Methods and systems of testing software and modeling user actions are described. In some embodiments, multiple different algorithms are provided for operating on a software model. The software model describes behavior associated with software that is to be tested. Different sets of algorithms can be selected for operating on the software model to produce a sequence of test actions that are to be used to test the software. The algorithms can be mixed and matched to achieve a desired testing result. In some embodiments, the different algorithms comprise deterministic algorithms, random algorithms, and various types of algorithms therebetween. In one embodiment, the software model comprises a state graph having nodes that represent state, and links between the nodes that represent actions. The different algorithms that are available for selection can have different graph traversal characteristics such that the state graph can be traversed in different manners.