Almost always, notes like these are going to be about greenfield projects.

Trying to incorporate it in existing codebases (esp when the end user is a support interaction or more away) is still folly, except for closely reviewed and/or non-business-logic modifications.

That said, it is quite impressive to set up a simple architecture, or just list the filenames, and tell some agents to go crazy to implement what you want the application to do. But once it crosses a certain complexity, I find you need to prompt closer and closer to the weeds to see real results. I imagine a non-technical prompter cannot proceed past a certain prototype fidelity threshold, let alone make meaningful contributions to a mature codebase via LLM without a human engineer to guide and review.

I'm using it on a large set of existing codebases full of extremely ugly legacy code, weird build systems, tons of business logic and shipping directly to prod at neckbreaking growth over the last two years, and it's delivering the same type of value that Karpathy writes about.

That was true for me, but is no longer.

It's been especially helpful in explaining and understanding arcane bits of legacy code behavior my users ask about. I trigger Claude to examine the code and figure out how the feature works, then tell it to update the documentation accordingly.

These models do well changing brownfield applications that have tests because the constraints on a successful implementation are tight. Their solutions can be automatically augmented by research and documentation.