Constant GRADIENT_TOLERANCE 

const GRADIENT_TOLERANCE: f64 = 1e-4;

First-order optimality: the fit has converged — after one final step — when the residual is this close to orthogonal to every Jacobian column — the largest |Jₐᵀr| / (‖Jₐ‖·‖r‖), the cosine MINPACK calls gtol.

Scale-free in both the parameters and the residuals, and it has a reading: under the local linear model, no single parameter can reduce ‖r‖ by more than cos²/2 of itself — 5e-9 here. The relative-improvement test cannot say that. An ill-conditioned form creeps along a curved valley, each accepted step improving by more than it and none by anything that matters: nine exponential splines on sixty-five government bonds reach this bound at step 16 and were still creeping at the 200-step cap, 0.05% lower, reported as a failure.