How We Refactored Our API from Express 4 to NestJS 11 and Cut Bugs by 30%

For three years, our core REST API ran on Express 4. It powered 12 downstream services, handled 40k requests per minute, and served 2M+ monthly active users. But as our team grew from 4 to 14 backend engineers, Express’s unopinionated structure started to hurt: inconsistent error handling, scattered middleware, no built-in dependency injection, and a 22% monthly bug rate tied to routing conflicts and untested edge cases.

After 6 months of planning and incremental migration, we fully moved to NestJS 11. The result? A 30% reduction in production bugs, 40% faster onboarding for new engineers, and a 25% drop in time spent on API maintenance. Here’s how we did it.

Why We Outgrew Express 4

Express 4 served us well early on.…