Research indicating the educational value of classroom dialogue, in which participants engage critically and constructively with other perspectives, is long established but classroom practice evolves slowly. Outcomes of practitioner professional development in this area are inconsistent and often dependent on costly, external input. Our study aimed to understand whether and how practitioner-led inquiry may offer an alternative, sustainable and scalable way of developing dialogic practices, characterising effective organisational models. The Teacher Scheme for Educational Dialogue Analysis (T-SEDA) resource pack was designed to support iterative cycles of practitioner inquiry based on systematic analysis of classroom dialogue and reflecting critically with peers, using customisable coding tools and templates. This open resource embeds research findings about dialogue forms that are productive for student learning. We report our design-based research comprising nested inquiry cycles involving 74 practitioners from early years to tertiary levels. Data were derived from surveys, inquiry reports and interviews. Participants successfully used and adapted the resources for their own goals, needs and diverse contexts across seven countries. The largely autonomous process was typically supported by local facilitators working with groups of practitioners; data analysis focused on illuminating models of institutional organisation of inquiry, uses of T-SEDA resources, participant perceptions and factors underlying (dis)engagement. The findings offer insights into knowledge mobilisation and educational change processes. They yield design principles for scalability and sustainability based on a non-prescriptive model of local ownership and facilitation of self-directed practitioner inquiry and purpose-driven adaptation in complex educational circumstances.