The discovery of skeletons of the related oviraptorids revealed that they were in fact non-avian theropods,[7] and the discovery of more complete caenagnathid remains [4][8] revealed that Chirostenotes pergracilis, originally named on the basis of a pair of hands, and Citipes elegans, originally thought to be an ornithomimid, named from a foot, were caenagnathids as well.

The inside of the lower jaws also bore a complex series of ridges and toothlike processes, as well as a pair of horizontal, shelf-like structures.

They had slender arms and long, gracile legs,[8] although they lacked the extreme cursorial specializations seen in avimimids and Caudipteryx.

In phylogenetic taxonomy, the clade Caenagnathidae is defined as the most inclusive group containing Caenagnathus collinsi but not Oviraptor philoceratops.

[19] Hendrickx and colleagues (2015) defined a subgroup of Caenagnathidae, the Caenagnathinae, as all caenagnathids more closely related to Caenagnathus collinsi than to Elmisaurus rarus.

[20] The group Elmisaurinae is defined as including all species more closely related to Elmisaurus rarus than to Caenagnathus collinsi.

[22] Microvenator celer Gigantoraptor erlianensis Anomalipes zhaoi Chirostenotes pergracilis Hagryphus giganteus Nomingia gobiensis Citipes elegans Elmisaurus rarus Apatoraptor pennatus Caenagnathasia martinsoni Epichirostenotes curriei Anzu wyliei Caenagnathus collinsi The earliest known caenagnathid is Microvenator celer, from the Early Cretaceous Cloverly Formation.

[23] Their beaks also show considerable variation; that of Caenagnathasia is relatively short and deep, while that of Caenagnathus is long and shovel-shaped.

This variation in size and beak shape suggests that caenagnathids evolved to exploit a range of ecological niches.