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Codex CLI is OpenAI's open-source coding agent that runs directly in your terminal. It can read, modify, and execute code on your machine. Written in Rust, it starts fast and performs well.
Installation
# npm
npm i -g @openai/codex
# Homebrew
brew install --cask codexSupports macOS and Linux; use via WSL on Windows. Codex is included with ChatGPT Plus, Pro, Business, Edu, and Enterprise plans.
Core Features
| Feature | Description |
|---|---|
| MCP Support | Configure STDIO or streaming HTTP servers in ~/.codex/config.toml; Codex itself can also act as an MCP server |
| Subagents | Spawns sub-agents only when explicitly requested, enabling parallel execution of large tasks |
| Image Input | Paste screenshots or design mockups — Codex reads and incorporates visual details |
| Code Review | A dedicated Codex agent reviews your code before commits or pushes |
| Automation | Use the exec command to script repetitive workflows |
| Skills | Bundle instructions, resources, and scripts so Codex can reliably connect to tools and run workflows |
Model Evolution
codex-1 (early 2025)
The earliest cloud version of Codex, an o3 model optimized for software engineering. It handled tasks in parallel — writing features, answering codebase questions, fixing bugs, opening PRs.
GPT-5-Codex (late 2025)
GPT-5 further optimized for agentic coding, good at both fast interactive conversation and long-running autonomous work. OpenAI rebuilt Codex CLI around agentic coding workflows based on community feedback.
The three GPT-5.6 tiers (current)
The main line is now Sol / Terra / Luna: Sol for depth, Terra as the everyday default, Luna for speed and cost. You pick via the Power setting (Smarter ↔ Faster); the default is gpt-5.6-sol at medium reasoning.
Two dates matter: GPT-5.4 and GPT-5.4 mini retire from ChatGPT-signed-in Codex on 2026/8/31 (replaced by Terra and Luna), and gpt-5.2 and gpt-5.3-codex were deprecated earlier. Update any script or codex exec --model invocation that hardcodes an old model ID. The bring-your-own-API-key path is unaffected.
For model and billing detail, see OpenAI Codex: A Complete Plan Analysis in this series.
Codex App vs. Codex CLI
| Codex CLI | Codex App | |
|---|---|---|
| Interface | Terminal | Desktop app (macOS / Windows) |
| Highlights | Lightweight, scriptable | Multi-agent management, parallel tasks, long-running collaboration |
| Open Source | ✅ | ❌ |
The Codex App added Windows support in March 2026, offering a more visual interface for managing multiple agents.
Typical Use Cases
- Bug fixing: Describe the problem — Codex reads the relevant files, pinpoints the issue, and proposes a fix
- Feature development: Provide a spec and Codex generates code step by step for you to review
- Codebase Q&A: Ask "what does this function do?" and Codex reads the source code and answers
- Automation scripts: Use
codex execto chain CI/CD pipelines or repetitive daily tasks
Positioning vs. Other Tools
Codex CLI's core advantage lies in its deep integration with OpenAI's own models (codex-1, GPT-5-Codex) and the complete ecosystem spanning from the cloud-based Codex App to the local CLI. If you're already using the OpenAI API, Codex CLI is the most seamless choice.
Resources
- GitHub - openai/codex
- Codex CLI Official Docs
- Codex CLI Features
- Introducing Codex
- Codex Product Page
References
- OpenAI Codex CLI GitHub: openai/codex open-source terminal coding agent
- Introducing Codex: OpenAI official blog announcement for Codex CLI
Changelog
- 2026-08-18: Added the current GPT-5.6 Sol / Terra / Luna line and the 2026/8/31 GPT-5.4 retirement date (the post previously stopped at codex-1 and GPT-5-Codex); noted that the three surfaces share one
config.toml; added to the Choosing an Agent CLI series
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