CLAUDE.md is useful only when it changes behavior.
A vague instruction does not help much. A concrete rule does.
Bad instructions are too broad
Examples like “write good code” or “be careful” sound reasonable.
But they do not tell Claude Code what to do differently.
Good instructions are testable
A useful instruction is specific.
For example:
- Run this command before finishing.
- Do not edit this folder.
- Use this naming pattern.
- Keep the final report short.
These rules are easy to follow.
Put local facts first
The best CLAUDE.md files explain the project.
Where are the important files? How do you test? What should never be changed without asking?
Keep it short
A long rule file can become noise.
Write the rules that prevent real mistakes. Leave out the rest.
Original Japanese Article
This article is based on the Japanese post: CLAUDE.mdの書き方：効く文と効かない文の違い.
