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The Materials Project (MP) has become the de facto benchmark for machine learning models in computational materials property prediction. Its vast repository of density-functional-theory (DFT) data has enabled rapid progress in graph neural networks, representation learning, and transfer-learning approaches. Yet this critique demonstrates that the field is systematically overfitting to MP’s structural and compositional artifacts. Far from being a neutral or representative sample of chemical space, MP is heavily skewed toward thermodynamically stable compounds lying near the convex hull, high-symmetry crystal systems, near-equiatomic stoichiometries, and a narrow set of well-studied elemental combinations. Models that achieve state-of-the-art mean absolute errors on MP test sets therefore learn dataset-specific regularities rather than transferable physical principles. Six interlocking forms of bias are identified: stability bias, symmetry bias, composition bias, prototype bias, element-frequency bias, and relaxation bias. Additional absence of disordered structures compounds the problem. The consequences are severe. Reported generalization performance is illusory when evaluated only on MP-held-out data; dramatic performance drops occur on independent databases, low-symmetry subsets, dilute alloys, and experimental formation energies. State-of-the-art claims based solely on MP benchmarks therefore mislead the community, waste experimental validation resources, and retard genuine advances in extrapolation and disorder handling. Drawing exclusively on peer-reviewed analyses published between 2017 and 2025, this work argues that the community’s MP-centric evaluation culture has created a benchmark artifact that masquerades as scientific progress. Debiased sampling, cross-dataset validation, subgroup reporting, and multi-database training are shown to be essential corrective measures. Until these practices become standard, claims of robust materials AI will remain premature. The critique concludes with concrete recommendations for benchmark designers, researchers, and journal editors to restore scientific integrity to property-prediction evaluation.