So you want Aider, Claude Code or opencode.ai it seems. I use opencode.ai a lot nowadays and am really happy and productive.
Like, I gave it access to our code base, wanted to try a very simple bug fix. I only told it to look at one service I knew needed changes, because it says it works better in smaller code bases. It wanted to send so many tokens to sonnet that I hit the limits before it even started actually doing any coding.
Instant fail.
Then I just ran Claude Code, gave it the same instructions and I had a mostly working fix in a few minutes (never mind the other fails with Claude I've had - see other comment), but Aider was a huge disappointment for me.
It would be good to define what's "smaller code bases". Here is what I am working one: 10 years old project full of legacy consisting of about 10 services and 10 front-end projects. As well tried it on project similar to MUI or Mantine UI. Naturally on many smaller projects. As well tried it on TypeScript codebase where it has failed for me (but it is hard to judge from one attempt). Lastly I am using it on smaller projects. Overall question is more about task than about code base size. If the task does not involve loading too much context when code base size might be irrelevant.
Monorepo.
And that was me already knowing what I wanted and just telling it to do it. I never added any files but the one service but it knows the entire repo.
Claude code on the other I tell "I need to do X thing across the code base" and it just goes and does it (sometimes badly of course) but it has no issue just "doing its agent thing" on it.
Aider wouldn't even be able to do both code and tests in one go? Like the whole idea of "only add the files you need" makes no sense to me for agentic use.