5.3 Walkthrough: Shipping a Feature

Everything up to this point has been individual tools. This lesson is where they become one loop — pick a real, small-to-medium feature you actually need (an API endpoint, a new page, a migration you've been putting off) and run it through all six steps, in order, on one real task before you move on.

Start in plan mode. Describe the feature and let Codex propose an approach before a single file changes — this is Lesson 2.2's habit, and it's the one step people skip when they're in a hurry, which is exactly when skipping it costs the most.

Check that AGENTS.md is still current for this part of the repo before you go further. If Codex gets a convention wrong here, that's not a one-off correction to make in the chat and forget — it's an AGENTS.md gap, and fixing the file instead of just the moment is what prevents the same mistake from happening again next week.

Delegate the boring-but-necessary parts to a subagent — writing the test suite, researching an unfamiliar library you need to pull in. This is Lesson 4.1's context-management instinct applied to a real task instead of a hypothetical one.

Use /diff continuously as changes land, not just once at the very end when there's a wall of changes to review at once and your attention is already fading.

Run a Codex Security pass before you consider it done — especially, non-negotiably, if it touches auth, payments, or user data.

And close the loop: update AGENTS.md, or write a Skill, if this is a pattern you're going to repeat. Turn today's one-off into tomorrow's reusable shortcut.

This loop — plan, delegate, review, encode what you learned — is the actual gap between someone who used Codex once for a demo and someone who's technically great at it day to day. Everything in Modules 1 through 4 is the vocabulary. This loop is the sentence you keep writing, on every feature, until it's not a checklist anymore, it's just how you work.