arXiv:2602.08062v2 Announce Type: replace-cross Abstract: Large Language Models have demonstrated remarkable capabilities in natural language understanding. However, they remain susceptible to malicious prompts that induce unsafe or policy-violating behavior. Existing defenses in black-box moderation APIs offer limited transparency and adapt poorly to evolving threats, while white-box approaches using large LLM judges impose prohibitive computational costs and require expensive retraining for new attacks. We present BAGEL (Bootstrap AGgregated Ensemble Layer), a modular, lightweight, and incrementally updatable framework for malicious prompt detection. BAGEL employs a bootstrap aggregation and mixture-of-experts-inspired ensemble of finetuned models, each specialized on a different attack dataset. At inference, BAGEL aggregates predictions from a stochastically selected subset of ensemble members, optionally augmented by a lightweight learned router that predicts the most suitable member to include. When new attacks emerge, BAGEL updates incrementally by finetuning just a single small classifier (86M parameters) and adding it to the ensemble. We evaluate BAGEL on a held-out, source-disjoint set constructed from datasets never used for training or calibration, ensuring that performance reflects generalization to out-of-distribution attacks. In single-turn evaluation, BAGEL achieves an F1 score of 0.757 at a size equivalent to 430M parameters, improving to 0.772 when augmented with the router using fewer ensemble members (324M parameters). These results outperform popular black-box moderation APIs and most white-box baselines at lower costs. In multi-turn evaluation, BAGEL achieves an F1 score of 0.660, outperforming billion-parameter methods. Our results show ensembles of small finetuned classifiers can match or exceed the performance of billion-parameter guardrails while offering the adaptability and efficiency required for production systems.