Multi-agent large language model (LLM) systems have emerged as a promising approach for clinical diagnosis, leveraging collaboration among agents to refine medical reasoning. However, most existing frameworks rely on single-vendor teams (e.g., multiple agents from the same model family), which risk correlated failure modes that reinforce shared biases rather than correcting them. We investigate the impact of vendor diversity by comparing Single-LLM, Single-Vendor, and Mixed-Vendor Multi-Agent Conversation (MAC) frameworks. Using three doctor agents instantiated with o4-mini, Gemini-2.5-Pro, and Claude-4.5-Sonnet, we evaluate performance on RareBench and DiagnosisArena. Mixed-vendor configurations consistently outperform single-vendor counterparts, achieving state-of-the-art recall and accuracy. Overlap analysis reveals the underlying mechanism: mixed-vendor teams pool complementary inductive biases, surfacing correct diagnoses that individual models or homogeneous teams collectively miss. These results highlight vendor diversity as a key design principle for robust clinical diagnostic systems.

Large language models (LLMs) increasingly exhibit sycophancy—the tendency to conform to user beliefs rather than provide factually accurate information—posing significant risks in healthcare applications where reliability is paramount. We evaluate sycophantic behavior in ten LLMs from OpenAI, Google, and Anthropic across multi-turn medical conversations using an escalatory pushback framework. To enable fine-grained analysis, we introduce Resistance, a metric that measures nonconformity to user stances at each conversational turn, providing insights beyond existing flip-based metrics. Evaluating on MedCaseReasoning (open-ended diagnostic questions) and PubMedQA (clear-answer biomedical questions), we find that Gemini models exhibit the highest Resistance, followed by OpenAI and Claude models. We further observe that response patterns ("Yes, but..." vs. "Yes, and...") may be more predictive of sycophancy than specific phrases. Notably, all models are more easily persuaded to change their answers on clear multiple-choice questions than on ambiguous diagnostic cases. Our findings highlight critical vulnerabilities in deploying LLMs for clinical decision support and suggest that training toward contradiction-maintaining response patterns may serve as a potential mitigation strategy.