<p>This study explores the assumptions made by pre-service primary teachers during mathematical modeling activities using Fermi problems. While mathematical modeling is emphasized in curricula worldwide, little attention has been given to how teachers can effectively support students in making the necessary assumptions to solve these problems. This research addresses this gap by analyzing 100 Fermi Problem Activity Templates produced by 35 pairs of pre-service teachers, focusing on the assumptions made during model formulation, mathematical processing, and strategic choices. Results reveal participants’ challenges in identifying key quantities and constructing coherent models, particularly in distant and complex contexts. While accessible activities like guesstimation and data searching dominate, strategic decisions like experimentation and statistical polling remain underutilized. Findings highlight the potential of FPATs in teacher training to foster modeling competencies and address contextual problem-solving skills.</p>