One metric I would add is mastery under decreasing assistance. A student who answers correctly after three increasingly specific hints has shown something useful, but it is not the same evidence as solving a fresh version unaided. If both outcomes update the skill score equally, the personalized path can advance the student too early.
For each attempt, I would keep the skill, item variant, number and specificity of hints, revisions, elapsed time, and whether the final reasoning came from the student. Advancement could require an unaided success on a new item plus a delayed check later in the session or on another day. The tutor's own explanation should never count as evidence that the learner mastered the step it just supplied.
This also gives teachers and families a more useful explanation than a single mastery percentage. “Can solve independently, but does not retain it after a day” and “can solve with one conceptual hint” point to different next lessons. That distinction seems especially important when the chat interface is both teaching the skill and measuring it.