Polar is an open-source, full-stack monetization platform designed for AI-era software developers. It serves as a Merchant of Record, handling checkout flows, subscriptions, usage-based billing, seats, discounts, and global tax compliance. The system is built as a monorepo with a Python/FastAPI backend (async SQLAlchemy, Dramatiq workers, PostgreSQL, Redis, Tinybird for event analytics), a Next.js web dashboard and customer portal, a React Native mobile app (Expo), a design system (Orbit), and multiple SDKs (TypeScript, Python) with a CLI tool. Architecture follows a service-oriented pattern with versioned REST APIs (2026-04, 2026-10, 2027-01), a repository pattern for persistence, benefit fulfillment strategies for integrations (Discord, GitHub, Slack, license keys), and framework adapters (Next.js, Nuxt, TanStack Start, Better Auth) for easy integration. The platform also includes an internal backoffice (HTMX/tagflow), observability (SLOs, invariants, Prometheus metrics), and merchant migration tooling from Stripe.