Cal.diy is a fully open-source, MIT-licensed fork of Cal.com, providing a self-hostable scheduling platform. It is a large monorepo containing a Next.js web application (apps/web) for booking pages, settings, and admin, a NestJS API (apps/api/v2) for platform REST endpoints, and extensive packages for business logic (packages/features), integrations (packages/app-store with 50+ apps), embeddable booking widgets (packages/embeds), and reusable React atoms (packages/platform/atoms). The architecture uses tRPC for client-server communication, Prisma ORM with PostgreSQL, Redis caching, and a custom dependency injection system (ioctopus) alongside NestJS DI. Key patterns include a tasker abstraction for async/sync background jobs (webhooks, emails, calendar sync), a versioned webhook payload builder factory, repository pattern over Prisma, and a consistent app-store integration structure. The platform exposes a public API with OAuth2, webhooks, and embeddable components for third-party developers. The codebase is actively maintained with frequent schema migrations and CI/CD pipelines, but exhibits some technical drift due to dual API surfaces (tRPC vs REST), stub implementations for enterprise features, and generated files that require regeneration.