I used a combination of OpenAI's online Codex, and Claude Sonnet 4 in VSCode agent mode. It was nice that Codex was more automated and had an environment it could work in, but its thought-logs are terrible. Iteration was also slow because it takes awhile for it to spin the environment up. And while you _can_ have multiple requests running at once, it usually doesn't make sense for a single, somewhat small project.
Sonnet 4's thoughts were much more coherent, and it was fun to watch it work and figure out problems. But there's something broken in VSCode right now that makes its ability to read console output inconsistent, which made things difficult.
The biggest issue I ran into is that both are set up to seek out and read only small parts of the code. While they're generally good at getting enough context, it does cause some degradation in quality. A frequent issue was replication of CSS styling between the Rust side of things (which creates all of the HTML elements) and the style.css side of things. Like it would be working on the Rust code and forget to check style.css, so it would just manually insert styles on the Rust side even though those elements were already styled on the style.css side.
Codex is also _terrible_ at formatting and will frequently muck things up, so it's mandatory to use it with an autoformatter and instructions to use it. Even with that, Codex will often say that it ran it, but didn't actually run it (or ran it somewhere in the middle instead of at the end) so its pull requests fail CI. Sonnet never seemed to have this issue and just used the prevailing style it saw in the files.
Now, when I say "almost 100% AI", it's maybe 99% because I did have to step in and do some edits myself for things that both failed at. In particular neither can see the actual game running, so they'd make weird mistakes with the design. (Yes, Sonnet in VS Code can see attached images, and potentially can see the DOM of vscode's built in browser, but the vision of all SOTA models is ass so it's effectively useless). I also stepped in once to do one major refactor. The AIs had decided on a very strange, messy, and buggy interpreter implementation at first.