How OpenAI Codex Works Behind-the-Scenes (and How It Compares to Claude Code)

OpenAI Codex and Anthropic's Claude Code are two AI-assisted development tools with distinct approaches, each suited to different workflows. Codex uses an agentic architecture with a single-agent ReAct loop, focusing on a shell-centric design that leverages familiar CLI utilities for tasks like reading files and executing commands. This approach prioritizes simplicity, flexibility, and strong sandboxing for security, making it ideal for developers who prefer control and transparency in local development. In contrast, Claude Code employs a structured toolset with explicit planning, proactive codebase scanning, and a polished permission model, excelling in handling large codebases and complex refactoring tasks. Both systems embrace a diff-centric philosophy for code changes, encouraging iterative development through small, reviewable modifications and immediate testing. Codex's strength lies in its ability to perform quick, surgical edits, while Claude Code is praised for its comprehensive understanding of larger projects, despite being more verbose. Overall, both tools are likened to "extremely capable interns," offering significant time savings while requiring user guidance and output verification.