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).

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.