OpenAI Codex

OpenAI Codex is OpenAI's AI coding platform, entirely rebuilt in 2025. The former Codex API was replaced by a system integrated into ChatGPT comprising three distinct products: Codex Web, Codex CLI, and an IDE extension.

Codex Web is a cloud-based autonomous agent capable of working in parallel on multiple coding tasks. Powered by a specialized version of o3 optimized for code, it can write features, debug, answer codebase questions, and propose pull requests — all in a secure sandboxed environment. Tasks take 1 to 30 minutes and execute completely autonomously.

Codex CLI is an open-source command-line tool that runs locally on your machine. Using GPT-5 by default, it offers an experience comparable to Claude Code but within the OpenAI ecosystem. The CLI is available on GitHub and can be freely extended.

The IDE extension integrates directly into VS Code and other editors, offering a similar experience to GitHub Copilot but with Codex's agentic capabilities.

Codex stands out for its token efficiency (3x more efficient than competitors) and native cloud-side parallelism, allowing you to launch multiple agents on different tasks simultaneously.