Codex CLI: A Complete Guide to OpenAI's Open-Source Terminal Coding Agent

TL;DR Codex CLI is OpenAI's open-source terminal coding agent built in Rust. It supports MCP, subagents, image input, and code review. Paired with the codex-1 (o3-optimized) or GPT-5-Codex model, it can read, write, and execute code directly on your local machine.

#codex #openai #ai-tools #cli #coding-agent #open-source

Table of Contents

Installation
Core Features
Model Evolution
1. codex-1 (early 2025)
2. GPT-5-Codex (late 2025)
Codex App vs. Codex CLI
Typical Use Cases
Positioning vs. Other Tools
Resources
References

🌏 中文版

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 codex

Supports 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, built on the o3 model and optimized for software engineering. Capable of handling multiple tasks in parallel — writing features, answering codebase questions, fixing bugs, and opening PRs.

GPT-5-Codex (late 2025)

A version of GPT-5 further optimized for agentic coding. Excels at both fast interactive conversations and autonomous execution of long, complex tasks. OpenAI rebuilt Codex CLI from the ground up around agentic coding workflows, based on community feedback.

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 exec to 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.