To simulate the fragmentation of grains in cold deformed polycrystals, the underlying heterogeneity of crystal slip patterns should be quantified. Although it is understood that such patterns split owing to both the grain interaction and tendency to weaken the local strain hardening, properly justified numerical models for the considered effect are still wanted. This paper briefly surveys relevant extensions of rate-independent Taylor's theory and currently popular rate-sensitive formulations coupled with finite element modeling. Then, to exclude the limitations of such efforts, a novel rate-independent model is formulated that accommodates macroscopic deformation by the interfacial skeleton while suggesting specific slip patterns to keep the local strain compatibility across any grain boundary facet. Based on this approach, the fragmentation onset at grain junctions is predicted, and refinements of the model which should extend it to grain interiors are discussed.