It's very interesting. I think Anthropic's early success in coding/tooling resulted in a lot of workflows using claude. I have started using every bit of my spare capacity to now move off these workflows. It's almost at a point now that if I use anything but Fable, the quality is subpar, Compared to alternatives (closed and open). The only reason I use Fable is because my harnesses still depend on claude code.
You can use Claude Code directly with any provider that supports Anthropic's API by setting some environment variables, and indirectly via a proxy with pretty much anything else.
I think there are some anti-patterns in CC that cause the issue - less a deficiency with other models.
Not to mention a lot of the harness is just built around the misbehaviors of anthropics models.
It's a lot of the instruction when it gets given to other models actually degrades their performance, not because the models are bad, but because they don't have the same underlying issues as Claude.