PÉPITE is a multidisciplinary project involving researchers in computer science and in didactics for mathematics. It consists in building a computerised environment able to model the competence of 15-year-old students of French secondary schools in elementary algebra. This work is based on a didactical analysis whose result is a multidimensional model of competence in algebra and a validated pencil and paper diagnosis tool. The PÉPITE project aims to automate this tool. In this paper we present the prototype developed to automatically build students’ profiles and the results that we have already obtained. We put emphasis on the methodology adopted to design and validate this tool and the model of student’s competence built by PÉPITE.