How I Built an AI Bookkeeping SaaS as a Solo Developer (And What I'd Do Differently)

TL;DR: Got laid off from my dev job. Built an AI invoicing for freelancers tool plus full bookkeeping in 4 months. Tech stack: Next.js 16, Prisma, OpenAI, Clerk, Inngest. Launched on Product Hunt. Here's the architecture, the mistakes, and the one decision that saved me months.

The Backstory

I'm Dominic, a full-stack developer from Chengdu, China 🇨🇳. I spent years freelancing for international clients before taking a full-time programming job. Then I got laid off.

Instead of job hunting immediately, I decided to solve the problem that annoyed me most during my freelance years: bookkeeping.

Every month, I'd download CSVs from Stripe, PayPal, Wise, and my Chinese bank — each in a completely different format. I'd spend hours reformatting dates, converting European commas, matching currencies, and manually creating invoices. Then I'd awkwardly chase clients who were late on payments.