<p>When adopting the role of a teacher in learning-by-teaching environments, students often struggle to engage in knowledge-building activities, such as providing explanations and addressing misconceptions. Instead, they frequently default to knowledge-telling behaviors, where they simply dictate what they already know or what to do without deeper reflection, thereby limiting learning. Teachable agents, particularly those capable of posing persistent follow-up questions, have been shown to encourage students (tutors) to shift from knowledge-telling to knowledge-building and enhance tutor learning. Tutor learning encompasses two interrelated types of knowledge: conceptual and procedural knowledge. This study examines the role of knowledge-building in supporting these domains. Our findings revealed that tutors’ ratio of knowledge-building responses to an agent’s follow-up questions was positively associated with conceptual knowledge and procedural skill post-test scores, and this relationship was not explained by pre-test performance—a well-established predictor of learning outcomes. Critically, tutors with low prior knowledge who managed to produce more knowledge-building responses to an agent’s follow-up questions achieved post-test scores comparable to their high prior peers. These findings suggest knowledge-building can be a key mechanism through which teachable agents can bridge performance gaps between students with differing prior knowledge.</p>