In this paper we describe the functional features and the architecture of a tool implementing the Stocks-Carrington framework (TTF) for model based testing (MBT). The resulting prototype, called Fastest, makes it easy to generate test cases from Z specifications. We not only apply the ideas of the referred framework but we also use a technique based on finite models to find test cases, which has proved to increase the level of automation during the whole testing process. The paper also discusses problems and challenges that have appeared during the development of the tool, and introduces real case studies and an analysis of the results obtained so far.