I believe there are a number of cli tools which also use Anthropic's Max plan (subscription) - this isn't just an OpenCode issue.

I had the TaskMaster AI tool hooked up to my Anthropic sub, as well as a couple of other things - Kilo Code and and Roo Code iirc?

From discussions at the time (6 months ago) this "use your Anthropic sub" functionality was listed by at least one of the above projects as "thanks to the functionality of the Anthropic SDK you can now use your sub...." implying it was officially sanctioned rather than via a "workaround".