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.

You can, but claude is better at working with it's own tool calls. Other models work great as a drop-in into omp/opencode etc, but in my experience not as much with CC.

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.