Generally end-user applications, depending on how you classify internal tools. I'm generally very happy with the output of Opus 4.X with a moderately structured CLAUDE.md and some investment in detection/avoidance of anti-patterns (Next.js+ts). I imagine that the bitter lesson is true here, and these heuristic guidelines will become increasingly unnecessary w/ smarter models.

Library-type work has mostly been side/toy projects, although fwiw, with a standard/spec on hand (CommonMark for example), I'm also happy w/ the output. It's often possible to "close the loop" and have the coding agent autonomously iterate until the standard is adhered to.