> When I tried using Codex on my existing codebases, with or without guardrails, half of my time went into fixing the subtle mistakes it made or the duplication it introduced.

If you want to get good at this, when it makes subtle mistakes or duplicates code or whatever, revert the changes and update your AGENTS.md or your prompt and try again. Do that until it gets it right. That will take longer than writing it yourself. It's time invested in learning how to use these and getting a good setup in your codebase for them.

If you can't get it to get it right, you may legitimately have something it sucks at. Although as you iterate might also have some other insights into why it keeps getting it wrong and can maybe change something more substantial about your setup to make it able to get it right.

For example I have a custom xml/css UI solution that draws inspiration both from XML and SwiftUI, and it does an OK job of making UIs for it. But sometimes it gets stuck in ways it wouldn't if it was using HTML or some known (and probably higher quality/less buggy) UI library. I noticed it keeps trying things, adding redundant markup to both the xml and css, using unsupported attributes that it thinks should exist (because they do in HTML/CSS), and never cleans up on the way.

Some amount of fixing up its context made it noticeably better at this but it still gets stuck and makes a mess when it does. So I made it write a linter and now it uses the linter constantly which keeps it closer to on the rails.

Your pet feeding app isn't in this category. You can get a substantial app pretty far these days without running into a brick wall. Hitting a wall that quickly just means you're early on the learning curve. You may have needed to give it more technical guidance from the start, and have it write tests for everything, make sure it makes the app observable to itself in some way so it can see bugs itself and fix them, stuff like that.