Hey, this is a known limitation of the ACP integration. Third-party ACP clients like Zed and IntelliJ don’t declare the capability our backend uses to show model variants, so every model collapses to its default variant. For Composer 2.5, the default is Fast, and the non-Fast variant isn’t sent to the client at all. That’s why favorite_config_option_values in Zed doesn’t change anything, there’s no choice list on the server side.
We’re working on a fix by moving to the standard ACP Session Config Options, so any spec-compliant client can natively render variant selection and thinking level. I can’t share an ETA yet.
Related thread with the same question and explanation: Can't select thinking level or variant in Cursor ACP. I’ll post an update when there’s news.