Parallel AI agents built a fitness attendance app after a shared contract was set

A developer describes building an attendance app for a fitness coach using Claude, with one orchestrator directing four agents working on separate parts simultaneously. Before any agents started, the project required a written plan with business rules, a shared TypeScript API contract of 28 types, a design system, and stub screens assigned to owners. Each agent owned specific files and reported needed shared changes instead of editing them, which avoided merge conflicts. The first full version, about 53 files and 8,900 lines, arrived 16 minutes after the foundation commit. Later work covered deployment, a security and GDPR pass, and participant account features, with the author concluding that defining the shape and rules remains the human's job.