Just a recent anecdote, I asked the newest Codex to create a UI element that would persist its value on change. I'm using Datastar and have the manual saved on-disk and linked from the AGENTS.md. It's a simple html element with an annotation, a new backend route, and updating a data model. And there are even examples of this elsewhere in the page/app.

I've asked it to do why harder things so I thought it'd easily one-shot this but for some reason it absolutely ate it on this task. I tried to re-prompt it several times but it kept digging a hole for itself, adding more and more in-line javascript and backend code (and not even cleaning up the old code).

It's hard to appreciate how unintuitive the failure modes are. It can do things probably only a handful of specialists can do but it can also critical fail on what is a straightforward junior programming task.