> do I have to pay API based costs
Usually, yes, you do.
However, in this case, opencode kinda cheats by using Antropic client ID and pretending to be Claude Code, so it can use your existing subscription.
> We recommend signing up for Claude Pro or Max, running opencode auth login and selecting Anthropic. It’s the most cost-effective way to use opencode.
https://opencode.ai/docs/