The Codex CLI course

This is a hands-on course, not a reference. You’ll spend a week with Codex turning an inherited side project — budgetcli, a small self-hosted personal-finance API — into something you’d actually trust with your own money. Every lesson takes a problem you’ll really hit (the agent reaches for the network and can’t, it keeps reintroducing the same money bug, a refactor sprawls across dozens of files) and shows the exact Codex move that fixes it.

If you want the spec-level “what is this primitive” view, that’s Foundations. This course is the other half: the muscle memory. Read it start to finish to build up from zero, or drop into any lesson when a specific problem is in front of you.

Already use Anthropic’s CLI? The Claude Code course is its sibling — same teach-by-doing spine, a different tool.

The curriculum

Ten chapters, in reading order. Each is a self-contained chapter that carries the budgetcli project one stretch further — start at the top, or pick the problem in front of you.

1 · Getting started Install Codex, authenticate (ChatGPT sign-in vs API key), understand one turn of the loop, and ship a first reviewed fix to the inherited repo.

2 · Sessions & context Resume across days, fork to try two approaches, branch off for a tangent, compact a full thread, and see what's eating the context window.

3 · Approvals & sandbox Codex's signature: the two-axis approval × sandbox model. Set exactly how much the agent can do and reach — and meet the network-off-by-default wall.

4 · Rules (AGENTS.md) Stop the agent reintroducing the same bugs. Write AGENTS.md, learn the git-root discovery walk and overrides, and where the size cap bites.

5 · Models & effort Dial reasoning effort to the task, switch models per job, and bundle a whole posture into a profile — spending where it actually pays.

6 · Subagents Push noisy work into isolated context and fan a big job across parallel workers in git worktrees, then gate and merge the results.

7 · Skills Package a repeatable procedure as a SKILL.md the agent invokes itself — built around onboarding each bank's differently-shaped CSV.

8 · Extending Codex Reach external systems over MCP, and gate risky moves with deterministic hooks on the tool lifecycle.

9 · Automation Run Codex headless with codex exec, wire it into CI, make runs reproducible, and know when to reach for the Agents SDK.

10 · Daily workflow The habits that compound: profiles as reflex, prompting well, and editing fast — then a look back across the whole week.

How to read it

Each chapter carries one running project — you’ll follow budgetcli across a week of real work — and the lessons build on each other: each one picks up a problem the last one left you with, adds a move to your repertoire, and hands off to the next.

So you get the most out of a chapter by reading it in order. But each lesson still stands on its own if you drop in from a search with one specific problem in front of you — you’ll just miss the running story.

What you won’t find is a fixed template stamped onto every lesson. Some need a long worked transcript, some a single command and a warning, some a digression into why the tool behaves the way it does. The shape follows the content, the way a good book varies its chapters.