  In the mental health domain, Large Language Models (LLMs) offer promising new
opportunities, though their inherent complexity and low controllability have
raised questions about their suitability in clinical settings. We present
MindfulDiary, a mobile journaling app incorporating an LLM to help psychiatric
patients document daily experiences through conversation. Designed in
collaboration with mental health professionals (MHPs), MindfulDiary takes a
state-based approach to safely comply with the experts' guidelines while
carrying on free-form conversations. Through a four-week field study involving
28 patients with major depressive disorder and five psychiatrists, we found
that MindfulDiary supported patients in consistently enriching their daily
records and helped psychiatrists better empathize with their patients through
an understanding of their thoughts and daily contexts. Drawing on these
findings, we discuss the implications of leveraging LLMs in the mental health
domain, bridging the technical feasibility and their integration into clinical
settings.
