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
│
▼
ACTIVEDB 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 constCheckout 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 9995Feature 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: truein 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
userIdin metadata; can't link subscription to user without it