We develop a semiclassical effective theory describing how geometric fluctuations modify reconstruction transitions near Quantum Extremal Surfaces (QESs). Rather than postulating a phenomenological smoothing law, we derive a Conditional Convolution Theorem from conditional recovery channels and local semiclassical regularity assumptions. This theorem establishes that the observed reconstruction fidelity is governed by a convolution between a geometric fluctuation measure and an intrinsic reconstruction profile. We then evaluate the framework within Jackiw-Teitelboim (JT) gravity coupled to a conformal matter sector. By explicitly analyzing the generalized entropy landscape along the stable island branch, we show that the associated Hessian remains strictly positive throughout the controlled semiclassical regime. This guarantees a finite QES susceptibility and excludes critical behavior within the validity domain of the effective theory. Using the Schwarzian boundary mode, we derive the leading gravitational contribution to the QES position variance and demonstrate that it is parametrically suppressed in the large-N limit. Under a set of axiomatic regularity conditions imposed on a generic intrinsic reconstruction profile, we prove a Universal Curvature Theorem showing that the leading gravitational correction to reconstruction fidelity is uniquely controlled by the local curvature of the intrinsic profile. The resulting correction scales as F '', where the competition parameter vanishes in the macroscopic semiclassical limit. Our results establish that semiclassical gravitational fluctuations act as a controlled finite-size deformation of reconstruction transitions rather than generating an independent universality class within the stable island regime. The framework provides a mathematically constrained effective description of reconstruction smoothing while clearly delineating its domain of validity and its separation from Planckian physics.
Sharukhan A (Mon,) studied this question.