Writing a useful CLAUDE.md file is essential for maximizing the effectiveness of the Claude coding agent. Since LLMs, like Claude, are stateless, they start each session without any knowledge of your codebase. The CLAUDE.md file acts as an onboarding tool, providing critical information about your project's tech stack, structure, and purpose. It should include what the project is about, why it exists, and how to work within it, but it shouldn’t be overloaded with every possible command. Simplicity is key. Research shows that LLMs can reliably follow around 150-200 instructions. Smaller models struggle with larger sets of instructions, while larger models handle them better, but even they lose effectiveness as instruction counts rise. Claude often disregards information in the CLAUDE.md file if it considers it irrelevant to current tasks. This means that the contents should be focused and universally applicable. Avoid specific instructions that may not apply to all tasks to reduce the chances of Claude ignoring relevant context. To further enhance the utility of CLAUDE.md, use the principle of Progressive Disclosure. Instead of cramming all instructions into one file, keep task-specific guidelines in separate markdown files. This allows Claude to access only the information it needs when it needs it. For example, you might have files for building the project, running tests, or coding conventions, and reference these in your CLAUDE.md. This strategy helps maintain clarity and ensures that the context presented to the model is relevant and concise, improving its overall performance.