I'm guessing the greatest reason behind each provider creating and agent harness is that (a) there is not a clear winner still and (b) it is harder to switch models with a competitor, as you also have to switch harness
Or by using a proxy, yeah. Personally I would still prefer a multi provider harness over CC when using it with another provider, if alone for the visible reasoning, model switcher, cost estimation and so on. So far I've only preferred CC when I needed to work with Jupyter Notebooks because it has built-in tools for that.