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For pairing by K-point optical phonons ($\\hbar \\omega \\sim 0.15\\,\\mathrm{eV}$ $>> \\text{ flat-band width}$ $W \\sim 10\\,\\mathrm{meV}$) the anti-adiabatic regime can give $\\alpha_{\\mathrm{iso}}$ well below the BCS value 0.5; a purely electronic mechanism gives $\\alpha_{\\mathrm{iso}} \\sim 0$. Since $\\ln(13/12) = 0.08$, the answer is $\\alpha_{\\mathrm{iso}}$ with an error bar small enough to separate the computed predictions, which requires a statistical ensemble of devices.","problem_ref":"69afa9fbcd1de23920428803e8538fb40733550a31ec3f4256640b9c953a2dff","references":"","settled_by":"Transport $T_c$ measured on ensembles of 12C and 13C magic-angle devices with matched angle and strain and quantified sample-to-sample scatter, compared with mechanism-specific calculations of $\\alpha_{\\mathrm{iso}}$ in the flat-band regime.","status_note":"No measurement on isotope-enriched magic-angle devices is known to this survey as of 2026.","title":"Carbon isotope effect on the magic-angle superconducting transition 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