Is that Open-Source like, run it locally, no phone home included, or open source like the thin front-end layer is all that is actually open-source but it’s an empty shell without the remote API it relies on?
It defaults to the API, but you can download the weights for the 1T-parameter Pro model and selfhost if you have the hardware for it.
The latter. It looks like it's meant to be a batteries-included agent to promote their free-for-a-limited time AI service that it connects to by default.
They default it to talking to a free version of their model (which is incredibly cheap if you decide you like it.)
But it seems trivially easy to run it against local models. Their onboarding guide offers that option, though I have no idea if it changes any functionality.