Prompt to Add Stripe Checkout to a Next.js App

AI agent prompt to add Stripe Checkout with webhook handling, customer portal, and subscription status to a Next.js App Router project.

Give this prompt to your agent to implement the complete Stripe billing flow — checkout session creation, webhook processing, customer portal, and subscription status gating — with proper webhook signature verification and no secrets in client code.

Main Prompt

You are working in a Next.js 15 App Router project with TypeScript and PostgreSQL.
Auth is already set up with a `getSession()` helper. The pricing page already defines
`PLANS` with Stripe Price IDs.

Task: wire up Stripe Checkout for subscription billing.

Requirements:
- Install `stripe` (server-only) and `@stripe/stripe-js` (client). Do NOT import `stripe` in
  Client Components or expose `STRIPE_SECRET_KEY` to the browser.
- Create `src/lib/stripe.ts`: export a singleton Stripe client using `STRIPE_SECRET_KEY`.
- Create a Server Action `src/lib/actions/create-checkout.ts`:
  - Get the current user session; return an error if not authenticated.
  - Create a Stripe Checkout Session in `subscription` mode.
  - Set `success_url` to `/dashboard?session_id={CHECKOUT_SESSION_ID}`.
  - Set `cancel_url` to `/pricing`.
  - Store the Stripe `customerId` in the `users` table (`stripe_customer_id` column).
  - Return the Checkout Session URL.
- Create `src/app/api/stripe/webhook/route.ts`:
  - Read the raw body using `request.text()`.
  - Verify the signature using `stripe.webhooks.constructEvent(body, sig, STRIPE_WEBHOOK_SECRET)`.
  - Handle events: `checkout.session.completed`, `customer.subscription.updated`,
    `customer.subscription.deleted`.
  - On each event, update the `users` table: set `subscription_status` and `subscription_tier`.
  - Return `{ received: true }` with status 200.
  - On signature failure, return 400.
- Create `src/lib/actions/create-portal.ts`: create a Stripe Customer Portal session and return the URL.
- Add a PostgreSQL migration `migrations/0011_add_stripe_columns.sql` adding
  `stripe_customer_id TEXT`, `subscription_status TEXT`, `subscription_tier TEXT` to `users`.
- Add all Stripe keys to `.env.example`: `STRIPE_SECRET_KEY`, `STRIPE_WEBHOOK_SECRET`,
  `NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY`.
- Do NOT use the deprecated `stripe.charges` API. Use Payment Intents / Checkout Sessions only.

Stop and list all planned file changes before writing any code.

Implementation Notes

Webhook signature verification requires the raw request body — if you parse it as JSON first, the signature check will fail. Use request.text() not request.json().
The webhook handler must be excluded from Next.js body size limits via a route config: export const config = { api: { bodyParser: false } } (Pages Router) — in App Router, use export const dynamic = 'force-dynamic' and read the stream directly.
Test webhooks locally with the Stripe CLI: stripe listen --forward-to localhost:3000/api/stripe/webhook.
Store stripe_customer_id on first checkout to avoid creating duplicate Stripe customers.

Expected File Changes

src/lib/stripe.ts                           (new)
src/lib/actions/create-checkout.ts          (new — Server Action)
src/lib/actions/create-portal.ts            (new — Server Action)
src/app/api/stripe/webhook/route.ts         (new — Route Handler)
migrations/0011_add_stripe_columns.sql      (new)
.env.example                                (edited)
package.json                                (edited)

Acceptance Criteria

Clicking a pricing CTA redirects to Stripe Checkout for the correct plan.
Completing a test checkout updates subscription_status = 'active' in PostgreSQL.
The Stripe Customer Portal link redirects to the Stripe-hosted portal.
Sending a test customer.subscription.deleted event via Stripe CLI sets subscription_status = 'canceled'.
A webhook with an invalid signature returns HTTP 400.

Test Commands

bun add stripe @stripe/stripe-js
psql "$DATABASE_URL" -f migrations/0011_add_stripe_columns.sql
bun run typecheck
bun run dev &
stripe listen --forward-to localhost:3000/api/stripe/webhook
stripe trigger checkout.session.completed
# verify users table updated
psql "$DATABASE_URL" -c "SELECT stripe_customer_id, subscription_status FROM users LIMIT 5;"

Common AI Mistakes

Importing stripe (server SDK) in a Client Component, exposing STRIPE_SECRET_KEY.
Parsing the webhook body as JSON before signature verification, causing all webhook validations to fail.
Creating a new Stripe customer on every checkout instead of reusing stripe_customer_id.
Using stripe.charges.create (deprecated) instead of stripe.checkout.sessions.create.

Fix Prompt

Webhook signature verification fails with `No signatures found matching the expected signature`.
Fix in order:
1. In the webhook route, replace `await request.json()` with `const body = await request.text()`.
2. Ensure `stripe.webhooks.constructEvent(body, sig, process.env.STRIPE_WEBHOOK_SECRET!)` receives
   the raw string body, not a parsed object.
3. Add `export const dynamic = 'force-dynamic'` at the top of the route file.
Show only the corrected route handler diff.