Billing Subscriptions

Billing & Subscriptions Skill

Multi-provider billing system with Gateway Factory pattern. Supports Stripe, Polar.sh, and future providers through a unified interface.

Architecture Overview

BILLING ARCHITECTURE:

Configuration Layer:
contents/themes/{theme}/config/billing.config.ts
├── provider: 'stripe' | 'polar'  # (paddle, lemonsqueezy, mercadopago: type defined, not yet implemented)
├── currency: 'usd' | 'eur' | ...
├── defaultPlan: 'free'
├── features: { featureSlug: FeatureDefinition }
├── limits: { limitSlug: LimitDefinition }
├── plans: PlanDefinition[]
└── actionMappings: ActionMappings

Core Library:
core/lib/billing/
├── config-types.ts      # BillingConfig, PlanDefinition interfaces
├── types.ts             # PlanType, SubscriptionStatus, PaymentProvider
├── schema.ts            # Zod validation schemas
├── gateways/
│   ├── types.ts         # Provider-agnostic result types
│   ├── interface.ts     # BillingGateway interface (contract)
│   ├── factory.ts       # getBillingGateway() factory
│   ├── stripe.ts        # StripeGateway implements BillingGateway
│   └── polar.ts         # PolarGateway implements BillingGateway
├── queries.ts           # Database queries
├── enforcement.ts       # Limit/feature enforcement
├── helpers.ts           # Utility functions
└── jobs.ts              # Background jobs

Services Layer:
core/lib/services/
├── subscription.service.ts  # Subscription CRUD (uses factory)
├── plan.service.ts          # Plan management (getPriceId generic)
└── usage.service.ts         # Usage tracking

API Endpoints:
app/api/v1/billing/
├── checkout/route.ts        # Create checkout session (via factory)
├── portal/route.ts          # Customer portal access (via factory)
├── plans/route.ts           # List available plans
├── cancel/route.ts          # Cancel subscription (via factory)
├── change-plan/route.ts     # Upgrade/downgrade
├── check-action/route.ts    # Permission check
├── webhooks/stripe/route.ts # Stripe webhooks (provider-specific)
└── webhooks/polar/route.ts  # Polar webhooks (provider-specific)

When to Use This Skill

• Implementing billing features
• Working with subscription management
• Configuring plans and features
• Adding a new payment provider
• Setting up webhooks (Stripe or Polar)
• Implementing usage limits
• Testing billing flows

Gateway Factory Pattern

Key Principle: Consumers never import from a specific provider. They use getBillingGateway() which returns the correct implementation based on billing.config.ts.

BillingGateway Interface

// core/lib/billing/gateways/interface.ts
export interface BillingGateway {
  // Checkout
  createCheckoutSession(params: CreateCheckoutParams): Promise<CheckoutSessionResult>
  createPortalSession(params: CreatePortalParams): Promise<PortalSessionResult>

  // Customers
  getCustomer(customerId: string): Promise<CustomerResult>
  createCustomer(params: CreateCustomerParams): Promise<CustomerResult>

  // Subscription Management
  updateSubscriptionPlan(params: UpdateSubscriptionParams): Promise<SubscriptionResult>
  cancelSubscriptionAtPeriodEnd(subscriptionId: string): Promise<SubscriptionResult>
  cancelSubscriptionImmediately(subscriptionId: string): Promise<SubscriptionResult>
  reactivateSubscription(subscriptionId: string): Promise<SubscriptionResult>

  // Webhooks (Stripe passes string signature, Polar passes headers Record)
  verifyWebhookSignature(payload: string | Buffer, signatureOrHeaders: string | Record<string, string>): WebhookEventResult

  // Dashboard & Metadata
  getProviderName(): string
  getSubscriptionDashboardUrl(externalSubscriptionId: string | null | undefined): string | null
  getResourceHintDomains(): { preconnect: string[]; dnsPrefetch: string[] }
}

Provider-Agnostic Types

// core/lib/billing/gateways/types.ts
// Return types - NO Stripe.* or Polar.* imports

export interface CheckoutSessionResult {
  id: string
  url: string | null
}

export interface PortalSessionResult {
  url: string
}

export interface SubscriptionResult {
  id: string
  status: string
  cancelAtPeriodEnd: boolean
}

export interface CustomerResult {
  id: string
  email: string | null
  name: string | null
}

export interface WebhookEventResult {
  id: string
  type: string
  data: Record<string, unknown>
}

Factory Usage

// CORRECT: Use factory everywhere
import { getBillingGateway } from '@nextsparkjs/core/lib/billing/gateways/factory'

const session = await getBillingGateway().createCheckoutSession(params)
const portal = await getBillingGateway().createPortalSession(params)
await getBillingGateway().cancelSubscriptionAtPeriodEnd(subId)

// Provider metadata
const name = getBillingGateway().getProviderName()             // "Stripe" or "Polar"
const url = getBillingGateway().getSubscriptionDashboardUrl(id) // Dashboard URL or null

// Resource hints (used in layout.tsx automatically)
import { getBillingResourceHints } from '@nextsparkjs/core/lib/billing/gateways/factory'
const { preconnect, dnsPrefetch } = getBillingResourceHints()

// WRONG: Import from specific provider
import { createCheckoutSession } from '.../gateways/stripe'  // DEPRECATED

Plan Price IDs

Plans use providerPriceIds for price configuration (works with any provider):

// PlanDefinition in config-types.ts
{
  slug: 'pro',
  providerPriceIds: {
    monthly: 'price_xxx_monthly',
    yearly: 'price_xxx_yearly',
  },
}

// PlanService.getPriceId() reads from providerPriceIds
const priceId = PlanService.getPriceId('pro', 'monthly')

Provider: Stripe

SDK & Packages

# Required
pnpm add stripe

Environment Variables

STRIPE_SECRET_KEY=sk_test_...
STRIPE_PUBLISHABLE_KEY=pk_test_...
STRIPE_WEBHOOK_SECRET=whsec_...

StripeGateway Class

// core/lib/billing/gateways/stripe.ts
export class StripeGateway implements BillingGateway {
  async createCheckoutSession(params) {
    const session = await getStripe().checkout.sessions.create({
      mode: 'subscription',
      payment_method_types: ['card'],
      line_items: [{ price: priceId, quantity: 1 }],
      success_url: params.successUrl,
      cancel_url: params.cancelUrl,
      metadata: { teamId, planSlug, billingPeriod },
    })
    return { id: session.id, url: session.url }  // Provider-agnostic
  }

  async createPortalSession(params) {
    const session = await getStripe().billingPortal.sessions.create({
      customer: params.customerId,
      return_url: params.returnUrl,
    })
    return { url: session.url }
  }

  verifyWebhookSignature(payload, signature) {
    const event = getStripe().webhooks.constructEvent(payload, signature, secret)
    return { id: event.id, type: event.type, data: event.data }
  }

  // ... other methods
}

Stripe Webhook Events

Stripe Webhook Route (Provider-Specific)

// app/api/v1/billing/webhooks/stripe/route.ts
// NOTE: Webhook routes stay provider-specific by design.
// They need raw provider types for proper type narrowing.
import Stripe from 'stripe'

const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!)

export async function POST(request: NextRequest) {
  const payload = await request.text()
  const signature = request.headers.get('stripe-signature')!
  const event = stripe.webhooks.constructEvent(
    payload, signature, process.env.STRIPE_WEBHOOK_SECRET!
  )

  switch (event.type) {
    case 'checkout.session.completed':
      const session = event.data.object as Stripe.Checkout.Session
      // Handle with full Stripe types
      break
  }
}

Provider: Polar.sh

SDK & Packages

# Core SDK
pnpm add @polar-sh/sdk

# Next.js adapter (optional - pre-built route handlers)
pnpm add @polar-sh/nextjs

# Better Auth plugin (optional - auth-integrated billing)
pnpm add @polar-sh/better-auth

Environment Variables

POLAR_ACCESS_TOKEN=pat_...          # Organization Access Token
POLAR_WEBHOOK_SECRET=whsec_...
POLAR_SERVER=sandbox                 # 'sandbox' or 'production'

Integration Path A: PolarGateway (via our factory)

This is the primary integration path. Consistent with Stripe, goes through getBillingGateway().

// core/lib/billing/gateways/polar.ts
import { Polar } from '@polar-sh/sdk'
import { validateEvent, WebhookVerificationError } from '@polar-sh/sdk/webhooks'
import type { BillingGateway } from './interface'

let polarInstance: Polar | null = null

function getPolar(): Polar {
  if (!polarInstance) {
    polarInstance = new Polar({
      accessToken: process.env.POLAR_ACCESS_TOKEN!,
      server: (process.env.POLAR_SERVER as 'sandbox' | 'production') || 'production',
    })
  }
  return polarInstance
}

export class PolarGateway implements BillingGateway {
  async createCheckoutSession(params) {
    const priceId = getPriceIdFromRegistry(params.planSlug, params.billingPeriod)
    const result = await getPolar().checkouts.create({
      productPriceId: priceId,           // Polar uses productPriceId
      successUrl: params.successUrl,
      returnUrl: params.cancelUrl,       // Polar calls it returnUrl
      customerEmail: params.customerEmail,
      metadata: { teamId: params.teamId, planSlug: params.planSlug },
    })
    return { id: result.id, url: result.url }
  }

  async createPortalSession(params) {
    const result = await getPolar().customerSessions.create({
      customerId: params.customerId,
      returnUrl: params.returnUrl,
    })
    return { url: result.customerPortalUrl }  // Different field name
  }

  async getCustomer(customerId) {
    const customer = await getPolar().customers.get({ id: customerId })
    return { id: customer.id, email: customer.email, name: customer.name }
  }

  async createCustomer(params) {
    const customer = await getPolar().customers.create({
      email: params.email,
      name: params.name,
      externalId: params.metadata?.userId,  // Map to app userId
    })
    return { id: customer.id, email: customer.email, name: customer.name }
  }

  async updateSubscriptionPlan(params) {
    const result = await getPolar().subscriptions.update({
      id: params.subscriptionId,
      subscriptionUpdate: { productPriceId: params.newPriceId },
    })
    return {
      id: result.id,
      status: result.status,
      cancelAtPeriodEnd: result.cancelAtPeriodEnd,
    }
  }

  async cancelSubscriptionAtPeriodEnd(subscriptionId) {
    // Polar: customer portal cancel = cancel at period end
    const result = await getPolar().subscriptions.update({
      id: subscriptionId,
      subscriptionUpdate: { cancelAtPeriodEnd: true },
    })
    return { id: result.id, status: result.status, cancelAtPeriodEnd: true }
  }

  async cancelSubscriptionImmediately(subscriptionId) {
    // Polar: revoke = immediate cancel
    const result = await getPolar().subscriptions.revoke({ id: subscriptionId })
    return { id: result.id, status: 'canceled', cancelAtPeriodEnd: false }
  }

  async reactivateSubscription(subscriptionId) {
    const result = await getPolar().subscriptions.update({
      id: subscriptionId,
      subscriptionUpdate: { cancelAtPeriodEnd: false },
    })
    return { id: result.id, status: result.status, cancelAtPeriodEnd: false }
  }

  verifyWebhookSignature(payload, signature) {
    // Polar validates against headers object, not a single signature string
    // The signature param here carries the headers as JSON for compatibility
    const headers = JSON.parse(signature as string)
    const event = validateEvent(payload, headers, process.env.POLAR_WEBHOOK_SECRET!)
    return { id: event.data.id, type: event.type, data: event.data as any }
  }
}

Polar Webhook Events

Polar Webhook Route (Provider-Specific)

// app/api/v1/billing/webhooks/polar/route.ts
import { validateEvent, WebhookVerificationError } from '@polar-sh/sdk/webhooks'

export async function POST(request: NextRequest) {
  const payload = await request.text()
  const headers = Object.fromEntries(request.headers.entries())

  try {
    const event = validateEvent(payload, headers, process.env.POLAR_WEBHOOK_SECRET!)

    switch (event.type) {
      case 'order.paid':
        await handleOrderPaid(event.data)
        break
      case 'subscription.active':
        await handleSubscriptionActive(event.data)
        break
      case 'subscription.canceled':
        await handleSubscriptionCanceled(event.data)
        break
      case 'subscription.revoked':
        await handleSubscriptionRevoked(event.data)
        break
      case 'subscription.uncanceled':
        await handleSubscriptionUncanceled(event.data)
        break
    }

    return NextResponse.json({ received: true })
  } catch (error) {
    if (error instanceof WebhookVerificationError) {
      return NextResponse.json({ error: 'Invalid signature' }, { status: 403 })
    }
    throw error
  }
}

Polar Next.js Adapter (Shortcut)

Pre-built route handlers for simple setups:

// app/api/polar/checkout/route.ts
import { Checkout } from '@polar-sh/nextjs'

export const GET = Checkout({
  accessToken: process.env.POLAR_ACCESS_TOKEN!,
  successUrl: process.env.NEXT_PUBLIC_APP_URL + '/dashboard/settings/billing?success=true',
  server: 'sandbox',
})

// app/api/polar/portal/route.ts
import { CustomerPortal } from '@polar-sh/nextjs'

export const GET = CustomerPortal({
  accessToken: process.env.POLAR_ACCESS_TOKEN!,
  getCustomerId: (req) => getCustomerIdFromSession(req),
  returnUrl: process.env.NEXT_PUBLIC_APP_URL + '/dashboard/settings/billing',
})

// app/api/polar/webhooks/route.ts
import { Webhooks } from '@polar-sh/nextjs'

export const POST = Webhooks({
  webhookSecret: process.env.POLAR_WEBHOOK_SECRET!,
  onOrderPaid: async (payload) => { /* sync subscription */ },
  onSubscriptionActive: async (payload) => { /* activate */ },
  onSubscriptionCanceled: async (payload) => { /* cancel */ },
  onSubscriptionRevoked: async (payload) => { /* revoke */ },
})

Integration Path B: Better Auth Plugin

For projects using Better Auth, Polar provides a first-party plugin that auto-creates customers on signup and provides client-side billing methods.

// lib/auth.ts (server)
import { betterAuth } from 'better-auth'
import { polar, checkout, portal, usage, webhooks } from '@polar-sh/better-auth'
import { Polar } from '@polar-sh/sdk'

const polarClient = new Polar({
  accessToken: process.env.POLAR_ACCESS_TOKEN!,
  server: 'sandbox',
})

export const auth = betterAuth({
  plugins: [
    polar({
      client: polarClient,
      createCustomerOnSignUp: true,   // Auto-create Polar customer
      use: [
        checkout({
          products: [
            { productId: 'prod_xxx', slug: 'pro' },
            { productId: 'prod_yyy', slug: 'enterprise' },
          ],
          successUrl: '/dashboard/settings/billing?success=true',
          authenticatedUsersOnly: true,
        }),
        portal(),
        usage(),
        webhooks({
          secret: process.env.POLAR_WEBHOOK_SECRET!,
          onOrderPaid: (payload) => { /* handle */ },
          onCustomerStateChanged: (payload) => { /* handle */ },
        }),
      ],
    }),
  ],
})

// lib/auth-client.ts (client)
import { createAuthClient } from 'better-auth/react'
import { polarClient } from '@polar-sh/better-auth/client'

export const authClient = createAuthClient({
  plugins: [polarClient()],
})

// Usage in components:
await authClient.checkout({ products: ['prod_xxx'], slug: 'pro' })
await authClient.customer.portal()
const { data: state } = await authClient.customer.state()
const { data: subs } = await authClient.customer.subscriptions.list({ query: { active: true } })

// Usage-based billing
await authClient.usage.ingest({ event: 'api-call', metadata: { endpoint: '/api/v1/products' } })
const { data: meters } = await authClient.usage.meters.list()

When to Use Path A vs Path B

For NextSpark core: Use Path A (PolarGateway) to maintain provider abstraction. Optionally layer Path B for customer auto-creation on signup.

Provider Comparison: API Mapping

Key Differences

1. Webhook verification: Stripe validates against a single stripe-signature header. Polar validates against ALL request headers.
2. Cancel semantics: Stripe uses cancel_at_period_end flag + cancel(). Polar uses revoke() for immediate, update for period-end.
3. Customer identity: Polar supports externalId to map customers to your app's userId. Stripe uses metadata.
4. Subscription update: Stripe requires finding the subscription item ID first. Polar just takes the new productPriceId directly.
5. Better Auth: Polar has a first-party Better Auth plugin. Stripe does not.

Three-Layer Permission Model

The billing system uses a three-layer model (provider-agnostic):

RESULT = Permission (RBAC) AND Feature (Plan) AND Quota (Limits)

Layer 1: RBAC Permissions

actionMappings: {
  permissions: {
    'team.billing.manage': 'team.billing.manage',
    'team.settings.edit': 'team.settings.edit',
  }
}

Layer 2: Plan Features

features: {
  advanced_analytics: { name: 'billing.features.advanced_analytics' },
  api_access: { name: 'billing.features.api_access' },
}

actionMappings: {
  features: {
    'analytics.view_advanced': 'advanced_analytics',
    'api.generate_key': 'api_access',
  }
}

Layer 3: Usage Limits (Quotas)

limits: {
  team_members: { name: 'billing.limits.team_members', unit: 'count', resetPeriod: 'never' },
  api_calls: { name: 'billing.limits.api_calls', unit: 'calls', resetPeriod: 'monthly' },
}

actionMappings: {
  limits: {
    'team.members.invite': 'team_members',
    'api.call': 'api_calls',
  }
}

Plans Configuration

Plan Definition Structure

// contents/themes/default/config/billing.config.ts
import type { BillingConfig } from '@/core/lib/billing/config-types'

export const billingConfig: BillingConfig = {
  provider: 'stripe',  // or 'polar'
  currency: 'usd',
  defaultPlan: 'free',

  plans: [
    {
      slug: 'free',
      name: 'billing.plans.free.name',
      type: 'free',
      visibility: 'public',
      price: { monthly: 0, yearly: 0 },
      features: ['basic_analytics'],
      limits: { team_members: 3, tasks: 50, api_calls: 1000 },
      // No price IDs for free plan
    },
    {
      slug: 'pro',
      name: 'billing.plans.pro.name',
      type: 'paid',
      visibility: 'public',
      price: { monthly: 2900, yearly: 29000 },  // in cents
      trialDays: 14,
      features: ['basic_analytics', 'advanced_analytics', 'api_access'],
      limits: { team_members: 15, tasks: 1000, api_calls: 100000 },

      // Price IDs from your payment provider dashboard
      providerPriceIds: {
        monthly: 'price_pro_monthly',
        yearly: 'price_pro_yearly',
      },
    },
    {
      slug: 'enterprise',
      name: 'billing.plans.enterprise.name',
      type: 'enterprise',
      visibility: 'hidden',
      features: ['*'],
      limits: { team_members: -1, tasks: -1, api_calls: -1 },
    },
  ],
}

Team-Based Subscriptions

Subscriptions are tied to teams, not users (provider-agnostic DB schema):

{
  id: string
  teamId: string
  planId: string
  status: SubscriptionStatus   // 'active' | 'trialing' | 'past_due' | 'canceled' | 'expired' | 'paused'
  billingInterval: 'monthly' | 'yearly'
  paymentProvider: PaymentProvider | null  // 'stripe' | 'polar' | 'paddle' | 'lemonsqueezy'
  externalSubscriptionId?: string         // Provider subscription ID
  externalCustomerId?: string             // Provider customer ID
  currentPeriodStart: Date
  currentPeriodEnd: Date
  cancelAtPeriodEnd: boolean
}

Checkout Flow (Provider-Agnostic)

// app/api/v1/billing/checkout/route.ts
import { getBillingGateway } from '@nextsparkjs/core/lib/billing/gateways/factory'

export async function POST(request: NextRequest) {
  // 1. Authenticate + validate + check permissions
  // ...

  // 2. Create checkout session via gateway (works for any provider)
  const session = await getBillingGateway().createCheckoutSession({
    teamId,
    planSlug,
    billingPeriod,
    successUrl: `${appUrl}/dashboard/settings/billing?success=true`,
    cancelUrl: `${appUrl}/dashboard/settings/billing?canceled=true`,
    customerEmail: user.email,
    customerId: existingCustomerId,
  })

  return Response.json({
    success: true,
    data: { url: session.url, sessionId: session.id }
  })
}

Customer Portal (Provider-Agnostic)

// app/api/v1/billing/portal/route.ts
import { getBillingGateway } from '@nextsparkjs/core/lib/billing/gateways/factory'

const session = await getBillingGateway().createPortalSession({
  customerId: subscription.externalCustomerId,
  returnUrl: `${appUrl}/dashboard/settings/billing`,
})

return Response.json({ success: true, data: { url: session.url } })

Webhook Security

// CRITICAL: Always verify webhook signatures regardless of provider

// Stripe: single signature header
const event = getStripeInstance().webhooks.constructEvent(payload, signature, secret)

// Polar: validates against all headers
import { validateEvent } from '@polar-sh/sdk/webhooks'
const event = validateEvent(payload, headers, secret)

// NOTE: Webhooks bypass RLS (no user context)
// Use direct query() calls, not queryWithRLS()

Database Schema

Why Plans/Subscriptions Use Inline JSONB

Migrations

-- Plans table
CREATE TABLE plans (
  id TEXT PRIMARY KEY DEFAULT gen_random_uuid()::text,
  slug TEXT UNIQUE NOT NULL,
  name TEXT NOT NULL,
  type plan_type NOT NULL DEFAULT 'free',
  visibility plan_visibility NOT NULL DEFAULT 'public',
  "priceMonthly" INTEGER DEFAULT 0,
  "priceYearly" INTEGER DEFAULT 0,
  "trialDays" INTEGER DEFAULT 0,
  features JSONB DEFAULT '[]',
  limits JSONB DEFAULT '{}',
  -- Price IDs stored in providerPriceIds (via billing.config.ts)
  -- Legacy: stripePriceIdMonthly/Yearly columns may exist in older migrations but are unused
  "createdAt" TIMESTAMPTZ DEFAULT NOW(),
  "updatedAt" TIMESTAMPTZ DEFAULT NOW()
);

-- Subscriptions table (provider-agnostic)
CREATE TABLE subscriptions (
  id TEXT PRIMARY KEY DEFAULT gen_random_uuid()::text,
  "teamId" TEXT REFERENCES teams(id) ON DELETE CASCADE,
  "planId" TEXT REFERENCES plans(id),
  status subscription_status NOT NULL DEFAULT 'active',
  "billingInterval" TEXT DEFAULT 'monthly',
  "paymentProvider" TEXT,                -- 'stripe' | 'polar' | etc.
  "externalSubscriptionId" TEXT,         -- Provider sub ID
  "externalCustomerId" TEXT,             -- Provider customer ID
  "currentPeriodStart" TIMESTAMPTZ,
  "currentPeriodEnd" TIMESTAMPTZ,
  "cancelAtPeriodEnd" BOOLEAN DEFAULT false,
  "createdAt" TIMESTAMPTZ DEFAULT NOW(),
  "updatedAt" TIMESTAMPTZ DEFAULT NOW()
);

Environment Variables

# Provider selection (in billing.config.ts, not env)
# provider: 'stripe' | 'polar'

# Stripe
STRIPE_SECRET_KEY=sk_test_...
STRIPE_PUBLISHABLE_KEY=pk_test_...
STRIPE_WEBHOOK_SECRET=whsec_...

# Polar
POLAR_ACCESS_TOKEN=pat_...
POLAR_WEBHOOK_SECRET=whsec_...
POLAR_SERVER=sandbox                  # 'sandbox' or 'production'

Anti-Patterns

// NEVER: Import from specific provider in consumers
import { createCheckoutSession } from '.../gateways/stripe'  // REMOVED

// CORRECT: Use factory
import { getBillingGateway } from '.../gateways/factory'
await getBillingGateway().createCheckoutSession(params)

// NEVER: Use Stripe-specific types in consumer code
const session: Stripe.Checkout.Session = ...

// CORRECT: Use provider-agnostic types
const session: CheckoutSessionResult = ...

// NEVER: Use getStripePriceId (removed)
PlanService.getStripePriceId('pro', 'monthly')

// CORRECT: Use generic getPriceId
PlanService.getPriceId('pro', 'monthly')

// NEVER: Hardcode plan prices in frontend
const price = '$29.00'

// CORRECT: Use plan config
const price = formatCurrency(plan.price.monthly / 100)

// NEVER: Check plan features manually
if (plan.slug === 'pro' || plan.slug === 'business')

// CORRECT: Use feature checks
const hasFeature = membership.hasFeature('advanced_analytics')

// NEVER: Skip webhook signature verification
const event = JSON.parse(payload)  // UNSAFE!

// CORRECT: Always verify signatures
// Stripe: constructEvent(payload, signature, secret)
// Polar:  validateEvent(payload, headers, secret)

// NEVER: Store prices in dollars
price: { monthly: 29.00 }

// CORRECT: Store prices in cents
price: { monthly: 2900 }

// NEVER: Forget to handle -1 (unlimited)
if (current >= limit) return false

// CORRECT: Check for unlimited
if (limit === -1) return true
if (current >= limit) return false

Checklist

General (all providers)

• Provider selected in billing.config.ts
• Plans defined with price IDs (providerPriceIds)
• Features and limits defined
• Action mappings configured
• Team-based subscription created on team creation
• Checkout flow tested (monthly and yearly)
• Portal flow tested
• Webhook endpoint configured in provider dashboard
• All webhook events handled and tested
• Usage tracking implemented
• Limit enforcement working
• Translations added for plan names/descriptions

Stripe-specific

• STRIPE_SECRET_KEY configured
• STRIPE_WEBHOOK_SECRET configured
• Stripe price IDs set in plan definitions
• Invoice sync working

Polar-specific

• POLAR_ACCESS_TOKEN configured (Organization Access Token)
• POLAR_WEBHOOK_SECRET configured
• POLAR_SERVER set to sandbox or production
• Polar product price IDs set in plan definitions
• Customer externalId mapping to userId working
• (Optional) Better Auth plugin configured for auto customer creation

Testing

Unit Tests

Both gateway implementations have comprehensive Jest unit tests:

packages/core/tests/jest/lib/billing/
├── stripe.test.ts    # 38 tests - StripeGateway, factory, deprecated compat
├── polar.test.ts     # 26 tests - PolarGateway, getPolarInstance
└── (billing-queries.test.ts)  # Billing query tests

Mock pattern: Mock the provider SDK, PlanService, and BILLING_REGISTRY before importing the gateway class.

// Example: Polar test mocks
jest.mock('@polar-sh/sdk', () => ({
  Polar: jest.fn().mockImplementation(() => ({
    checkouts: { create: mockCheckoutsCreate },
    customerSessions: { create: mockCustomerSessionsCreate },
    // ...
  }))
}))

jest.mock('@polar-sh/sdk/webhooks', () => ({
  validateEvent: mockValidateEvent,
  WebhookVerificationError: MockWebhookVerificationError,
}))

jest.mock('@/core/lib/services/plan.service', () => ({
  PlanService: { getPriceId: mockGetPriceId }
}))

Running tests:

# All billing tests
cd packages/core && npx jest --config jest.config.cjs tests/jest/lib/billing/

# Specific provider
npx jest --config jest.config.cjs tests/jest/lib/billing/polar.test.ts
npx jest --config jest.config.cjs tests/jest/lib/billing/stripe.test.ts

Related Skills

• permissions-system - RBAC integration
• better-auth - Authentication patterns (Polar Better Auth plugin)
• entity-api - API patterns for billing endpoints
• service-layer - Service class patterns
• database-migrations - Billing table migrations