Claude doesn't allow third party harnesses unless you pay the full API price. Attempting to avoid that by pretending to be Claude code gets your account banned. Google is the same and will ban your entire Google account.

AFAIK - Of the big three only OpenAI explicitly allows third party harnesses without paying exorbitant API rates. It's one of the primary reasons I still use them.

To avoid these problems some folks use tools like herdr which operate like tmux or via the SDK to drive the official harness, rather than speaking directly to the API routes.