This study explores the implementation of diagnostic assessment to support English as a Foreign Language (EFL) learning at SMAN 3 Semarang, focusing on the types of assessments used, the utilization of results to guide instruction, and the associated benefits and challenges. Using a qualitative descriptive approach, data were collected through teacher interviews and classroom observations. The findings reveal that cognitive diagnostic assessments follow three stages: preparation, implementation, and evaluation, while non-cognitive assessments are conducted through collaboration with counseling teachers, observations, and interviews. Cognitive diagnostic assessments help teachers identify students' academic strengths and weaknesses, allowing them to group students into advanced, intermediate, and beginner levels. This grouping facilitates differentiated instruction, ensuring students receive tailored support aligned with their abilities, improving engagement and comprehension. The results also highlight the benefits of diagnostic assessments, including increased student motivation, targeted learning strategies, and the ability to uncover students' hidden potential. However, challenges such as limited time, administrative workloads, and insufficient resources hinder their effective application. This study emphasizes the importance of diagnostic tests in creating inclusive and personalized learning. Future researchers are advised to involve perspectives from different teachers and schools to explore the application of diagnostic tests in different contexts. In addition, diagnostic tests should be formulated in a multi-tier system to provide more in-depth information about students' abilities and support more effective differentiated learning.