Codex in Detail

[ikaijua]

Codex in Detail

This post is maintained as the product evolves. The Codex entry in the repo README links here so the detailed intro can be updated without frequent repo changes.

What is Codex?

Codex is OpenAI's AI coding agent. Its core is an open-source (Apache-2.0) terminal CLI written in Rust. It can understand, edit, and run code directly in your local repository, and is also available through VS Code / Cursor / Windsurf / JetBrains IDE extensions, the Codex App (desktop), and Codex Web (cloud tasks). Active sessions can sync across phone, desktop, and web through the same ChatGPT account.

Core Philosophy

Terminal-Native

Codex CLI is built around the terminal workflow, letting you complete the full development loop—explore, edit, run, and review—inside your familiar shell environment.

Local-First with Optional Cloud

By default Codex runs on your local machine, keeping code and credentials local. You can also hand work off to Codex Cloud and let it finish asynchronously in a cloud sandbox.

Controllable Automation

Approval modes and sandbox settings let you control when Codex can read or write files, run commands, and access the network.

Extensible

Codex supports Skills (reusable instructions), Plugins (team tool integrations), MCP servers, subagents, and Hooks.

Available Surfaces

Surface	Description
Terminal CLI	Open-source Rust client with the full feature set
IDE Extensions	VS Code / Cursor / Windsurf / JetBrains
Desktop App	codex app for visual management and multiple sessions
Codex Web	chatgpt.com/codex for cloud tasks and collaboration
Mobile App	ChatGPT app can connect to and continue active sessions

Core Capabilities

Code Understanding & Modification

• Analyze project structure, dependencies, and call relationships
• Automatically fix bugs, refactor, and add features
• Execute and iterate on multi-step tasks

Command-Line & CI Integration

• Interactive TUI and one-shot prompt mode
• codex exec for scripts and pipelines
• Pipe and shell-script integration

Code Review

• codex review examines uncommitted changes, commits, or branches
• Does not modify the working tree; only reports risks and suggestions

Extensibility

• Skills / Plugins: reusable instructions and team tool integrations
• MCP servers: connect to external tools and data sources
• Subagents: delegate complex investigations to specialized agents
• Hooks: customize lifecycle behavior

Cross-Device & Remote

• ChatGPT relay: active sessions sync across phone, desktop, and web
• Remote SSH: connect directly to remote development environments
• Codex Cloud: submit tasks to a cloud environment that keeps running after you close your laptop

Quick Start

Installation

Official install script:

# macOS / Linux
curl -fsSL https://chatgpt.com/codex/install.sh | sh

# Windows PowerShell
powershell -ExecutionPolicy ByPass -c "irm https://chatgpt.com/codex/install.ps1 | iex"

Homebrew:

brew install --cask codex

npm (alternative):

npm install -g @openai/codex

Verify installation:

codex --version

Log In

codex

Choose Sign in with ChatGPT (recommended) or use an OpenAI API key.

Common Commands

codex                                      # Start an interactive session
codex "analyze the structure of this project"  # One-shot prompt
codex exec "run the test suite"            # Non-interactive / CI mode
codex review                               # Review uncommitted changes
codex resume                               # Resume the most recent session
codex cloud                                # Manage cloud tasks
codex mcp list                             # List configured MCP servers
codex --image error.png "how do I fix this error"  # Pass image context

Approval Modes & Sandbox

Codex uses an approval mode to decide when to pause for confirmation and a sandbox to define file and network access.

Approval Modes

Mode	Description
suggest (default)	Ask before every change
auto-edit	Auto-edit files, ask before running commands
full-auto	Auto-edit and run commands (use with caution)

codex --approval-mode auto-edit
codex --approval-mode full-auto

Sandbox Modes

Mode	Description
read-only	Read-only, no file modifications (similar to plan mode)
workspace-write	Read/write within the current working directory
danger-full-access	No sandbox restrictions

codex -s read-only
codex --sandbox workspace-write

Use /permissions in the interactive UI to switch quickly. On unfamiliar projects, start with suggest plus read-only or workspace-write.

Best Practices

1. Start with suggest mode: increase automation only after you know the project.
2. Scope tasks clearly: mention target files, expected behavior, and tests to run.
3. Review diffs and run tests: AI-generated changes still need human review.
4. Use Skills for repetitive workflows: reduce repeated prompting.
5. Use a strict sandbox for sensitive code: avoid accidental file or network access.

Related Resources

• GitHub Repository
• Codex CLI Documentation
• OpenAI: Work with Codex from Anywhere

License

Codex CLI is released under the Apache-2.0 license. Use of OpenAI model services is subject to the corresponding terms of service.