So, are you asking:
a) using an open-weight alternative to other Claude models?
b) using an open-source alternative to Claude Code, but still use Claude models and/or open-weight models
c) something else?
The harness provides a means for the LLM to write file output and run tools (commands) to test output. Alternative harnesses including things like OpenCode.
You can point OpenCode to your Anthropic account, but only if you have API usage (there are ways around this, but if Anthropic detects it you could get banned). Anthropic locks their subscription plans to THEIR harness and no others.
Most harnesses like OpenCode, Cursor, Pi, Hermes, etc. have settings to tie into a bunch of different providers (although they'll always try to promote their own by default, because they have to fund their own harness development of course).
I recommend OpenCode + OpenRouter. OpenRouter has a bunch of models and their web interface shows where those models are run (AWS Bedrock, xAI, Anthropic, etc.) and what data they collect from your prompts (although this is kinda a 'trust me bro' status as a provider could be lying about collecting data).
With OpenCode you can run /model and just select whatever your provider offers. I always recommend not using prompts. Create an AGENT.md with your project rules, then create a plan/01-initial-requirements.md. Write an entire requirements spec. Commit those. Then tell the LLM "Read AGENT.md for project rules and run plan/01-initial-requirements.md".
That way you can commit that output on a branch, jump back to the original commit, and run the plan again with a different model. You can do that multiple times and compare the output on all the branches. Some models might give you better UI, some better structure. You can just keep the one you like most, or ask a model to combine certain aspects of each branch into your final.
good out of box experience for the harness, cheap plan with generous quotas
You can use the Go plan with any harness, so you could keep the Claude CLI while using open models.