Doesn’t using the Anthropic subscription this way get charged as extra usage, and thus effectively billed at API rates?

Not if you run your own system prompt that does not explicitly contain the term "pi coding agent". Perhaps they string-match against some other stuff, too, but as long as you paraphrase the system prompt a bit, you should be fine.

That's hilarious

I use my Anthropic subscription with Pi. I thought they would start charging me API usage at some point, but it never happened. (If it did happen, I would switch to open weights models.)

...funnily enough, a few hours after posting this, it stopped working.

Oh, I stand corrected. It's been a while since I've used their models.

Glad we can still use OpenAI subscription through Pi.

They had plans to do that for CLaude SDK and claude -p usage, but paused them indefinitely.