Recent trends have shifted the focus of science, technology, engineering, and mathematics (STEM) education onto practice-based learning, to encourage opportunities for students to engage in science and engineering practices (SEPs) with the goal of more meaningful participation and engagement in authentic STEM experiences for all students. However, we do not fully understand how K-12 engineering students use the SEP of argumentation to design solutions or make problem-solving decisions. In this study, we examine middle school students' discourse to understand the epistemic criteria students use when making design decisions and how they meaningfully take up epistemic practices for engineering, specifically when engaging in engineering design. We find that students use five epistemic criteria ("nature," "creativity and innovation," "justification," "collaboration," and "the user") in ways meaningful to design-oriented goals, even at the beginning of the school year, and that consideration of these criteria varies according to the implicit goals in each stage of the design process. These findings suggest that (i) students took up epistemic goals for sensemaking and designing solutions, and (ii) students already possessed the skills and abilities to meaningfully engage in design work, suggesting an asset view of student epistemic practices should be adopted.