>4. Although it's hard to 100% confirm this, there's reason to believe that Claude itself works better with Claude Code than with other harnesses.

I thought this was pretty much well established that all of the labs have model - harness pairings for exactly this reason

Whale Paper. https://arxiv.org/abs/2609.00196

EMNLP talk from like 2 weeks ago https://aclanthology.org/2026.findings-emnlp.2207/

Yeah I thought so too, just didn't want to overstate it. So actually, a pretty big reason not to use an open-source harness if you're using a big lab's model.