OpenAI Codex

Codex is OpenAI's coding agent, available as a CLI, an IDE extension, and a cloud agent that works on GitHub issues.

The CLI

npm install -g @openai/codex
codex

Run it in a repo and give it tasks:

codex "write a function that validates emails"
codex -r "review my changes for bugs"   # read-only review
codex -p "explain this diff"            # non-interactive

Codex CLI runs in a sandboxed environment by default — it can read your repo and run commands, but filesystem access outside the workspace is restricted unless you approve it.

Cloud Codex

The cloud version lives at chatgpt.com/codex. You connect a GitHub repo or paste a task, and it works in its own isolated environment, opening a pull request when done. It's great for delegating whole issues.

Configuration

# ~/.codex/config.toml
model = "gpt-5-codex"
approval_policy = "on-request"

It supports custom instructions (AGENTS.md project memory) and MCP servers, just like the other agents.

When to pick Codex

• You're already in the OpenAI ecosystem.
• You want a strong cloud agent that opens PRs for you.
• You like a stricter sandbox-by-default security model.

Read your repo's AGENTS.md convention: Codex, like Claude's CLAUDE.md, uses it as persistent project memory.