Crypto Fintech — 13 Apps in 70 Days
The challenge
Build a complete crypto fintech — payment gateway, OTC exchange, fiat-to-crypto bridge. Solo. Production-ready. In 70 days.
The method: Spec Driven Development
Every feature started as a specification, not a prompt. AI agents (Claude Code) executed against those specs. I reviewed and shipped. No vibe coding — every line of generated code traced back to a spec decision.
What was built
- 13 applications in a Turborepo monorepo
- 3 Fastify APIs serving production traffic
- 3 PostgreSQL databases with multi-tenant architecture
- PIX integration — Brazil’s government-backed instant payment system (3B+ transactions/month, similar to UPI in India or FedNow in the US) — connected to 4 payment service providers via factory pattern, HMAC-SHA256 webhook validation, and mTLS client certificates
- OTC exchange engine with VWAP pricing (24h rolling window), cross-rates from 5 sources with median + outlier filtering, asymmetric bid/ask spreads
- Exchange settler with on-chain deposit detection (10s polling), 2-block confirmation, tolerance-based auto-refund, $10K auto-settle cap, atomic state transitions, crash recovery
- OAuth 2.1/OIDC auth server: 5 roles, 19 scopes, 18 granular permissions, JWT/JWKS, M2M tokens, 2FA (TOTP)
- Kubernetes (DigitalOcean DOKS, HPA 2-10 replicas), Terraform IaC, Prometheus/Grafana monitoring
- CI/CD: GitHub Actions + Vitest + Playwright E2E
Tech stack
TypeScript, Node.js, Fastify, PostgreSQL, Drizzle ORM, Redis, BullMQ, Kubernetes, Terraform, Docker, Claude Code.
Key insight
The more capable the model, the further it goes in the wrong direction with a bad instruction. That’s why specification matters more than prompting.