User Experience is becoming an increasing centre of interest both in the academic and industrial worlds. Design agencies are no strangers to this phenomenon and are willing to shift from product-oriented design toward user experience design. However, their design process is often loose which may explain a lack of final UX quality or proof of quality, and does not allow to diagnose misuses and improvement opportunities due to a lack of UX formulation and traceability. We propose in this paper to retrospectively represent a design project and its specified, designed and validated UX, in the perspective to conduct a posterior collective diagnosis of UX design in a design agency. The proposed representation model is used to analyse one design project. Results show that only a few UX traces are found in the presented project, and that many dimensions are not considered (like perceptions or affects). Finally, we discuss the next steps of this tool's deployment to create a shared mental model of the design process among the design players.