A model fine-tuned for this app
Not a general assistant asked to be supportive. A small model trained on what this product actually does, so that what it is good at — and what it refuses — would be built into the model rather than asked of it in a prompt.
This is what would make running on your own device a claim about quality as well as about privacy. A smaller model that is right about one thing beats a larger one that is approximately right about everything, and it is the only version of “more capable” that would not cost you your privacy.