Build your own /init command

Claude’s /init makes it easy to add clear repo instructions. Build your own and use it with any agent to add or improve on an existing AGENTS.md Here’s the one I came up with.

Claude Code really nailed the onboarding experience for agentic coding. Open it, type /init, and you get a CLAUDE.md that delivers better results than a repo without proper system instructions (or an AGENTS.md).

It’s a clever way to ramp a repo fast. As I wrote last time, it hits one of the three levers for successful AI coding - seeding the right context. Even Codex CLI now comes with a built-in init prompt.

There’s no secret1 sauce: /init is just a strong prompt that writes (or improves) an instructions file.

Here’s the prompt, per folks who’ve reverse‑engineered it:

Please analyze this codebase and create a CLAUDE.md file, which will be given to future instances of Claude Code to operate in this repository.

What to add:

1. Commands that will be commonly used, such as how to build, lint, and run tests. Include the necessary commands to develop in this codebase, such as how to run a single test.
2. High-level code architecture and structure so that future instances can be productive more quickly. Focus on the "big picture" architecture that requires reading multiple files to understand

Usage notes:

- If there's already a CLAUDE.md, suggest improvements to it.
- When you make the initial CLAUDE.md, do not repeat yourself and do not include obvious instructions like "Provide helpful error messages to users", "Write unit tests for all new utilities", "Never include sensitive information (API keys, tokens) in code or commits"
- Avoid listing every component or file structure that can be easily discovered
- Don't include generic development practices
- If there are Cursor rules (in .cursor/rules/ or .cursorrules) or Copilot rules (in .github/copilot-instructions.md), make sure to include the important parts.
- If there is a README.md, make sure to include the important parts.
- Do not make up information such as "Common Development Tasks", "Tips for Development", "Support and Documentation" unless this is expressly included in other files that you read.
- Be sure to prefix the file with the following text:

You can write your own and get the same result. I use a custom /initialize on new repos to get up and running fast2. I tweaked it to work across different coding agents and sprinkled in a few tips I collected along the way.

It should create a relevant AGENTS.md; if one exists, it updates it. Save this prompt as a custom command and use it with any tool — Gemini CLI, Codex, Amp, Firebender, etc. You aren’t stuck with any single tool.

One more tip: a reasoning model works best for these types of commands.

I must say: the more time I spend with these tools, the more “emperor‑has‑no‑clothes” moments I have. Some of the ways these things work are deceptively simple.

  1. Claude does a few other things, like instructing its inner agent tools (BatchTool & GlobTool) to collect related files and existing instructions (package*.json, *.md, .cursor/rules/**, .github/copilot-instructions.md, etc.) as context for generating or updating CLAUDE.md. But the prompt is the meat. ↩︎

  2. I used this prompt when I vibe‑engineered a maintainable Firefox add‑on. ↩︎