If you're building something new, stick with languages/problems/projects that have plenty of analogues in the opensource world and keep your context windows small, with small changes.

One-shotting an application that is very bespoke and niche is not going to go well, and same goes for working on an existing codebase without a pile of background work on helping the model understand it piece by piece, and then restricting it to small changes in well-defined areas.

It's like teaching an intern.