stripe-integration-expert

SKILL.md

Stripe Integration Expert

Tier: POWERFUL

Category: Engineering Team

Domain: Payments / Billing Infrastructure

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.

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

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

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 | paused | unpaid

Stripe Client Setup

// lib/stripe.ts
import Stripe from "stripe"

export const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!, {
  apiVersion: "2024-04-10",
  typescript: true,
  appInfo: {
    name: "myapp",
    version: "1.0.0",
  },
})

// Price IDs by plan (set in env)
export const PLANS = {
  starter: {
    monthly: process.env.STRIPE_STARTER_MONTHLY_PRICE_ID!,
    yearly: process.env.STRIPE_STARTER_YEARLY_PRICE_ID!,
    features: ["5 projects", "10k events"],
  },
  pro: {
    monthly: process.env.STRIPE_PRO_MONTHLY_PRICE_ID!,
    yearly: process.env.STRIPE_PRO_YEARLY_PRICE_ID!,
    features: ["Unlimited projects", "1M events"],
  },
} as const

Checkout Session (Next.js App Router)

// app/api/billing/checkout/route.ts
import { NextResponse } from "next/server"
import { stripe } from "@/lib/stripe"
import { getAuthUser } from "@/lib/auth"
import { db } from "@/lib/db"

export async function POST(req: Request) {
  const user = await getAuthUser()
  if (!user) return NextResponse.json({ error: "Unauthorized" }, { status: 401 })

  const { priceId, interval = "monthly" } = await req.json()

  // Get or create Stripe customer
  let stripeCustomerId = user.stripeCustomerId
  if (!stripeCustomerId) {
    const customer = await stripe.customers.create({
      email: user.email,
      name: "username-undefined"
      metadata: { userId: user.id },
    })
    stripeCustomerId = customer.id
    await db.user.update({ where: { id: user.id }, data: { stripeCustomerId } })
  }

  const session = await stripe.checkout.sessions.create({
    customer: stripeCustomerId,
    mode: "subscription",
    payment_method_types: ["card"],
    line_items: [{ price: priceId, quantity: 1 }],
    allow_promotion_codes: true,
    subscription_data: {
      trial_period_days: user.hasHadTrial ? undefined : 14,
      metadata: { userId: user.id },
    },
    success_url: `${process.env.NEXT_PUBLIC_APP_URL}/dashboard?session_id={CHECKOUT_SESSION_ID}`,
    cancel_url: `${process.env.NEXT_PUBLIC_APP_URL}/pricing`,
    metadata: { userId: user.id },
  })

  return NextResponse.json({ url: session.url })
}

Subscription Upgrade/Downgrade

// lib/billing.ts
export async function changeSubscriptionPlan(
  subscriptionId: string,
  newPriceId: string,
  immediate = false
) {
  const subscription = await stripe.subscriptions.retrieve(subscriptionId)
  const currentItem = subscription.items.data[0]

  if (immediate) {
    // Upgrade: apply immediately with proration
    return stripe.subscriptions.update(subscriptionId, {
      items: [{ id: currentItem.id, price: newPriceId }],
      proration_behavior: "always_invoice",
      billing_cycle_anchor: "unchanged",
    })
  } else {
    // Downgrade: apply at period end, no proration
    return stripe.subscriptions.update(subscriptionId, {
      items: [{ id: currentItem.id, price: newPriceId }],
      proration_behavior: "none",
      billing_cycle_anchor: "unchanged",
    })
  }
}

// Preview proration before confirming upgrade
export async function previewProration(subscriptionId: string, newPriceId: string) {
  const subscription = await stripe.subscriptions.retrieve(subscriptionId)
  const prorationDate = Math.floor(Date.now() / 1000)

  const invoice = await stripe.invoices.retrieveUpcoming({
    customer: subscription.customer as string,
    subscription: subscriptionId,
    subscription_items: [{ id: subscription.items.data[0].id, price: newPriceId }],
    subscription_proration_date: prorationDate,
  })

  return {
    amountDue: invoice.amount_due,
    prorationDate,
    lineItems: invoice.lines.data,
  }
}

Complete Webhook Handler (Idempotent)

// app/api/webhooks/stripe/route.ts
import { NextResponse } from "next/server"
import { headers } from "next/headers"
import { stripe } from "@/lib/stripe"
import { db } from "@/lib/db"
import Stripe from "stripe"

// Processed events table to ensure idempotency
async function hasProcessedEvent(eventId: string): Promise<boolean> {
  const existing = await db.stripeEvent.findUnique({ where: { id: eventId } })
  return !!existing
}

async function markEventProcessed(eventId: string, type: string) {
  await db.stripeEvent.create({ data: { id: eventId, type, processedAt: new Date() } })
}

export async function POST(req: Request) {
  const body = await req.text()
  const signature = headers().get("stripe-signature")!

  let event: Stripe.Event
  try {
    event = stripe.webhooks.constructEvent(body, signature, process.env.STRIPE_WEBHOOK_SECRET!)
  } catch (err) {
    console.error("Webhook signature verification failed:", err)
    return NextResponse.json({ error: "Invalid signature" }, { status: 400 })
  }

  // Idempotency check
  if (await hasProcessedEvent(event.id)) {
    return NextResponse.json({ received: true, skipped: true })
  }

  try {
    switch (event.type) {
      case "checkout.session.completed":
        await handleCheckoutCompleted(event.data.object as Stripe.Checkout.Session)
        break

      case "customer.subscription.created":
      case "customer.subscription.updated":
        await handleSubscriptionUpdated(event.data.object as Stripe.Subscription)
        break

      case "customer.subscription.deleted":
        await handleSubscriptionDeleted(event.data.object as Stripe.Subscription)
        break

      case "invoice.payment_succeeded":
        await handleInvoicePaymentSucceeded(event.data.object as Stripe.Invoice)
        break

      case "invoice.payment_failed":
        await handleInvoicePaymentFailed(event.data.object as Stripe.Invoice)
        break

      default:
        console.log(`Unhandled event type: ${event.type}`)
    }

    await markEventProcessed(event.id, event.type)
    return NextResponse.json({ received: true })
  } catch (err) {
    console.error(`Error processing webhook ${event.type}:`, err)
    // Return 500 so Stripe retries — don't mark as processed
    return NextResponse.json({ error: "Processing failed" }, { status: 500 })
  }
}

async function handleCheckoutCompleted(session: Stripe.Checkout.Session) {
  if (session.mode !== "subscription") return
  
  const userId = session.metadata?.userId
  if (!userId) throw new Error("No userId in checkout session metadata")

  const subscription = await stripe.subscriptions.retrieve(session.subscription as string)
  
  await db.user.update({
    where: { id: userId },
    data: {
      stripeCustomerId: session.customer as string,
      stripeSubscriptionId: subscription.id,
      stripePriceId: subscription.items.data[0].price.id,
      stripeCurrentPeriodEnd: new Date(subscription.current_period_end * 1000),
      subscriptionStatus: subscription.status,
      hasHadTrial: true,
    },
  })
}

async function handleSubscriptionUpdated(subscription: Stripe.Subscription) {
  const user = await db.user.findUnique({
    where: { stripeSubscriptionId: subscription.id },
  })
  if (!user) {
    // Look up by customer ID as fallback
    const customer = await db.user.findUnique({
      where: { stripeCustomerId: subscription.customer as string },
    })
    if (!customer) throw new Error(`No user found for subscription ${subscription.id}`)
  }

  await db.user.update({
    where: { stripeSubscriptionId: subscription.id },
    data: {
      stripePriceId: subscription.items.data[0].price.id,
      stripeCurrentPeriodEnd: new Date(subscription.current_period_end * 1000),
      subscriptionStatus: subscription.status,
      cancelAtPeriodEnd: subscription.cancel_at_period_end,
    },
  })
}

async function handleSubscriptionDeleted(subscription: Stripe.Subscription) {
  await db.user.update({
    where: { stripeSubscriptionId: subscription.id },
    data: {
      stripeSubscriptionId: null,
      stripePriceId: null,
      stripeCurrentPeriodEnd: null,
      subscriptionStatus: "canceled",
    },
  })
}

async function handleInvoicePaymentFailed(invoice: Stripe.Invoice) {
  if (!invoice.subscription) return
  const attemptCount = invoice.attempt_count
  
  await db.user.update({
    where: { stripeSubscriptionId: invoice.subscription as string },
    data: { subscriptionStatus: "past_due" },
  })

  if (attemptCount >= 3) {
    // Send final dunning email
    await sendDunningEmail(invoice.customer_email!, "final")
  } else {
    await sendDunningEmail(invoice.customer_email!, "retry")
  }
}

async function handleInvoicePaymentSucceeded(invoice: Stripe.Invoice) {
  if (!invoice.subscription) return

  await db.user.update({
    where: { stripeSubscriptionId: invoice.subscription as string },
    data: {
      subscriptionStatus: "active",
      stripeCurrentPeriodEnd: new Date(invoice.period_end * 1000),
    },
  })
}

Usage-Based Billing

// Report usage for metered subscriptions
export async function reportUsage(subscriptionItemId: string, quantity: number) {
  await stripe.subscriptionItems.createUsageRecord(subscriptionItemId, {
    quantity,
    timestamp: Math.floor(Date.now() / 1000),
    action: "increment",
  })
}

// Example: report API calls in middleware
export async function trackApiCall(userId: string) {
  const user = await db.user.findUnique({ where: { id: userId } })
  if (user?.stripeSubscriptionId) {
    const subscription = await stripe.subscriptions.retrieve(user.stripeSubscriptionId)
    const meteredItem = subscription.items.data.find(
      (item) => item.price.recurring?.usage_type === "metered"
    )
    if (meteredItem) {
      await reportUsage(meteredItem.id, 1)
    }
  }
}

Customer Portal

// app/api/billing/portal/route.ts
import { NextResponse } from "next/server"
import { stripe } from "@/lib/stripe"
import { getAuthUser } from "@/lib/auth"

export async function POST() {
  const user = await getAuthUser()
  if (!user?.stripeCustomerId) {
    return NextResponse.json({ error: "No billing account" }, { status: 400 })
  }

  const portalSession = await stripe.billingPortal.sessions.create({
    customer: user.stripeCustomerId,
    return_url: `${process.env.NEXT_PUBLIC_APP_URL}/settings/billing`,
  })

  return NextResponse.json({ url: portalSession.url })
}

Testing with Stripe CLI

# Install Stripe CLI
brew install stripe/stripe-cli/stripe

# Login
stripe login

# Forward webhooks to local dev
stripe listen --forward-to localhost:3000/api/webhooks/stripe

# Trigger specific events for testing
stripe trigger checkout.session.completed
stripe trigger customer.subscription.updated
stripe trigger invoice.payment_failed

# Test with specific customer
stripe trigger customer.subscription.updated \
  --override subscription:customer=cus_xxx

# View recent events
stripe events list --limit 10

# Test cards
# Success: 4242 4242 4242 4242
# Requires auth: 4000 0025 0000 3155
# Decline: 4000 0000 0000 9995
# Insufficient funds: 4000 0000 0000 9995

Feature Gating Helper

// lib/subscription.ts
export function isSubscriptionActive(user: { subscriptionStatus: string | null, stripeCurrentPeriodEnd: Date | null }) {
  if (!user.subscriptionStatus) return false
  if (user.subscriptionStatus === "active" || user.subscriptionStatus === "trialing") return true
  // Grace period: past_due but not yet expired
  if (user.subscriptionStatus === "past_due" && user.stripeCurrentPeriodEnd) {
    return user.stripeCurrentPeriodEnd > new Date()
  }
  return false
}

// Middleware usage
export async function requireActiveSubscription() {
  const user = await getAuthUser()
  if (!isSubscriptionActive(user)) {
    redirect("/billing?reason=subscription_required")
  }
}

Common Pitfalls

Webhook delivery order not guaranteed — always re-fetch from Stripe API, never trust event data alone for DB updates

Double-processing webhooks — Stripe retries on 500; always use idempotency table

Trial conversion tracking — store hasHadTrial: true in DB to prevent trial abuse

Proration surprises — always preview proration before upgrade; show user the amount before confirming

Customer portal not configured — must enable features in Stripe dashboard under Billing → Customer portal settings

Missing metadata on checkout — always pass userId in metadata; can't link subscription to user without it