# OpenAI Codex OpenAI Codex is [[OpenAI]]'s agentic coding product family; the umbrella brand under which OpenAI ships its [[AI Agent Harness|coding harnesses]]. It is a direct competitor to [[Claude Code]], [[Gemini CLI]], and [[Kimi CLI]]. The brand reuses the historical "Codex" name (the 2021 GPT-3 fine-tune that powered the original GitHub Copilot), but the modern Codex has nothing to do with that model; it is a multi-surface agent stack built on the GPT-5 family. ## The four surfaces Codex ships across four surfaces sharing the same authentication, configuration, and skill model: - **[[Codex CLI]]**; terminal-native agent. Built in Rust, open source. The default surface for power users and automation. - **[[Codex App]]**; full desktop application with project sidebar, thread list, review pane, in-app browser, computer use, automations, and worktrees. - **Codex IDE Extension**; integrates Codex inside editors (VS Code, JetBrains). - **Codex Cloud**; web-based platform with managed environments and internet access, used for async/cloud-hosted runs and team collaboration. The CLI can launch jobs in Codex Cloud, the App can resume CLI sessions, and the IDE extension can hand off to either; sessions are portable. ## Models Codex defaults to **GPT-5.5** (April 2026 frontier model) for most tasks, with GPT-5.4 and GPT-5.3-Codex still selectable. Models and reasoning levels switch via the `/model` command across all surfaces. A lighter "GPT-5.1-Codex-Mini" tier is recommended for routine tasks to control cost. ## Access Included with ChatGPT Plus, Pro, Business, Edu, and Enterprise plans. Authentication is shared across all four surfaces using the ChatGPT account or an OpenAI API key. ## Extensibility - **[[Model Context Protocol (MCP)]]**; connect third-party tools and data sources. - **AGENTS.md**; project-specific agent configuration; the format OpenAI contributed to the [[Agentic AI Foundation (AAIF)]]. - **[[AI Agent Skills]]**; supports the open Agent Skills standard for reusable capabilities. - **Plugin marketplace**; remote bundle caching and marketplace installs since 0.128.0. ## References - Official page: https://openai.com/codex/ - Documentation: https://developers.openai.com/codex/ - Source code: https://github.com/openai/codex - Skills: https://developers.openai.com/codex/skills - Changelog: https://developers.openai.com/codex/changelog/ ## Related - [[Codex CLI]] - [[Codex App]] - [[Codex Cloud]] - [[Codex IDE Extension]] - [[Codex plugin for Claude Code]] - [[AI Agent Harness]] - [[OpenAI]] - [[Claude Code]] - [[Gemini CLI]] - [[GitHub Copilot CLI]] - [[Kimi CLI]] - [[AI Agents]] - [[AI Agent Skills]] - [[Model Context Protocol (MCP)]] - [[Agentic AI Foundation (AAIF)]]