This paper presents Diagram, a learning environment for object-oriented modelling with UML class diagrams. Diagram reifies an interaction model that supports the learner’s metacognitive activity. This model relies on a task organization, on specific modelling graphic tools and on contextual helps. Diagram includes a diagnostic module which compares the student diagram with a reference diagram, and produces the list of the differences between these diagrams. The diagnostic algorithm is inspired from graph comparison methods, and uses structural patterns that direct the matching of the diagrams to be compared. The differences that are noticed in the student diagram give rise to feedbacks. We illustrate the whole feedback elaboration process by giving an example of a student diagram with the diagnostic results and the messages generated by the learning environment. We describe an experimentation in ecological context and analyze the effects of the feedbacks.