Unfortunately it's currently very utopian for (I would assume) most devs to use something like this when API cost is so prohibitively expensive compared to e.g. Claude Code. I would love to use a lighter and better harness, but I wouldn't love to quintuple my monthly costs. For now the pricing advantage is just too big for me compared to the inconvenience of using CC.
You technically still use CC, it's not via SDK but via CLI programmatically triggered via pi.
OpenAI officially supports using your subscription with pi. Same for OpenCode and other 3rd party harnesses.