If you're using AI coding agents, you've probably encountered multiple configuration formats: SKILL.md for skills, CLAUDE.md for Claude Code project instructions, and .cursorrules for Cursor. They serve different purposes and understanding when to use each one saves confusion and improves your workflow efficiency.

Understanding the Differences

SKILL.md, CLAUDE.md, and .cursorrules all aim to configure AI agents but differ in format, scope, persistence, and compatibility. Knowing these differences helps you pick the right tool for your specific needs. | Feature | SKILL.md | CLAUDE.md | .cursorrules | |---|---|---|---| | Format | YAML frontmatter + markdown | Plain markdown | Plain text | | Scope | Specific capability or workflow | Whole project | Whole project | | Persistence | Loaded on demand by trigger | Loaded every session | Loaded every session | | Cross-agent support | 20+ agents (Claude Code, OpenClaw, Codex, Cursor, Gemini CLI) | Claude Code, OpenClaw | Cursor only | | Use case | Add a reusable skill (e.g. code review, PR writer) | Onboard the agent to your codebase | Onboard the agent to your codebase |

SKILL.md: Portable, Task-Specific Automation

SKILL.md files are designed to add a reusable skill or capability to your AI coding agent. They are formatted with YAML frontmatter followed by markdown content. This structure allows you to define triggers, parameters, and step-by-step instructions for complex tasks. For example, if you want your AI agent to perform code reviews, generate pull request summaries, or refactor code in a specific way, you can create a SKILL.md file that encapsulates this workflow. Because SKILL.md supports 20+ agents like Claude Code, OpenClaw, Codex, and Cursor, it is highly portable across different environments.

Practical Steps to Create a SKILL.md

1. Define the skill name and description in the YAML frontmatter. 2. Write clear instructions or prompts in markdown explaining how the agent should perform the task. 3. Specify any input parameters or triggers that activate the skill. 4. Save the file in a directory accessible to your AI agent. 5. Invoke the skill via your agent’s interface or command.

Tip: Modularize Your Skills

Break down complex workflows into multiple SKILL.md files focused on atomic tasks, making them easier to maintain and reuse.

Real-world Use Case

A company wants to automate their PR review process. They create a SKILL.md titled `pr-review.md` that instructs the agent to analyze changes, check for code style, and provide suggestions. This skill is then used across multiple projects and agents, streamlining reviews and reducing manual effort.

CLAUDE.md: Whole Project Onboarding for Claude Code

CLAUDE.md is intended to onboard the Claude Code AI agent to your entire project. Placed at the root directory, it is read at the start of every session, ensuring the AI understands your codebase's structure, conventions, and team preferences. Unlike SKILL.md, CLAUDE.md is plain markdown without YAML frontmatter and is designed for broad instructions rather than task-specific automation. For example, you might describe the tech stack, testing frameworks, styling conventions, and directory layout.

How to Write Effective CLAUDE.md Files

1. Begin with a summary of the project: language, framework, and version. 2. List architectural patterns or important design decisions. 3. Describe folder structures and key files. 4. Outline coding standards and best practices. 5. Include any relevant links to documentation or resources. 6. Keep it updated as the project evolves.

Tip: Use Clear, Concise Language

The AI agent relies on this file to guide its decisions, so clarity helps reduce misunderstandings and improves output quality.

Real-world Use Case

A team working on a Next.js TypeScript project uses CLAUDE.md to inform Claude Code that tests are located in the `__tests__` folder, Tailwind CSS is used for styling, and all API calls must handle errors gracefully. This onboarding ensures the AI generates code consistent with project standards.

.cursorrules: Project Instructions for Cursor

.cursorrules is a plain text file aimed at configuring the Cursor AI coding agent. Like CLAUDE.md, it provides whole-project context and instructions that are loaded every session. .cursorules typically contain rules, preferences, or guidelines specific to the Cursor environment. Because it is plain text, it’s less structured than SKILL.md but serves well for straightforward onboarding.

Steps to Create .cursorrules

1. Create a `.cursorrules` file in the root of your project. 2. Write simple, clear rules or instructions relevant to your coding style or project requirements. 3. Save and commit it to your repository so that Cursor agents can access it.

Tip: Focus on Key Guidelines

Since .cursorrules lacks formatting, prioritize the most critical instructions to avoid ambiguity.

Real-world Use Case

An open-source project uses .cursorrules to specify that all code contributions must include unit tests and follow PEP8 style guidelines. When Cursor loads the project, it respects these rules, ensuring consistency.

When to Use Which Configuration?

Choosing between SKILL.md, CLAUDE.md, and .cursorrules depends on your goals and the AI agent you’re working with. 1. Use SKILL.md if you want to add a reusable, task-specific skill that can be triggered on demand and works across multiple AI coding agents. 2. Use CLAUDE.md to onboard Claude Code or OpenClaw agents at the project level, giving them a comprehensive understanding of your codebase. 3. Use .cursorrules if you are working exclusively with Cursor and need a straightforward way to provide project-specific rules and guidelines.

Example Scenario

Suppose you manage a large monorepo with multiple teams and AI agents. You might use CLAUDE.md to onboard Claude Code across the repo, .cursorrules for Cursor-specific guidelines, and SKILL.md files to share reusable code analysis or refactoring skills across teams.

Tips for Maximizing AI Agent Effectiveness

- Keep your instructions up to date: Outdated configuration files can mislead AI agents. - Be explicit but concise: Provide enough detail to guide the agent without overwhelming it. - Test changes incrementally: When modifying any of these files, test the AI’s behavior to confirm improvements. - Combine approaches when needed: Don’t hesitate to use SKILL.md alongside CLAUDE.md or .cursorrules for a layered approach.

Conclusion

In summary, SKILL.md, CLAUDE.md, and .cursorrules serve distinct but complementary roles in configuring AI coding agents. SKILL.md excels at modular, reusable skills across multiple agents; CLAUDE.md shines at comprehensive project onboarding for Claude Code; and .cursorrules provides simple project rules for Cursor. Understanding these differences empowers you to tailor AI behavior precisely, boosting productivity and code quality across your development workflows.