Build Hour: API & Codex – Agentic AI Development Workshop is an advanced technical session focused on the evolution of AI-powered software engineering. The workshop introduces Codex as an agentic development tool that can handle complex programming tasks, from planning and coding to testing and deployment. It demonstrates how modern AI agents are moving beyond simple code suggestions to full task delegation, where systems can independently execute software engineering workflows. Learners are introduced to OpenAI API capabilities and how they integrate with Codex to build intelligent development environments. The session also explores key concepts such as hosted execution environments, agent skills, and real-time communication between AI systems. A major focus is placed on production-grade AI workflows, including how to evaluate agent reliability using structured methods like agent legibility and harness engineering principles. The workshop includes practical demonstrations of how AI agents can improve developer productivity by automating repetitive tasks, managing large codebases, and generating pull requests. It is designed for developers and engineers interested in building scalable AI-powered systems. By the end of the course, learners will understand how Codex and OpenAI APIs work together to enable the next generation of autonomous software