OpenAI Codex for Code Review: Developer Guide

What Is OpenAI Codex? OpenAI Codex is an AI-powered coding agent that operates inside a sandboxed cloud environment, capable of executing multi-step software engineering tasks autonomously. Unlike conversational AI tools that simply suggest code snippets, Codex can read your codebase, write and modify files, install dependencies, run tests, and submit its work as pull requests or branches. It launched in 2025 as part of OpenAI's push into agentic AI, and by early 2026 it has become one of the most discussed tools in the developer ecosystem. The name "Codex" has a history at OpenAI. The original Codex model, released in 2021, was the AI engine behind GitHub Copilot's autocomplete. That model was eventually deprecated in favor of GPT-3.5 and GPT-4. The current Codex is a fundamentally different product - not a model, but a full agent platform built on top of OpenAI's latest models (including the o3 and o4-mini reasoning models). The shared name causes some confusion, but the 2025-era Cod