Start with Codex CLI
Good if you already use OpenAI, ChatGPT, Codex, AGENTS.md, code review, and local repositories.
AI CLIs: getting started
Beginner
15 min
AI CLIs are terminal tools that act as local coding agents. They can read your project, propose changes, write files, and run commands. That makes them useful, but also worth starting with carefully.
This section starts with three tools:
- Codex CLI by OpenAI
- Claude Code by Anthropic
- Gemini CLI by Google
Shortest Path
Section titled “Shortest Path”| If you… | Do this first |
|---|---|
| Have never used an AI CLI | Read this page, start in a test repository, and ask the tool to explain the project without changes. |
| Already use ChatGPT/Codex | Start with Codex CLI and run a local /review after the first change. |
| Already use Claude | Start with Claude Code and create a short CLAUDE.md. |
| Already use Google/Gemini | Start with Gemini CLI and choose your auth method before API automation. |
| Want to compare them | Read Codex vs Claude Code vs Gemini CLI after trying one small task. |
The First Hour
Section titled “The First Hour”- Choose one tool. Do not start with all three at once.
- Use a real Git repository, preferably one where mistakes are cheap.
- Run
git status, so you know what is already changed. - Ask the CLI to explain the project without running commands.
- Let it propose test/build commands.
- Approve one small change.
- Run build/test and review the diff.
5 Terms to Know
Section titled “5 Terms to Know”| Term | Meaning |
|---|---|
| Workspace | The folder the agent works from. Use the concrete project root. |
| Sandbox | Technical limits on what commands can read, write, and access over the network. |
| Approval | The prompt where you approve a command, file edit, or other action. |
| Instruction file | AGENTS.md, CLAUDE.md, or GEMINI.md, used for stable project rules. |
| Context | Everything the agent receives: prompt, files, instructions, logs, MCP data, and prior conversation. More context can help, but costs more. |
Quick comparison
Section titled “Quick comparison”| Topic | Codex CLI | Claude Code | Gemini CLI |
|---|---|---|---|
| Command | codex | claude | gemini |
| Install | npm or Homebrew | Native installer | npm, Homebrew, or npx |
| Login | ChatGPT or OpenAI API key | Claude/Console or provider | Google sign-in, API key, or Vertex AI |
| Instruction file | AGENTS.md | CLAUDE.md | GEMINI.md |
| Script mode | codex exec | claude -p | gemini -p |
| MCP | Yes | Yes | Yes |
Which one should you start with?
Section titled “Which one should you start with?”Start with Claude Code
Good if you want strong conversation workflows, project memory, and clear permissions.
Start with Gemini CLI
Good if you want an open source terminal agent with Google sign-in or a Gemini API key.
Safe baseline
Section titled “Safe baseline”Before you start:
- Open a real Git repository.
- Run
git status. - Create a commit or branch if needed.
- Start your AI CLI.
- Ask it to read and explain first, not edit.
Good first prompt:
Explain this project from the repository files. Do not make changes.Tell me which test or build commands you would run, but do not run them yet.Starter guides
Section titled “Starter guides” Codex CLI Install OpenAI Codex CLI and run your first safe session.
Claude Code Install Claude Code and understand CLAUDE.md, permissions, and first prompts.
Gemini CLI Install Gemini CLI, sign in with Google or API key, and get started.
Pricing and plans Understand subscriptions, API keys, token prices, and how to avoid expensive surprises.
Project folders and workflows Place AGENTS.md, CLAUDE.md, and GEMINI.md correctly, and start the CLI in the right folder.
Sources
Section titled “Sources”Last checked: April 11, 2026.