How coding agents work
Simon Willison Blog
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Generative AI
Agentic Engineering Patterns > As with any tool, understanding how coding agents work under the hood can help you make better decisions about how to apply them. A coding agent is a piece of software that acts as a harness for an LLM, extending that LLM with additional capabilities that are powered by invisible prompts and implemented as callable tools. Large Language Models At the heart of any coding agent is a Large Language Model, or LLM. These have names like GPT-5.4 or Claude Opus 4.6 or Gemini 3.1 Pro or Qwen3.5-35B-A3B.