In this paper we assess the functionality and therapeutic potential of ambient smart environments. We argue that the language of affordances alone fails to do justice to the peculiar functionality of this ambient technology, and draw from theoretical approaches based on the free energy principle and active inference. We argue that ambient smart environments should be understood as playing an'upstream' role, shaping an agent's field of affordances in real time, in an adaptive way that supports an optimal grip on a field of affordances. We characterise this optimal grip using precision weighting, and in terms of allostatic control, drawing an analogy with the role of precision weighting in metacognitive processes. One key insight we present is that ambient smart environments may support allostatic control not only by simplifying an agent's problem space, but by increasing uncertainty, in order to destabilise calcified, sub-optimal, psychological and behavioural patterns. In short, we lay an empirically-grounded theoretical foundation for understanding ambient smart environments, and for answering related philosophical questions around agency, trust, and subjective wellbeing.