OICC Press Journal of Applied Linguistic Studies 2820-9974 5 2 2025 12 30 AI-Assisted Lesson Planning and Teacher Development: A Mixed-Methods Study on Critical Thinking and Creativity 10.57647/jals.2025.0502.03 EN Roghaye Torki Department of English, ShK.C., Islamic Azad University, ShahreKord, Iran Fariba Rahimi Esfahani Department of English, ShK.C., Islamic Azad University, ShahreKord, Iran Farshad Kiyoumarsi Department of Computer, ShK.C., Islamic Azad University, ShahreKord, Iran Journal Article 2025 12 30 The rapid integration of generative Artificial Intelligence, such as ChatGPT, in education is reshaping teachers’ professional practices. However, its influence on teachers’ higher-order cognitive processes during instructional design remains insufficiently understood. This study examined the impact of ChatGPT-assisted lesson planning on EFL teachers’ critical thinking, creativity, and overall instructional design quality. Using an explanatory sequential mixed-methods quasi-experimental design, the research combined quantitative and qualitative strands to determine how AI-supported planning influenced teachers’ cognitive engagement. Forty-two in-service EFL teachers were purposively selected and randomly assigned to an experimental group, that received structured training on integrating ChatGPT into lesson planning, and a control group, that planned lessons using conventional methods. Participants completed pretest and posttest lesson-planning tasks evaluated through three validated instruments: The Critical Thinking Rubric, the Creativity Rubric, and the Lesson Plan Quality Scale. Repeated-measures MANOVA results revealed significant time × group interactions across all three domains, with the experimental group demonstrating substantially higher posttest gains. To enrich these findings, semi-structured interviews with a subsample of twelve teachers explored their planning experiences and cognitive processes. Thematic analysis showed that AI-supported planning enhanced teachers’ ability to justify instructional decisions, expanded their creative repertoires, and fostered more deliberate pedagogical reasoning, while also highlighting the importance of critical engagement to avoid passive reliance on AI-generated content. Collectively, the results suggest that when used interactively and reflectively, ChatGPT can function as a productive cognitive scaffold supporting teachers’ instructional design. Implications include the need for targeted professional development to ensure that AI is integrated responsibly and in ways that strengthen rather than diminish teachers’ pedagogical agency.