Building AI agents requires more than prompt engineering - it demands thoughtful architectural decisions. This talk explores the system design choices that make or break production AI. This talk navigates the key decisions that we faced at HappyFox: choosing between workflows and multi-agent systems, engineering context for reliability, and building your agent toolbox right. We'll tackle the hard problems - maintaining systems across model changes, implementing security without breaking functionality, and handling privacy in production - covering the full stack of production AI architecture.