Express.js HTTP APIs AI Skill Guide (Claude)

Overview & Engine Architecture

Express is a minimal Node HTTP framework of middleware chains and routers. Requests flow top-to-bottom; handlers must end the cycle or call next(). Express 5 forwards rejected promises from async handlers to error middleware automatically; Express 4 needs an async wrapper or explicit .catch(next). Agents validate input (zod/joi), mount 4-arg error middleware last, set trust proxy to hop count—not true, and keep persistence in @prisma / @postgresql with workers for slow jobs (@redis queues).