<p>The purpose of the study is to investigate how Generative Artificial Intelligence (GenAI) tools can be integrated into language lesson planning within the Lesson Study (LS) framework in a language teacher education program and to determine the challenges and benefits faced by the participants while they integrate these tools into their lesson planning practices. The study used a qualitative case study research design to explore the use of GenAI tools and the challenges and benefits faced by pre-service language teachers. This method allows an in-depth exploration of the interactions between practicum students, GenAI tools, and classroom practices. The case study approach captures the nuances of individual experiences and provides rich, contextualized data, which is essential for understanding how GenAI technologies shape teaching and learning processes. The findings showed that the participants used a variety of tools for creating exercises and lesson plans within the LS framework. The participants highly valued GenAI tools, as these tools helped them create efficient lesson plans and exercises, especially when there were functions and topics that coursebooks failed to cover. The quality of these lesson plans was evaluated during the stages of the LS framework and after the implementation in the real classes. The findings also showed that the participants faced several issues, such as over-reliance, bias, and the quality of outputs.</p>