Machine-learning models routinely outperform the generalized linear models (GLMs) that dominate regulated underwriting practice, yet they rarely reach production because their predictions cannot be decomposed into the auditable rating factors that internal validation, filing, and governance processes require, and because their behaviour in thinly supported regions of the portfolio is difficult to certify. We propose Credibility-Gated Anchored Boosting (CGAB), a hybrid architecture with three tightly coupled components: (i) a rating-band GLM whose linear predictor anchors a gradient-boosting machine through its offset, so that the machine-learning layer learns only a multiplicative correction to the regulatory tariff; (ii) domain monotonicity constraints imposed on the correction, followed by a one-constant balance recalibration of the composite model; and (iii) a novel credibility gate that shrinks every leaf of the fitted ensemble by the Bühlmann factor zℓ = Eℓ/(Eℓ +κ), where Eℓ is the training exposure routed to leaf ℓ, implemented by direct model surgery on the fitted ensemble. Because the gate acts linearly on leaf values, the gated model remains a standard tree ensemble: exact TreeSHAP attributions in the log link decompose every prediction, without approximation error, into per-factor multiplicative relativities that combine with the GLM tariff into a policy-level audit sheet. On 677,991 French motor third-party liability policies, CGAB improves test Poisson deviance by 0.83% and Gini lift by nine percent relative to the GLM, while conceding 1.38% of deviance to an unconstrained LightGBM of identical capacity-an explicit, quantified price for the guarantee bundle of monotonicity, tariff traceability, and credibility control, with each component's individual cost isolated by ablation. The credibility gate implements graceful degradation towards the filed tariff: under 5-10% training subsamples it improves deviance over ungated boosting by more than one percent, and on a small Australian portfolio, where no machine-learning model beats the GLM, it recovers most of the lost ground while unanchored boosting falls furthest behind; on heavy-tailed severity data, unanchored gamma boosting diverges catastrophically while the anchored model remains close to the GLM and attains the best mean absolute error. The audit decomposition is exact to machine precision (maximum reconstruction error 3.8 × 10-15), turning every prediction into a verifiable rate calculation rather than a post-hoc approximation.