How we turned a broken, vibe-coded MVP
into $18,400 in 30 days
InvoiceFlo (a B2B invoice generation and automated matching SaaS) was losing customers to failed payments, security warnings, and 8-second query times. We rebuilt their core infrastructure in 5 days.
Built by AI, broken under pressure.
InvoiceFlo’s founder used AI code generation to spin up a gorgeous MVP in under a week. On the surface, the UI looked beautiful, the marketing worked, and early users were eager to subscribe.
But underneath, the AI-generated code was full of critical logic gaps. When the founder launched, the app immediately began buckling under real-world traffic:
- Session Collisions: A misconfigured caching layer caused users to randomly see other companies' sensitive invoicing dashboards.
- Silent Stripe Failures: Webhooks failed signature verification, meaning users were successfully charged by Stripe but their accounts were never upgraded in the database.
- API Tokens Leaked: The database connection string and Stripe secret keys were accidentally exposed in the client-side JavaScript bundle, visible to anyone opening Chrome DevTools.
// ❌ AI-generated webhook handler (VULNERABLE)
export async function POST(req) {
const body = await req.json();
// WARNING: Exposing private secret on client
const STRIPE_SECRET = "sk_live_51P...";
// ⚠️ Missing Stripe signature validation!
// Anyone can spoof post requests to fake payments
const userId = body.data.object.metadata.userId;
await db.user.update({
where: { id: userId },
data: { plan: "pro" }
});
return Response.json({ success: true });
}How we fixed InvoiceFlo in 5 days
We audited the entire codebase, isolated the vulnerable parts, and replaced the AI shortcuts with production-grade architecture.
Auth Overhaul & Secret Isolation
Replaced the insecure token setup with robust, encrypted session cookies using iron-session. We removed all backend keys from client-side bundles and configured Next.js environment variable routing to guarantee that private credentials never touch the user's browser.
Stripe Webhook signature verification
Rewrote the Stripe handler from scratch. Configured webhook signature checks, set up retry queues for database synchronization, and added idempotency keys to ensure users are immediately, reliably upgraded when they pay.
Database indexing & query audit
Audited their database schema. Added missing indexes on key query tables (`userId`, `invoiceId`, and `status`). Resolved three nested N+1 loops in the invoice matching cron job, dropping page load latency from 8.2 seconds to under 150ms.
Testing & Production Handoff
Set up automated integration tests checking the payment and authentication pipelines, and configured a multi-stage GitHub Actions CI/CD deployment pipeline to prevent future AI bugs from breaking staging.
"Our prototype was in absolute pieces after launch. Payments were failing, and users were seeing incorrect dashboards. CompleteMySaaS jumped in, audited the mess, and had a rock-solid system running in 5 days. We processed $18,400 in our first month with zero customer complaints."
Zero bugs. Real subscription revenue.
With a secure, reliable backend, InvoiceFlo successfully launched on Product Hunt. The page-load speedups increased conversion rates by 22%, and the reliable payment pipeline ensured every user who signed up converted into recurring cash.
Instead of spending 8 hours a day responding to angry support tickets about broken billing, the founder spent their first 30 days focusing entirely on onboarding customers and refining their product strategy.