Just use the ENV vars, https://kkovacs.eu/opencode-go-with-claude-code/ | https://code.claude.com/docs/en/env-vars

You can use a project like GoModel to add a great gateway where you can swap out models without touching CC (or any harness) after setup. Their "virtual models" are the bee's knees: https://gomodel.enterpilot.io/docs/features/virtual-models