CoFiBreak: A Coarse-to-Fine Diversity Reward for Effective Multi-Turn Jailbreak

Summary

CoFiBreak is a coarse-to-fine diversity reward for Group Relative Policy Optimization (GRPO) that trains an attacker LLM to produce more varied multi-turn jailbreaks. The coarse reward favors strategies underrepresented within each GRPO group, and the fine reward discourages reuse of the same personas, framings, and output formats. Against Llama3.1-8B-Instruct on HarmBench it finds 1.6-2.0x as many distinct successful fine-grained strategies as the strongest baseline, and it reaches 98.8% ASR@10 on AdvBench. The attacks transfer to unseen frontier models, reaching 71.0% ASR@10 on AdvBench against Claude-Opus-4.6.

Topics

• Prompt injection and jailbreaks

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