Circularity in agri-food systems (AFS) has gained popularity as a concept promoted to decrease negative environmental impacts of agriculture and value chains, from the perspective of either nutrient cycling or a circular economy. However, at the global scale, evidence of increased circularity in AFS remains scarce. Research has been critical of the concept of a circular economy, especially socio-metabolic research. Yet, the latter provides a comprehensive framework for analyzing AFS in relation to planetary boundaries through the quantification of material and energy flows. The present review aimed to understand how socio-metabolic research addresses circularity in AFS, renders circularity operational and supports the transition towards strong sustainability. A total of 78 case studies were selected and characterized in terms of their (i) key methodological features, especially whether they addressed circularity as a context, purpose or means, and (ii) their pathways to action through the place of stakeholders and the key drivers of changes related to livestock production. We discuss knowledge gaps for acting on AFS and identify three priorities for transformative socio-metabolic research of AFS: (i) reveal hidden flows, identify system and compartment diversity (ii) assess trade-offs of transitioning to a circular system, including external impacts beyond the territorial scale (iii) strengthen methods for engaging local stakeholders in the design and implementation of the transition.