Ever since Claude banned this, I haven't found one. Which harnesses have you found that allows using Claude through the subscription plan?
The active project seems to use Claude code's agent API https://github.com/anthropics/claude-agent-sdk-typescript so yes you can issue commands to Claude code from opencode, but the harness in that case is still ultimately Claude code, with all the big fat system prompts that come with it.
Did you use some kind of plugin? I'd like to use opencode with my Claude subscription.