stripe-integration-expert

# Stripe Integration Expert

**Tier:** POWERFUL  
**Category:** Engineering Team  
**Domain:** Payments / Billing Infrastructure

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## Overview

Implement production-grade Stripe integrations: subscriptions with trials and proration, one-time payments, usage-based billing, checkout sessions, idempotent webhook handlers, customer portal, and invoicing. Covers Next.js, Express, and Django patterns.

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## Core Capabilities

- Subscription lifecycle management (create, upgrade, downgrade, cancel, pause)
- Trial handling and conversion tracking
- Proration calculation and credit application
- Usage-based billing with metered pricing
- Idempotent webhook handlers with signature verification
- Customer portal integration
- Invoice generation and PDF access
- Full Stripe CLI local testing setup

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## When to Use

- Adding subscription billing to any web app
- Implementing plan upgrades/downgrades with proration
- Building usage-based or seat-based billing
- Debugging webhook delivery failures
- Migrating from one billing model to another

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## Subscription Lifecycle State Machine

```
FREE_TRIAL ──paid──► ACTIVE ──cancel──► CANCEL_PENDING ──period_end──► CANCELED
     │                  │                                                    │
     │               downgrade                                            reactivate
     │                  ▼                                                    │
     │             DOWNGRADING ──period_end──► ACTIVE (lower plan)           │
     │                                                                        │
     └──trial_end without payment──► PAST_DUE ──payment_failed 3x──► CANCELED
                                          │
                                     payment_success
                                          │
                                          ▼
                                        ACTIVE
```

### DB subscription status values:
`trialing | active | past_due | canceled | cancel_pending | pa...