What problem does it solve? Building recurring billing systems involves financial correctness risks like duplicate charges, lost webhooks, incorrect proration, and unreconciled ledgers that are hard to get right without a disciplined model. ## Core Features & Use Cases - Billing State Machine Design: Model the full subscription lifecycle including trials, renewals, plan changes, pauses, cancellations, failed payments, refunds, and disputes with explicit effective dates. - Correctness Invariants: Enforce idempotent event handlers, verified webhook authenticity, integer minor-unit monetary values, and traceable financial adjustments. - Reconciliation & Operations Guidance: Reconcile provider transactions, invoices, entitlements, and accounting exports while monitoring renewal success, involuntary churn, and webhook lag. - Use Case: When integrating a payment provider like Stripe into a SaaS product, use this Skill to design the subscription lifecycle, verify webhook handling against duplicate and out-of-order delivery, and define a proration policy before writing code. ## Quick Start Ask the assistant to design a subscription billing lifecycle with proration, dunning, and webhook idempotency for your payment provider integration.
