As an experiment over the holidays I had Opus create a coding agent in a Prolog DSL (more than 200 lines though) [0] and I was surprised how well the agent worked out of the box. So I guess that the latest one or two generations of models have reached a stage where the agent harness around the model seems to be less important than before.

[0] https://news.ycombinator.com/item?id=46527722