Shopify's 12-week rewrite: agents wrote code, humans rebuilt the loop
Thirty Shopify engineers moved a 300+ screen app from React Native to Swift and Kotlin in 12 weeks, with agents writing most of the implementation. The post is about what surrounds the code: business logic runnable without a device, the Reflex architecture and a CLI that loads any screen in milliseconds, one shared architecture across iOS and Android, an example and .md file per screen, and CI that fails when the two platforms diverge. Spec-driven development was abandoned because specs went stale in days; the legacy React Native app became the reference. The second half covers the bottleneck shift — laptops saturating on emulators, review and merge queues, tickets lagging behind Slack-reported fixes — and the responses: a remote dev environment for agents, adversarial agent-first review, and a self-healing loop that reproduces crashes every 15 minutes. Results: Android launch 1.71s to 0.87s, binary 80.4MB to 47.6MB.