/stripe-integration-expert

name: "stripe-integration-expert"
description: "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. Use when integrating Stripe for the first time, debugging webhook reliability issues, migrating from a different payment provider, or adding usage-based billing to an existing subscription product."

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.

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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 | 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.da