Rewriting a Node.js Service in Go With AI Agents

Checkly rewrote its Node.js Results Daemon, which processes roughly 92 million messages daily, in Go using Claude Code, achieving a production launch without incidents while reducing running pods by 70%, database active sessions by 60%, and database CPU use by about 15%. The effort depended on a strict black-box test harness built before coding, using golden-file comparisons against the legacy service, real infrastructure containers, controlled boundary emulators, Playwright, Docker Compose, and Toxiproxy to validate normal and failure scenarios. Test cases were generated from production-like data extracted from the company’s data lake and assessed with high code-coverage targets, though the team found that coverage alone could miss edge cases. An initial rollout exposed a major mismatch between local and production queue topology, causing retry failures and demonstrating the importance of modeling every infrastructure boundary precisely and verifying assumptions. After correcting the harness and retry logic, Checkly gradually migrated internal users and then customer cohorts through feature flags, monitoring each stage while CI required both old and new implementations to pass the same tests. Human review remained necessary to remove inherited design problems, simplify runtime configuration, and improve observability, while Go’s type system and the harness increased confidence in agent-generated changes and accelerated subsequent development.

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