We Let AI Agents Rewrite a 92M-Message-a-Day Service in Go

Checkly rewrote its Node.js Results Daemon, which processes 92 million messages daily, in Go using Claude Code. The key was a test harness built before the agent wrote code, ensuring byte-for-byte parity. The rewrite shipped with zero incidents, cut running pods by 70%, and reduced database load. Go's type system also made the agent's output safer. The article details the harness design, test case generation from production data, and lessons learned from the rollout.