Evaluating Diagnostic Accuracy and Clinical Reasoning of Multiple Large Language Models in Psychiatry

Summary Background Existing large language model (LLM) evaluations rely on accuracy benchmarks that fail to capture whether models reason well while making diagnoses. Studies that do analyse reasoning focus on post hoc explanations accompanying model outputs rather than distinct, clinician-visible artifacts such as detailed reasoning traces. This creates a translational gap in domains such as psychiatry, where diagnosis relies on narrative interpretation, diagnostic reasoning, and clinical judgment under uncertainty. Methods We conducted a mixed-methods evaluation of four state-of-the-art LLMs using a clinician-curated dataset of 196 psychiatric case vignettes, including 135 published cases and 61 novel clinician-authored vignettes. Diagnostic accuracy was assessed using multiple metrics (top-1 accuracy, top-5 accuracy, recall@5, and mean reciprocal rank) based on ranked lists of five differential diagnoses per vignette. Clinical reasoning quality was evaluated on a randomly selected subset of 30 vignettes through clinician assessment of model-generated diagnostic reasoning traces alongside qualitative commentary from board-certified psychiatrists. We examined the association between clinician-rated reasoning quality and diagnostic correctness and included an illustrative comparison with psychiatry residents. Findings Clinician-rated diagnostic reasoning quality was strongly associated with diagnostic correctness in mixed-effects logistic regression analyses (β = 1·80; p < 0·001), whereas data extraction quality alone was not. Across the full vignette set, models demonstrated moderate to high diagnostic accuracy. The highest-performing model achieved a top-5 accuracy of 0·801 and also received the highest clinician-rated reasoning scores. In an illustrative comparison, model diagnostic accuracy fell within the range observed for psychiatry residents. Interpretation Diagnostic reasoning quality captures clinically meaningful variation in LLM performance beyond accuracy metrics. Psychiatry may represent a stringent testbed for evaluating reasoning in narrative-driven clinical domains. Evaluation frameworks for LLM-based clinical decision support should incorporate structured assessment of reasoning processes, not accuracy alone. Research in Context Evidence before this study We searched PubMed and Scopus for studies evaluating large language models for psychiatric diagnosis and/or differential diagnosis from text vignettes. Searches were run from database inception to February 6, 2026, using terms including (“large language model” OR LLM “artificial intelligence” OR “generative AI” OR AI OR ChatGPT OR GPT OR Claude OR Gemini OR DeepSeek OR Llama) AND (psychiatr* OR mental OR DSM OR “differential diagnosis” OR diagnos*) AND (vignette OR case OR “case report”). We included empirical studies that evaluated model diagnostic outputs using psychiatric cases/vignettes and excluded editorials/commentaries and studies that did not report case-level diagnostic performance. We did not formally assess risk of bias/quality; studies were heterogeneous in vignette sources, model access, and outcome definitions, precluding quantitative pooling. Prior studies have typically examined small vignette sets, focused on narrow diagnostic domains, evaluated single models, or relied primarily on outcome-based accuracy metrics. When diagnostic reasoning has been assessed, it has usually been inferred from post hoc explanations accompanying model outputs rather than evaluated as a distinct, clinician-visible artifact. Clinician-grounded evaluations of diagnostic reasoning across multiple contemporary models remain limited. Added value of this study This study provides a large-scale, clinician-grounded evaluation of diagnostic accuracy and diagnostic reasoning quality across four contemporary large language models using a diverse dataset of psychiatric case vignettes. Rather than relying solely on outcome-based explanations, we directly evaluated model-generated diagnostic reasoning traces as clinician-visible artifacts using structured clinician ratings and qualitative analysis. By integrating multiple accuracy metrics with clinician assessment of reasoning coherence, flexibility, and plausibility, we demonstrate that clinician-rated reasoning quality is strongly associated with diagnostic correctness, whereas data extraction quality alone is not. Our analysis also identifies recurrent reasoning failure modes not captured by accuracy metrics, highlighting psychiatry as a stringent testbed for evaluating reasoning in narrative-driven clinical domains. Implications of all the available evidence Evaluations of large language models for clinical decision support should extend beyond accuracy to include systematic assessment of clinician-visible diagnostic reasoning. Mixed-methods, clinician-grounded evaluation frameworks that examine both diagnostic outcomes and reasoning artifacts may be critical for responsible assessment of LLMs in psychiatry and other areas of medicine where diagnosis depends on interpretation, judgment, and tolerance of uncertainty.