP. The process has been interesting enough that I think it deserves a series of posts about how I set out to build a sandbox for the AI era.

By the time of writing, there have been around 20 releases. From here on I’ll walk through Kobako’s evolution as close to chronological order as I can, along with the decisions made along the way.

Developing Kobako has surfaced quite a few interesting cases. Continuing from the previous post, Building Kobako with AI: Will It Eventually Crash?, I ran into yet another new problem afterward—and this time it was the Segmentation Fault error I had always dreaded seeing during development. That signals a high chance something went wrong in the non-Ruby territory between Rust and WebAssembly.

At RubyKaigi 2026 this year, while chatting with friends about Harness Engineering and the Sandbox aspect of it, I realized that Ruby still lacks a solid solution in this space. On the second night, I happened to be assigned to the dRuby (Ruby’s built-in distributed object system) group at the Code Party — and that’s how the idea for Kobako was born.

The Ralph Loop technique — a trick for making AI Coding Agents automatically repeat execution — was popular last year, but I didn’t trust whether it was safe enough or sufficiently stable.

A few months ago, Claude Code’s update introduced the /loop skill, which essentially uses Claude Code’s built-in Cron tool to repeat specific prompts at fixed intervals, achieving a similar effect. At least it’s safer and more controllable.

However, things are rarely that simple.