This paper presents a cooperative consultation system on a restricted domain. The system builds hypotheses on the user's plan and avoids misunderstandings (with consequent repair dialogues) through clarification dialogues in case of ambiguity. The role played by constraints in the generation of the answer is characterized in order to limit the cases of ambiguities requiring a clarification dialogue. The answers of the system are generated at different levels of detail, according to the user's competence in the domain.