Whether the static 0 K defect picture remains valid for strongly anharmonic, entropy-stabilized $\alpha \text{−}{\mathrm{CsPbI}}_3$ is a key open question. To address this, by combining long-time molecular dynamics with machine-learned interatomic potentials, and vibrational entropy analysis, we develop a finite-temperature thermodynamic framework that explicitly captures dynamic disorder and anharmonic lattice fluctuations. We find that the charge-transition level of $V_{\mathrm{I}}$ in α-${\mathrm{CsPbI}}_3$ lies above the conduction-band minimum, demonstrating that the vacancy behaves as an extremely shallow donor, not a deep recombination center. Short Pb-Pb dimer configurations are only rare, subpicosecond fluctuations and do not correspond to a metastable minimum on the finite-temperature free-energy landscape. Crucially, we uncover a mechanism we term entropic suppression: Strong low-frequency anharmonic fluctuations reduce the vibrational entropy available to locally contracted configurations, thereby raising their free energy at finite temperature. This vibrational-entropy-driven suppression of the dimer basin resolves the long-standing discrepancy between static deep-defect predictions and experimentally observed benign behavior. Our work establishes a thermodynamic framework for understanding defect tolerance in soft-lattice semiconductors, where vibration entropy, not configurational entropy, plays the dominant role.

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• Anharmonic lattice dynamics
• Entropy
• Phonons
• Point defects
• Machine learning
• Molecular dynamics