We present the first numerically stable nonlinear evolution for leading-order gravitational effective field theory (quadratic gravity) in spherically-symmetric sector. The formulation relies on (i) harmonic gauge to cast system into quasilinear form (ii) Cartoon method reduce spherical sym\ifmmode \bar{\imath}\else \={\i}\fi{}metry keeping with gauge, and (iii) order reduction (in time) by mean...