The harnesses have helped in training the models themselves (i.e. every good trace was "baked in" the model) and have improved in enabling test time compute. But at the end of the day this is all put back into the models, and they become better.
The simplest proof of this is on benchmarks like terminalbench and swe-bench with simple agents. The current top models are much better than their previous versions, when put in a loop with just a "bash tool". There's a ~100LoC harness called mini-swe-agent [1] that does just that.
So current models + minimal loop >> previous gen models with human written harnesses + lots of glue.
> Gemini 3 Pro reaches 74% on SWE-bench verified with mini-swe-agent!