For anyone coming looking for a solution; I peeked around the OC repository, and a few PRs got merged in. Add this to $HOME/.config/opencode/opencode.json: plugin = ["opencode-anthropic-auth"]

That is, if that's not pulled in to latest OC by the time I post this. Not sure what the release cycle is for builtin plugins like that, but by force specifying it it definitely pulls master which has a fix.

https://opencode.ai/docs/plugins/