The Fix for Hallucinating Medical Models Isn’t Bigger Data

The hospital problem Medical AI has a credibility crisis. The systems that perform brilliantly on standardized benchmarks often fail catastrophically in real hospitals. A model might achieve 95% accuracy on a test set of curated X-rays, then hallucinate a diagnosis when faced with an actual patient whose symptoms don't fit neatly into its training distribution. It confidently generates medical reports filled with plausible-sounding but fabricated details. It skips diagnostic reasoning entirely, jumping from image to conclusion without showing its work. Worst of all, when something goes wrong, there's no trace to follow, no way for a clinician to understand where the logic broke down. This gap between benchmark success and clinical failure isn't a minor engineering problem. It's the reason AI hasn't transformed medical practice despite a decade of promises. The problem runs deeper than data quality or model size. It's architectural. Most medical language models were trained on text or images in isolation, learning associations (shadows on lung scans often indicate infection) but missing the deeper reasoning that characterizes medical expertise. Real clinical work isn't pattern matching. A radiologist examining a chest X-ray doesn't just look once and guess. The patient's age is already known. Recent symptoms have been documented. The radiologist zooms in on suspicious areas. They consider what's absent as much as what's present. They form multiple hypotheses and mentally test each one. They explain their reasoning in ways other doctors can verify and challenge. absent The closed-source systems that dominate the landscape, GPT-4V and Claude, perform reasonably on benchmarks but struggle with long-form medical reasoning, generating confident claims they cannot actually support. Open-source medical models remain smaller and less capable. What was needed was a foundation model that could genuinely bridge this gap, performing at the frontier of capability while remaining transparent enough to use in clinical workflows. That's what MedXIAOHE attempts to deliver. Rethinking medical AI from first principles Building a medical AI turns out to require thinking about four distinct problems, none of which reduce to simply scaling up existing approaches. The first is knowledge breadth. Medical conditions follow a power law. Common diseases like hypertension, diabetes, and pneumonia appear thousands of times in any large dataset. Rare diseases, the ones where diagnosis is most crucial and most difficult, appear a handful of times or sometimes not at all. If a model trains exclusively on raw frequency, it becomes excellent at common presentations and blind to rare ones. That's like training a doctor entirely on typical cases then sending them to a specialty hospital. The solution isn't just collecting more data, it's organizing data to ensure comprehensive coverage. knowledge breadth organizing The second is reasoning capability. Diagnosis isn't prediction. It's reasoning under uncertainty. A pathologist examining a blood smear doesn't announce a diagnosis immediately. They follow a mental protocol: What categories of disease cause these findings? Which is most likely given the patient's presentation? What other abnormalities would I expect to see? What tests would confirm or rule out each hypothesis? Teaching this requires more than labeled examples. It requires teaching reasoning patterns. reasoning capability patterns The third is agency. Real doctors use tools. They order labs, consult reference materials, search literature, zoom in on imaging details, solicit specialist opinions. A model that can only generate text and look at images is missing half of clinical practice. Teaching agency means teaching the model when to admit uncertainty and reach for external evidence. agency The fourth is calibration. A hallucinating AI is eloquently, confidently wrong. It generates text that sounds medically plausible because it's absorbed patterns from thousands of medical texts, but it's assembled from pattern-matching rather than reasoning. Preventing this requires building honest uncertainty into the training objective itself, not hoping the model will learn to be humble. calibration MedXIAOHE addresses all four dimensions through a unified pipeline. The architecture itself is noteworthy: a Multimodal Native-Resolution Transformer that processes medical images at their natural resolution rather than forcing everything into uniform patches. MedXIAOHE architecture showing multimodal input processing through a native-resolution transformer that preserves image fidelity across diverse medical imaging modalities. MedXIAOHE architecture showing multimodal input processing through a native-resolution transformer that preserves image fidelity across diverse medical imaging modalities. Medical images are not created equal. X-rays, CT scans, pathology slides, and charts arrive at wildly different sizes and aspect ratios, each with its own information density. Downsampling a pathology slide destroys diagnostic detail. Forcing a wide panoramic X-ray into square patches loses anatomical context. The native-resolution approach respects this heterogeneity rather than smoothing it away. This architectural choice enables the model to work with medicine's visual complexity rather than fighting it. Teaching comprehensive medical knowledge The entity-aware continual pretraining framework solves the knowledge breadth problem. Medical knowledge cannot be organized by treating all examples equally. Instead, the authors built a Medical Entity Tree, a hierarchical taxonomy organizing medical concepts into aligned categories. The Medical Entity Tree structures medical knowledge hierarchically across conditions, symptoms, treatments, and anatomical structures to enable balanced training and precise knowledge coverage. The Medical Entity Tree structures medical knowledge hierarchically across conditions, symptoms, treatments, and anatomical structures to enable balanced training and precise knowledge coverage. This isn't merely a database schema. It's a teaching structure. By training on data organized hierarchically, the model learns that certain concepts belong together, that rare entities deserve explicit attention, and that medical knowledge is structured. The tree enables the pretraining process to actively balance coverage of common conditions against rare ones. The data construction pipeline itself is unglamorous but essential. Text cleaning runs in parallel with multimodal data production, combining automated quality filters with human curation to ensure coherence. The result is a pretraining corpus organized not by raw frequency but by medical completeness. The data-cleaning pipeline for pretraining combines text cleaning and multimodal data production workflows to construct a high-quality corpus suitable for entity-aware learning. The data-cleaning pipeline for pretraining combines text cleaning and multimodal data production workflows to construct a high-quality corpus suitable for entity-aware learning. Long-tail entities require not just frequency but contextually rich examples. A rare disease appearing once as an isolated mention teaches nothing. The same rare disease appearing in realistic case reports, clinical literature, and expert-annotated contexts teaches structure and reasoning patterns. The team synthesized training data explicitly to this standard, aggregating diverse sources to ensure even uncommon conditions appeared in meaningful clinical contexts. contextually rich examples This approach solves a subtle but critical problem: by the time the model reaches mid-training, it has learned not just facts but the landscape of medical knowledge. It understands which concepts belong together and where gaps exist. This foundation matters when the next phase begins. Learning to reason like a clinician After pretraining on medical knowledge, the model needs to learn how to use that knowledge. This is where diagnosis shifts from pattern-matching to reasoning. how Mid-training data construction synthesizes step-by-step reasoning sequences. Rather than showing the model thousands of images and labels, the team created high-fidelity reasoning chains by aggregating medical literature and clinical notes, then using expert supervision to construct sequences showing diagnostic workflow. An example flows like this: "Patient presents with dyspnea. The differential diagnosis includes pneumonia, congestive heart failure, and pulmonary embolism. The examination findings suggest..." The model then learns to generate the next reasoning step. This teaches that diagnosis is sequential, iterative, and conditional. It's not "see symptoms, output diagnosis." It's "see symptoms, gather evidence, refine hypotheses, reach conclusion." Mid-training data construction creates high-fidelity medical reasoning sequences from diverse unsupervised sources augmented with expert supervision to teach step-by-step diagnostic workflows. Mid-training data construction creates high-fidelity medical reasoning sequences from diverse unsupervised sources augmented with expert supervision to teach step-by-step diagnostic workflows. But real doctors don't reason in isolation. They use tools. The next phase teaches this explicitly through agentic data synthesis. The model learns to reason sequences where it actively calls tools: general search through Google and Scholar, medical-specific search through drug labels and clinical databases, image manipulation through zoom, rotation, and cropping. This is where hallucination gets prevented, not through perfect training data but through teaching the model to externally verify claims. Rather than expecting the model to know everything from pretraining, it learns when to admit uncertainty and reach for evidence. It's the difference between a doctor confidently stating a drug interaction from memory versus one who says, "Let me check the literature on that" and actually looks it up. Agentic data synthesis teaches the model to use tools including general search, medical search, and image editing to verify reasoning and resolve uncertainty. Agentic data synthesis teaches the model to use tools including general search, medical search, and image editing to verify reasoning and resolve uncertainty. Post-training then optimizes for what actually matters. Using supervised fine-tuning followed by reinforcement learning, the model learns to navigate complex trade-offs between accuracy, explainability, and confidence calibration. This phase combines diverse guidance signals, each shaped by what clinicians actually need. Post-training pipeline applies supervised fine-tuning and reinforcement learning to optimize diagnostic reasoning for clinical utility and reliability. Post-training pipeline applies supervised fine-tuning and reinforcement learning to optimize diagnostic reasoning for clinical utility and reliability. Making reasoning verifiable and external Teaching tools and grounding are inseparable. When the model learns to use external verification, it also learns to point to what triggered each conclusion. This serves two purposes simultaneously: it communicates clearly what finding prompted the diagnosis, and it makes reasoning auditable. Medical grounding reasoning takes a raw image, reasons through what it observes, and outputs bounding boxes marking the abnormalities. This isn't simply helpful visualization. It's the mechanism by which reasoning becomes verifiable. Another clinician can look at the marked region and independently judge whether the finding exists. Medical grounding reasoning marks abnormalities with bounding boxes, making diagnostic conclusions traceable and independently verifiable. Medical grounding reasoning marks abnormalities with bounding boxes, making diagnostic conclusions traceable and independently verifiable. Similarly, magnification-assisted reasoning mimics how radiologists actually work. When something appears ambiguous at normal magnification, you zoom in. The model learns this as a taught skill. When uncertain about a finding, it doesn't speculate. It calls a zoom tool, examines the magnified region, and then reasons about what it observes. then Magnification-assisted reasoning shows the model analyzing an original image, calling a zoom tool to examine suspicious areas, and rendering conclusions based on the magnified view. Magnification-assisted reasoning shows the model analyzing an original image, calling a zoom tool to examine suspicious areas, and rendering conclusions based on the magnified view. These capabilities matter because they bridge the gap between "the model gave an answer" and "I understand why and can verify it." Without grounding and magnification, even correct diagnoses feel like black boxes. With them, the reasoning becomes transparent and subject to clinical judgment. Building trust through calibrated uncertainty The hardest problem in medical AI is preventing confident hallucination. A model that generates eloquent medical reasoning filled with plausible details but detached from actual evidence is worse than useless. It's dangerous. Traditional approaches assume the solution is better training data. MedXIAOHE's approach is different: teach the model that evidence-grounded reasoning with honest uncertainty is preferable to confident guessing. This gets built into the training objective itself through a multi-layered reward system. preferable The multi-layered hybrid reward system directs inputs to parallel rule-based and rubric-based reward modules, combining signals to optimize for trustworthy reasoning. The multi-layered hybrid reward system directs inputs to parallel rule-based and rubric-based reward modules, combining signals to optimize for trustworthy reasoning. The rule-based reward module checks for specific desirable properties: Is evidence cited? Is reasoning step-by-step? Are there any unsupported claims? The rubric reward module captures more holistic judgments: Does the reasoning match how human experts reason? Is uncertainty appropriately calibrated? These reward signals aren't combined into a single score. Instead, a data router directs different inputs to different modules, and an aggregation layer combines signals. The model doesn't learn a single "correct answer." It learns a preference landscape: evidence-grounded reasoning with honest uncertainty beats confident hallucination. User-preference rubrics make this even more concrete. Rather than defining "correct medical reasoning" abstractly, the team collected rubrics from actual clinicians describing what they want from an AI assistant. This grounds the training objective in clinical usefulness rather than arbitrary benchmark metrics. When generating diagnostic reports, the model learns to cite evidence explicitly. "The findings suggest pneumonia because of X, which is supported by Y." This makes reasoning auditable. Even if a conclusion proves wrong, the logic chain remains visible. Another clinician can identify where reasoning broke down and correct it. These mechanisms don't eliminate errors. They change the type of error. Instead of confidently wrong conclusions, the model makes transparently reasoned claims. When mistakes occur, they're usually in the reasoning chain rather than in fundamental understanding. And reasoning chains can be corrected. type Verification in practice The benefits become concrete when examining actual diagnostic reasoning. Tool-assisted medical reasoning shows the model retrieving and verifying evidence before producing a final answer. Rather than generating a report entirely from the image, it systematically checks claims. Tool-assisted medical reasoning retrieves and verifies evidence before producing diagnostic conclusions, creating an auditable reasoning chain. Tool-assisted medical reasoning retrieves and verifies evidence before producing diagnostic conclusions, creating an auditable reasoning chain. Complex diagnostic cases demonstrate why these capabilities matter. Consider a case presenting pancytopenia with splenomegaly. A pattern-matching approach might recognize common causes: aplastic anemia, leukemia, infection. But reasoning-based diagnosis goes further. The model identifies hairy lymphoid cells on the peripheral smear, connects them to the systemic findings, recognizes the pattern as Hairy Cell Leukemia, and confirms with TRAP staining. The diagnosis emerges from reasoning through related findings, not pattern-matching against memorized presentations. A complex diagnostic case showing Hairy Cell Leukemia diagnosed through reasoning that connects pancytopenia, splenomegaly, and characteristic lymphoid cell morphology. A complex diagnostic case showing Hairy Cell Leukemia diagnosed through reasoning that connects pancytopenia, splenomegaly, and characteristic lymphoid cell morphology. This reasoning quality becomes possible because the model has learned that medical diagnosis is structured reasoning, not image classification. It understands that findings relate to each other, that some explanations are more parsimonious than others, and that verification matters. Measuring what matters How do you evaluate whether a medical AI actually works? Benchmark metrics are convenient but misleading. A model can perform well on standardized test sets while failing in real clinical workflows. MedXIAOHE was evaluated across more than 30 benchmarks spanning diagnostic accuracy on diverse conditions, reasoning quality, tool usage patterns, uncertainty calibration, and report usefulness as judged by clinicians. Performance was compared against closed-source systems like GPT-4V and against specialized medical models. Performance comparison showing MedXIAOHE's results across comprehensive medical benchmarks, with average scores and performance across specialized domains. Performance comparison showing MedXIAOHE's results across comprehensive medical benchmarks, with average scores and performance across specialized domains. The evaluation framework matters more than any single number. Rather than treating evaluation as a checklist of benchmark scores, the work treats it as measuring whether the model exhibits capabilities needed in real practice: Can it reason through ambiguous cases? Does it know when to use tools? Is it appropriately calibrated? Does it generate reports clinicians actually find useful? A model that reasons transparently, even if slightly less accurate on narrow benchmarks, might be more valuable in practice than one that's slightly more accurate but inexplicable. The evaluation framework makes space for this kind of reasoning about trade-offs. The recipe, not the trick The title uses the word "recipe" deliberately. This work shares not a single architectural breakthrough but a design philosophy for how to build medical AI that works in practice. Each component addresses a specific problem: entity-aware pretraining ensures knowledge breadth, multi-phase training teaches reasoning workflows, tool-augmented training creates agency, and multi-layered rewards build calibration. Together they create a system that thinks like a clinician rather than a classifier. The practical design choices are thoroughly documented. How should medical pretraining corpora be organized? What reasoning patterns should be taught? Which tools actually reduce hallucination? How do you integrate human preferences into the training objective? The paper treats these as engineering questions with practical answers, not philosophical mysteries. This approach connects to broader work in medical AI reasoning. Systems like MedXAI have explored retrieval-augmented reasoning for medical tasks. MMED-Expert R1 has investigated reinforcement learning for multimodal medical reasoning. LingShu has worked on building generalist foundation models for medicine. MedXIAOHE synthesizes insights from this landscape while adding its own contributions around entity-aware knowledge organization and multi-layered reward systems. MedXAI MMED-Expert R1 LingShu The work ultimately makes a claim about what medical AI needs to become clinically useful: not bigger models or better benchmarks, but systems that reason transparently, verify their reasoning, acknowledge uncertainty honestly, and work with tools rather than pretending to omniscience. That's a high bar to clear, but it's the bar that matters in actual hospitals. This is a Plain English Papers summary of a research paper called MedXIAOHE: A Comprehensive Recipe for Building Medical MLLMs. If you like these kinds of analysis, join AIModels.fyi or follow us on Twitter. This is a Plain English Papers summary of a research paper called MedXIAOHE: A Comprehensive Recipe for Building Medical MLLMs. If you like these kinds of analysis, join AIModels.fyi or follow us on Twitter. This is a Plain English Papers summary of a research paper called MedXIAOHE: A Comprehensive Recipe for Building Medical MLLMs. If you like these kinds of analysis, join AIModels.fyi or follow us on Twitter.