arXiv:2607.24509v3 Announce Type: replace Abstract: Return mapping keeps an exact elastic domain but switches its response tangent at yield, while smoothing laws with a sub-yield tail admit plastic flow below the threshold. We present SECA, a Strict-Elastic Compact Activation method for differentiable elastoplastic simulation. SECA prescribes a bounded $C^2$ activation over a finite positive-overstress interval and integrates it into a Perzyna-type flow law, so the implicit update returns exactly zero plastic increment for sub-yield trials while smoothing the onset. For scalar and proportional isotropic $J_2$ updates we derive an exact decomposition of the plastic-increment discrepancy into finite-width and viscosity contributions, and an explicit algebraic budget for the transition width. The update is embedded in finite-deformation loading histories with contact and complete unloading, and differentiated to released equilibria. Controlled indentation results stay closely matched. In the tested coarse cube-indentation run, where the transition is crossed without intermediate increments, SECA reduces whole-run Newton iterations by 47% and backtracking halvings by 93% against return mapping. A four-method comparison evaluates released-shape error and computational cost across loading partitions, and on a contact scene the path derivative drives a bounded control task to its target in three iterations. Further examples show elastic recovery and residual deformation under compression, tension and bending.