本研究采用案例研究法，追踪一位初中地理教师使用自建智能体设计“探究教室西晒问题”跨学科实践活动的过程，以人机对话日志、方案迭代版本和教师反思日志为数据来源，揭示了人机协同设计的决策机制，凝练出三域决策机制模型。研究发现，教师对智能体方案表现出采纳、修正和拒斥三类行为，完全采纳集中于程序性任务，划定了智能体的“效能边界”；局部修正聚焦内容补充、任务分层和表述优化等维度，体现了教师专业判断的核心“介入点”；完全拒斥则源于智能体对学情与资源的预设，暴露了其“认知盲区”。研究表明，人机协同设计的本质不是“机器输出、教师执行”，而是教师通过修正实现专业主导；智能体的价值不在于替代教师，而在于诱发教师专业决策。本研究为理解人机协同的教学设计机制提供了微观证据，也为教师AI素养培训与智能体迭代优化提供了启示。

This study employs the case study method. It traces how a junior high geography teacher uses a self-developed AI agent to design an interdisciplinary practical activity themed “Exploring Classroom West-Facing Sun Exposure.” Drawing on human-AI dialogue logs, iterative versions of the instructional plan, and teacher refl ection journals, this study reveals the decision-making mechanisms underlying human-AI collaborative design and summarizes a three-domain decision-making model. The findings reveal three distinct response behaviors exhibited by the teacher toward the AI-generated proposals: adoption, modifi cation, and rejection. Full adoption occurred exclusively in procedural tasks, thereby delineating the “effi cacy boundary” of the AI agent. Partial modifi cation focused on dimensions such as content supplementation, task hierarchy, and phrasing refi nement, refl ecting the core intervention point of teachers’ professional judgment. Total rejection was based on the presupposition of learning situation and resources, which revealed the cognitive blind spot of the agent. The study shows that human-AI design should not be a “Machine Output, Teacher Execution” model. Instead, teachers achieve professional leadership through revision. The value of an AI agent is not to replace teachers, but to stimulate their professional judgment. This study off ers micro-level evidence for understanding the mechanisms of human-AI collaborative design. It also inspires practical directions for both AI literacy training for teachers and the ongoing refi nement of educational agents.