This paper explores how mathematical praxeologies are integrated into an interdisciplinary STEM assessment in the LabSTEM+ projects. Despite the growing emphasis on integrated STEM education, assessing both subject-specific and interdisciplinary competencies remains challenging. Using the Anthropological Theory of the Didactic (ATD), the study examines how students incorporate mathematical praxeologies into their assessments and integrate them with scientific disciplines. Videorecording from the exploratory case study reveals that while students generally attempt to link mathematical models with scientific content, their grasp of the underlying mathematical theory varies. Some groups effectively integrate mathematics, but many do not include the theoretical aspects. The findings highlight the importance of providing more effective guidance to students on incorporating the assessment of mathematical praxeologies within interdisciplinary STEM practices.