This article analyzes the design decisions of a team developing diagnostic assessments for a learning trajectory focused on rational number reasoning. The analysis focuses on the design rationale for key decisions about how to develop the cognitive assessments and related validity arguments within a fluid state and national policy context. The study draws on ethnographic methods adapted from science, technology, and society studies to document key rationales for decisions. For this team, concerns about the validity of both the assessments and the hypothetical trajectory, anticipated uses of the assessments, and the available distribution of resources and expertise to different project activities were all considerations for significant design decisions throughout the project. The study findings suggest that success in the design of trajectories-based assessments depends on teams' attention to balancing core design activities with engagement in the external policy environment, balancing precision with utility for diagnosis in defining the levels of a trajectory, and balancing the goals of supporting and assessing student learning.