When designing backend system architecture, choosing between API styles, or making database scaling decisions. When reviewing backend code for production readiness, performance bottlenecks, or architectural anti-patterns. Also when planning authentication architecture or error handling strategy across service boundaries. For code quality patterns use clean-code instead. For database-specific queries use database instead. For security reviews use security-best-practices instead. For containerization use docker-expert instead. For NestJS-specific work use nestjs-expert instead. For debugging errors use error-resolver instead.