Until recently, caenagnathids were a family of oviraptorosaurs represented only by fragmentary material. As such, caenagnathid biology has never been studied in depth. A well-preserved mandible provides new information on the anatomy and dietary habits of Chirostenotes. The mandible is edentulous, has a completely fused symphysis, with sharp occlusal margins and complex lingual surfaces. Finite element analysis shows that the lingual ridges are reinforced. This suggests that they had a function in food processing. These and other features suggest adaptations for an efficient shearing mechanism, and the overall morphology is poorly adapted for durophagous behaviour. Comparisons with three groups with convergently similar mandibles, especially dicynodonts, indicate caenagnathids were capable of handling an herbivorous diet. Here, an omnivorous diet is proposed for Chirostenotes, including folivory and small prey.