From what I can see in the code, this library is not set up to handle soft constraints. Additionally, while, WCSPs are NP-Hard, they are not in the class of NPC (which CSPs are), because you cannot verify a solution in polynomial time. However, given a k-value (cost < k), they do become NPC, but I don't know how you would use this library to solve them. You would need to incorporate the cost into another constraint that would calculate the cost of all constraints which kind of defeats the purpose. Further, it would probably reduce the efficiency of search, because I don't think that look-ahead would work well with that constraint.