> it wasn't managing goroutines or waitgroups well, and the whole thing was a buggy mess and could not be gracefully killed

First pass on a greenfield project is often like that, for humans too I suppose. Once the MVP is up, refactor with Opus ultrathink to look for areas of weakness and improvement usually tightens things up.

Then as you pointed out, once you have solid scaffolding, examples, etc, things keep improving. I feel like Claude has a pretty strong bias for following existing patterns in the project.