This study investigates the conversation strategies that Korean middle school English learners employ when interacting with AI-based writing tutor chatbots. A total of 79 eighth-grade students at a middle school in Seoul engaged in English process writing tasks with ChatGPT API-powered writing tutor chatbots, composing an argumentative and an expository essay. Their dialogues during the pre-writing and drafting stages were analyzed using content analysis, with a focus on linguistic modification strategies within situated writing contexts. The results reveal that learners actively interacted with the AI writing tutor chatbots, employing various strategies to refine chatbot responses, clarify misunderstandings, and negotiate meaning. These strategies demonstrated both backward- and forward-oriented approaches, allowing students to iteratively improve their writing through dynamic interaction. Based on these findings, the study provides pedagogical recommendations for optimizing AI chatbot-mediated conversations in L2 English writing instruction.