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 covers four tools:

Codex CLI by OpenAI
Claude Code by Anthropic
Gemini CLI by Google
Antigravity CLI (agy) by Google — the successor to Gemini CLI

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 Antigravity CLI (agy) — Gemini CLI’s free access shuts down June 18, 2026.
Want to compare them	Read Codex vs Claude Code vs Gemini CLI vs agy after trying one small task.

The First Hour

Choose one tool. Do not start with all four 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

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

Topic	Codex CLI	Claude Code	Gemini CLI	Antigravity CLI (agy)
Command	`codex`	`claude`	`gemini`	`agy`
Install	npm or Homebrew	Native installer	npm, Homebrew, or npx	Official install script
Login	ChatGPT or OpenAI API key	Claude/Console or provider	Google sign-in, API key, or Vertex AI	Google sign-in
Instruction file	`AGENTS.md`	`CLAUDE.md`	`GEMINI.md`	`.md` rule files (auto-discovery)
Script mode	`codex exec`	`claude -p`	`gemini -p`	`agy -p`
MCP	Yes	Yes	Yes	Yes

Which one should you start with?

Start with Codex CLI

Good if you already use OpenAI, ChatGPT, Codex, AGENTS.md, code review, and local repositories.

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. Note: the free Google sign-in access shuts down on June 18, 2026 — after that a paid Gemini API key or Code Assist Standard/Enterprise is required.

Start with Antigravity CLI (agy)

Good if you have a Google account and want a fast, lightweight agent. agy is Google’s successor to Gemini CLI — a single binary with Google sign-in and MCP support.

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

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.

Antigravity CLI (agy) Install Google's new agy CLI — the successor to Gemini CLI. A single fast binary with MCP support.

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

Last checked: April 11, 2026.