Understanding the thermal stability of hollow metal nanoframes remains a significant challenge due to strongly coupled effects such as porosity, curvature, and defect migration. Using tight-binding Monte Carlo simulations, we systematically investigate gold and silver nanoframes with cavity radii of 7.0-19.9 Å and quantitatively resolve their structural evolution through a shape-parametrization method that tracks cavity collapse and global flattening with ångström-level precision. Silver nanoframes exhibit lateral pore closure at 605-785 K, cavity collapse at 783-844 K, and melting near 852-900 K. Gold analogues show earlier pore closure (585-720 K) but higher collapse temperatures (688-823 K) and melting at 772-825 K, consistent with more coordinated recrystallization. The flattening parameter rises to 0.18-0.22 before global deformation, serving as a universal geometric indicator. Together, these quantitative criteria advance mechanistic understanding and enable the predictive design of thermally robust porous metal nanostructures.