SelfGrader: LLM Jailbreak Detection via Anchored Token-Level Logits

Zikai Zhang, Rui Hu, Olivera Kotevska, Jiahao Xu


Abstract
Large Language Models (LLMs) are powerful tools for answering user queries, yet they remain highly vulnerable to jailbreak attacks. Existing guardrail methods typically rely on internal features or textual responses to detect malicious queries, which either introduce substantial latency or suffer from randomness in text generation. To overcome these limitations, we propose SelfGrader, a lightweight guardrail method that formulates jailbreak detection as a numerical grading problem using anchored token-level logits. Specifically, SelfGrader evaluates the safety of a user query within a compact set of numerical tokens (NTs) (e.g., 0–9) and interprets their logit distribution as an internal safety signal. To align these signals with the target safety rubric, SelfGrader constructs Probably Approximately Correct-guided ICL anchor examples and introduces a dual-perspective scoring rule that considers both the maliciousness and benignness of the query, yielding a stable and interpretable score that reflects harmfulness and reduces the false positive rate simultaneously. Extensive experiments across diverse jailbreak benchmarks, adaptive attacks, benign prompt benchmarks, multiple LLMs, and state-of-the-art guardrail baselines demonstrate that SelfGrader achieves strong robustness with low false positive rates, memory overhead, and latency. Warning: Some content generated by LLMs may be offensive.
Anthology ID:
2026.findings-aacl.27
Volume:
Findings of the Association for Computational Linguistics: IJCNLP-AACL 2026
Month:
November
Year:
2026
Address:
Hengqin, China
Editors:
Yue Zhang, Ruifeng Xu, Ayu Purwarianti, Tong Rong, Jun Suzuki
Venue:
Findings
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
493–523
Language:
URL:
https://aclanthology.org/2026.findings-aacl.27/
DOI:
Bibkey:
Cite (ACL):
Zikai Zhang, Rui Hu, Olivera Kotevska, and Jiahao Xu. 2026. SelfGrader: LLM Jailbreak Detection via Anchored Token-Level Logits. In Findings of the Association for Computational Linguistics: IJCNLP-AACL 2026, pages 493–523, Hengqin, China. Association for Computational Linguistics.
Cite (Informal):
SelfGrader: LLM Jailbreak Detection via Anchored Token-Level Logits (Zhang et al., Findings 2026)
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PDF:
https://aclanthology.org/2026.findings-aacl.27.pdf