Claude Code の CLAUDE.md に書くべきルール4選 ― 調整して使うテンプレート付きThis article distills hands-on experience across multiple projects into four…

Anyone who has adopted Claude Code, Anthropic's command-line coding agent, or Cursor with its .cursorrules mechanism, has probably hit the same wall: you carefully set up a configuration file, yet the assistant still does not behave the way you intended. A recently published Zenn post addresses that gap directly, distilling the author's hands-on experience across multiple projects into four rule categories worth writing into CLAUDE.md, each paired with a copy-paste template. For developers experimenting with agentic coding tools, the piece is positioned as a practical starting point rather than an abstract discussion of prompt design.

CLAUDE.md is the file that Claude Code reads to learn "the rules of your project." It functions as persistent context that the agent consults before acting, covering things a model cannot infer on its own, such as preferred conventions, project structure, and how the developer wants tasks carried out. The article's premise is that many people create this file but fill it with vague or arbitrary content because they are unsure what actually belongs there. The result is a configuration that exists on paper but does little to steer the model's behavior, which is the very outcome the author says prompted them to write the guide.

Rather than offering an exhaustive checklist, the post narrows its recommendations to four rule types the author found consistently useful, and it emphasizes that the templates are meant to be dropped in and adapted. The framing suggests that concreteness and specificity matter more than volume: an instruction the agent can act on unambiguously appears to be more effective than a long, loosely worded document. This aligns with a broader lesson in prompt engineering, where clearly scoped constraints and explicit expectations tend to produce more predictable output than open-ended requests. Because the article's value lies in the ready-made templates, readers are likely to get the most out of it by copying the examples and tailoring them to their own stack and workflow.

Some background helps explain why this topic resonates. Configuration files like CLAUDE.md and Cursor's .cursorrules belong to a growing pattern in AI-assisted development: giving the model a durable, project-level source of truth so that guidance does not have to be repeated in every session. The underlying constraint is the context window. Because a model can only consider a limited amount of text at once, a well-structured rules file acts as a compact, high-signal briefing rather than forcing the developer to re-explain the project each time. Poorly organized or bloated instructions can crowd out more useful information, which is part of why curation, not comprehensiveness, is often the goal.

The tooling landscape around these files has expanded quickly. Cursor popularized the .cursorrules approach within its editor, while Claude Code brings a similar idea to a terminal-based workflow, and other agentic tools such as aider and various IDE integrations have adopted comparable conventions. GitHub Copilot and similar assistants have likewise moved toward repository-aware and instruction-file features. The common thread is a shift from single-shot autocompletion toward agents that plan, edit multiple files, and run commands, which raises the stakes for clear, machine-readable project rules. In that context, advice on what to write in these files has become a small but active genre among developer bloggers.

It is worth treating the four rules as one practitioner's distilled opinion rather than an official standard. Anthropic and other vendors continue to iterate on how their agents parse and prioritize such files, so behavior can change across model and tool versions, and what works well in one repository may need adjustment in another. Readers should also remember that a rules file guides an agent's behavior but does not guarantee it; verification, testing, and human review remain necessary.

For anyone frustrated that their setup is not paying off, the article's contribution is straightforward: a focused, reusable template set grounded in real project work, aimed at turning CLAUDE.md from a neglected placeholder into a file that measurably shapes how Claude Code and similar agents operate. The specifics of each rule are best read in the original post, but the general takeaway is that deliberate, concrete instructions are what make these configuration files worth maintaining.