always has been
claude code has opusplan — uses opus while in plan mode, switches to sonnet for execution.
https://code.claude.com/docs/en/model-config#opusplan-model-...
edit: you can make it work with sonnet for planning, and haiku for execution, or any other combination you fancy to work with.
https://code.claude.com/docs/en/model-config#control-the-mod...