This is exactly the structural gap that makes agent accountability hard. An agent executing a trade and writing its own audit log has the same circularity problem you described: the log proves consistency with itself, not correctness. Breaking it requires a separate execution environment that cryptographically signs every action at the infrastructure level, decoupled from the agent's own reporting. That way you don't need to trust the agent's version of events, you check the signed receipt from the wallet that authorized the action.