Words to Describe What I'm Feeling: Exploring the Potential of AI Agents for High Subjectivity Decisions in Advance Care Planning

Serious illness can deprive patients of the capacity to speak for themselves. As populations age and caregiver networks shrink, the need for reliable support in Advance Care Planning (ACP) grows. To probe this fraught design space of using proxy agents for high-risk, high-subjectivity decisions, we built an experience prototype (\acpagent{}) and asked 15 participants in 4 workshops to train it to be their personal proxy in ACP decisions. We analysed their coping strategies and feature requests and mapped the results onto axes of agent autonomy and human control. Our findings argue for a potential new role of AI in ACP where agents act as personal advocates for individuals, building mutual intelligibility over time. We conclude with design recommendations to balance the risks and benefits of such an agent.