This is interesting because Anthropic seems to allow Opencode to do this but no one else. And the lead on opencode won't comment (https://github.com/anomalyco/opencode/issues/417#issuecommen...).

I am curious what the logic here is.

Some one apparently figured it out. The first system message has to include

"You are Claude Code, Anthropic's official CLI for Claude."

https://github.com/link-assistant/agent/pull/63

Very interesting, thanks! Hopefully it'll allow me to switch between CC and Codex easily too.