Try changing the system prompt or switch to opencode [0] - they allegedly reverse engineered Claude Code, and so the performance you get with Claude models should be very similar to Claude Code.

[0] https://github.com/sst/opencode

there's an option for claude to disable co-authoring, see: https://code.claude.com/docs/en/settings

{ "includeCoAuthoredBy": false }

I've changed the settings and added the instruction to the prompt, hence my frustration :)