Ask HN: Are LLMs good for implementing features in existing web apps?

So I have a typical medium-sized Rails codebase, but with significant code that's not the dead simple CRUD stuff. For example, it talks with third-party APIs, does user authentication, uses Postgres FTS plus some other things.

I'm bored to work on it lately, and I was thinking whether using an LLM I could implement new features for it.

Are any of the current models at this point yet? I guess it should need to have a big context so that it reads all the views/controllers/models.

If so, which tool would you use? Something like Codex or Claude Code?