Built a full-stack real-time messaging system with WebSocket (Socket.IO) bidirectional event architecture achieving sub-100ms message delivery latency under local load tests. • Designed RESTful APIs with Express.js for user auth, chat history retrieval, and room management; persisted messages in MongoDB with indexed queries for fast history pagination. • Implemented JWT-based authentication with token refresh logic, protecting all API routes and Socket namespaces from unauthorized access. • Managed global frontend state (active rooms, online presence, message threads) using zustand, eliminating the need for prop drilling across deeply nested components.