["British Journal of Educational Technology, EarlyView. ", "\nAbstract\n\nThis study investigated using generative artificial intelligence (GAI) as a learning companion for elementary school teachers to understand, design and implement the knowledge building (KB) approach in their school. The KB learning companion for teachers (KB LCT), a GAI chatbot, was developed to support teachers, integrating human–AI and human–human interactions. The cultural‐historical activity theory (CHAT) was used as a methodological lens to examine the professional learning of the teachers. We studied three lead teachers who first experienced the chatbot and facilitated the other 17 teachers in their learning. We uncovered how these lead teachers leveraged the affordances of the chatbot within the community for learning and exercised their professional agency to reconcile contradictions in the system for other teachers. Consequently, they showed evidence of learning about KB, deepening understanding and contextualising KB principles in classrooms, and transforming professional learning structures and activities for their colleagues.\n\n\n\n\nPractitioner notes\nWhat is already known about this topic?\n\nA community approach to teacher professional development is effective.\nGAI has been used to support teachers' work by generating lesson plans.\nThe knowledge building approach is effective in improving teachers' professional agency.\n\nWhat this paper adds?\n\nThe use of GAI as a learning companion for teachers.\nAffordances and contradictions of the GAI learning companion in a community experienced by the teachers.\nTeachers' manifestation of agency in learning related to the GAI and the learning community.\n\nImplications for practice and/or policy\n\nUsing a GAI learning companion in a community is effective for teacher learning.\nTeacher professional agency in learning can be manifested in how teachers leverage the affordances of the learning environment.\nTeacher professional agency in learning can be facilitated by providing the opportunity to reconcile contradictions they experience.\n\n\n\n\n"]