This article explores how software developers can integrate ChatGPT Codex into their development workflows, from initial code generation to pull request creation. Codex is OpenAI’s code-focused large language model, capable of reading and writing code, generating test cases, and interacting with GitHub repositories. We explain how to get the most out of Codex by combining it with unit tests and test-driven development (TDD), ensuring reliable and verifiable results. Drawing on real-world advice from Simon Willison, we emphasize why automated testing is not just a complement but a critical enabler of safe and effective AI-assisted software engineering.