Agentic Engineering Patterns - Simon Willison's Weblog

Simon Willison’s Weblog

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Agentic Engineering Patterns

Patterns for getting the best results out of coding agents like Claude Code and OpenAI Codex. See my introduction for more on this project.

Principles
What is agentic engineering?
Agentic engineering
Isn't this just vibe coding?
About this guide
Writing code is cheap now
Good code still has a cost
We need to build new habits
Hoard things you know how to do
Recombining things from your hoard
Coding agents make this even more powerful
AI should help us produce better code
Avoiding taking on technical debt
Coding agents can handle these for us
AI tools let us consider more options
Embrace the compound engineering loop
Anti-patterns: things to avoid
Inflicting unreviewed code on collaborators
Working with coding agents
How coding agents work
Large Language Models
Chat templated prompts
Token caching
Calling tools
The system prompt
Reasoning
LLM + system prompt + tools in a loop
Using Git with coding agents
Git essentials
Core concepts and prompts
Rewriting history
Subagents
Claude Code’s Explore subagent
Parallel subagents
Specialist subagents
Official documentation
Testing and QA
Red/green TDD
First run the tests
Agentic manual testing
Mechanisms for agentic manual testing
Using browser automation for web UIs
Have them take notes with Showboat
Understanding code
Linear walkthroughs
An example using Showboat and Present
Interactive explanations
Understanding word clouds
Annotated prompts
GIF optimization tool using WebAssembly and Gifsicle
The follow-up prompts
Adding a new content type to my blog-to-newsletter tool
Appendix
Prompts I use
Artifacts
Proofreader
Alt text
Podcast highlights
Disclosures
Colophon
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