The /goal command in Codex is amazing

I usually use Codex for my hobby programming.
I used to use Claude Code, but when I tried Codex around autumn last year (2025), I found it produced higher quality code, and have been using it ever since.
I feel Codex is more likely to correctly fix complex errors and bugs.

Recently, the /goal command has been added to Codex.

Since I didn’t fully trust AI, I hadn’t used it before, but when creating this blog (built with Ruby on Rails), I tried using /goal and was amazed it worked almost perfectly on the first attempt.

The usage is like this:

/goal Implement an application that fulfills all requirements in the requirements document (docs/requirements.md)

My steps were:

1. Chat with ChatGPT to create all screen designs as images (home, article display, edit…).
2. Create a new app with rails new and create a docs/images directory to store all the images I made.
3. Launch Codex and have it create a requirements document (docs/requirements.md) based on these images.
4. Give the /goal command with instructions to fulfill all requirements from the created requirements document and match the visual appearance of the views provided through images.
5. Go to sleep.

The model used was GPT-5.5 Middle.

I gave the final instruction around 2 AM, and when I woke up at 7 AM the next day, it was still working. It used up 5 hours of processing time and stopped once, but since the reset remained, I resumed it. Eventually, it finished around 11 AM before noon.

There were some sections that needed adjustments, and while it took quite some time for just a blog, it turned out pretty good. I felt it’s important to thoroughly define the specifications and pass them to /goal.

Funny story

Well, this is just a funny story, but the interface design images I gave to Codex were reproduced extremely meticulously by Codex, resulting in a web application that even had Mac-like window graphics and a browser URL bar-like element.
Of course, these aren’t needed for an app, but it was somewhat amusing and showed how seriously Codex takes its tasks.