Model assist is a feature of modelling environments aiding their users with entering well-formed models into an editor. Current implementations of model assist are mostly hard-coded in the editor and duplicate the logic captured in the environment's validation methods used for post hoc checking of models for well-formedness. We propose a fully declarative approach which computes legal model assists from a modelling language's well-formedness rules via constraint solving, covering a large array of assistance scenarios with only minor differences in the assistance specifications. We describe an implementation of our approach and evaluate it on 299 small to medium size open source models. Although more research will be needed to explore the boundaries of our approach, first results presented here suggest that it is feasible.