Medical errors in large language models revealed using 1,000 synthetic clinical transcripts

Current clinical evaluations of large language models (LLMs) rely on datasets which fail to reflect real-world medical complexity. We developed a high-throughput simulation of patients presenting with headache and generated 1,000 doctor-patient transcripts, enabling an unprecedented mapping of clinical reasoning failures across a vast spectrum of demographic and clinical phenotypes. While GPT-5.2 achieved 97.5% diagnostic accuracy with full histories (95% confidence interval: 95.0-99.5), incomplete transcripts triggered hazardous recommendations. Instead of requesting further information, both models discouraged essential investigations, including 100% of lumbar punctures in subarachnoid haemorrhage cases. For life- or sight-threatening emergencies, models inappropriately downgraded triage to self-management or routine follow-up in up to 54.8% cases (GPT-5-mini; 95% CIs: 40.5-69.0). GPT-5.2 triage was significantly less safe for females than males (odds ratio 3.2 95% CI 1.4-7.1). Our methodology transforms medical AI evaluation from simple snapshots into a comprehensive safety stress-test, revealing critical failures in risk calibration despite high diagnostic accuracy.