Are we justified attributing a mistake in diagnosis to an AI diagnostic system?

Responsible professional use of AI implies the readiness to respond to and address—in ethically appropriate manner—harm that may be associated with such use. This presupposes the ownership of mistakes. In this paper, I ask if a mistake in AI- enhanced decision making—such as AI-aided medical diagnosis—can be attributed to the AI system itself, and answer this question negatively. I will explore two options. If AI systems are merely tools, then we are never justified to attribute mistakes to them, because their failing does not meet rational constraints on being mistaken. If, for the sake of the argument, we assume that AI systems are not (mere) tools, then we are faced with certain challenges. The first is the burden to explain what this more-than-a-tool role of an AI system is, and to establish justificatory reasons for the AI system to be considered as such. The second is to prove that medical diagnosis can be reduced to the calculations by AI system without any significant loss to the purpose and quality of the diagnosis as a procedure. I will conclude that the problem of the ownership of mistakes in hybrid decision making necessitates new forms of epistemic responsibilities.