Patterns for getting the best results out of coding agents like [[Claude Code]] and [[OpenAI Codex]]. See [my introduction](https://simonwillison.net/2026/Feb/23/agentic-engineering-patterns/) for more on this project. 1. **Principles** 1. [["Writing code is cheap now" by Simon Willison|Writing code is cheap now]]. 2. [["Hoard things you know how to do" by Simon Willison|Hoard things you know how to do]] 3. [["Anti-patterns things to avoid" by Simon Willison|Anti-patterns: things to avoid]] 2. **Testing and QA** 1. [Red/green TDD](https://simonwillison.net/guides/agentic-engineering-patterns/red-green-tdd/) 2. [First run the tests](https://simonwillison.net/guides/agentic-engineering-patterns/first-run-the-tests/) 3. **Understanding code** 1. [Linear walkthroughs](https://simonwillison.net/guides/agentic-engineering-patterns/linear-walkthroughs/) 2. [Interactive explanations](https://simonwillison.net/guides/agentic-engineering-patterns/interactive-explanations/) 4. **Annotated prompts** 1. [GIF optimization tool using WebAssembly and Gifsicle](https://simonwillison.net/guides/agentic-engineering-patterns/gif-optimization/) 5. **Appendix** 1. [Prompts I use](https://simonwillison.net/guides/agentic-engineering-patterns/prompts/)