A solid understanding of subject matter is a fundamental requirement for pre-service mathematics teachers as they prepare for effective classroom instruction. To investigate their grasp of mathematical content and teaching skills, the development of valid and reliable assessment instruments is essential. This study focuses on the Knowledge of Content and Teaching (KCT) instrument as a tool to evaluate these competencies. Utilizing the Plomp model for educational design research, the study involved a systematic process of instrument development and validation. The findings indicate that the KCT instruments are valid, reliable, and appropriate for assessing pre-service teachers’ readiness in lesson planning and mathematics instruction. These results suggest that the KCT tools can play a crucial role in teacher education programs by supporting the evaluation and enhancement of pedagogical content knowledge. Future studies are recommended to explore broader applications of the instrument in various educational contexts