Claude code is fantastic. For me, the insight was that you have to give it the ability to close the loop. If it writes code it will try to reason about the code. "The button has the right CSS and so should be visible".

But everything is better if it can close the loop. So I instead instruct it to always use the puppeteer tool to launch the app and use some test credentials and see if the functionality works.

That's for a web app but you can see how you can do this for other things. Either unit tests, integration tests, or the appropriate MCP.

It needs to see what it's done and observe the resulting world. Not just attempt to reason to it.

Claude also leans towards what it's good at. Repetition costs it nothing so it doesn't mind implementing the same 5 times. One thing it did when I started is implement a sidebar on every page rather than using a component. So you need to provide some pressure against that with your prompts or at least force it to refactor at the end.