stripe-integration-expert

Stripe Integration Expert

Security Scan

The skill is a developer guide for Stripe integrations and appears legitimate in purpose, but its runtime instructions reference many secrets, environment variables, and application internals that are not declared in the skill metadata — this mismatch is concerning and should be resolved before trusting the skill with credentials or automated invocation.

Purpose & Capability

The SKILL.md is a full Stripe integration guide and includes code that expects STRIPE_SECRET_KEY, various STRIPE_*_PRICE_ID values, NEXT_PUBLIC_APP_URL, webhook signing secrets, and access to application DB/auth helpers. The registry metadata declares no required environment variables or primary credential. That omission is incoherent: a Stripe integration legitimately needs Stripe credentials and related config, so the declared requirements are incomplete.

Instruction Scope

The instructions are focused on implementing Stripe flows (checkout sessions, subscription changes, idempotent webhook handlers, invoice previews). They do not appear to instruct the agent to exfiltrate data or call unknown endpoints, but they do assume access to application internals (getAuthUser, db) and environment variables. The guide may prompt the agent to ask for or use secrets and DB state at runtime — the SKILL.md itself does not explicitly limit how the agent should obtain or use those secrets.

Install Mechanism

There is no install spec and no code files executed by the platform; this is instruction-only, which minimizes supply-chain risk from downloads or extracted archives.

Credentials

The skill makes substantive use of environment variables and secrets (e.g., STRIPE_SECRET_KEY, price IDs, NEXT_PUBLIC_APP_URL, webhook secret) but the skill metadata declares none and no primary credential. That mismatch is disproportionate: the number and sensitivity of env vars shown in examples should be declared explicitly (STRIPE_SECRET_KEY at minimum).

Persistence & Privilege

The skill is not marked always:true and has no install behavior, so it does not request permanent system presence. Note: model invocation is not disabled (default), so if you grant the skill access to secrets the agent could potentially use them autonomously — consider this if you provide credentials.

What to consider before installing

This skill looks like a useful Stripe integration guide, but the SKILL.md expects multiple sensitive environment variables (Stripe secret key, price IDs, webhook signing secret, app URL) and access to your app's DB/auth helpers while the registry metadata lists none. Before installing or invoking it: (1) ask the publisher to update the metadata to declare required env vars and the primary credential (STRIPE_SECRET_KEY) so you know what will be requested; (2) never paste your Stripe secret key or webhook signing secret into an unfamiliar skill — treat them like passwords; (3) prefer running this guide locally and copying code manually rather than giving the skill live access to your environment or secrets; (4) if you must provide credentials for testing, use a throwaway/test Stripe account and isolated environment; (5) inspect any produced code for places that might read other files or send data to unexpected endpoints; and (6) if you want to allow autonomous invocation, only do so after the author declares required env vars and you are confident about how secrets will be used.

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