Prefilling the Reasoning Channel: Output-Prefix Attacks on Reasoning LLMs
Output-prefix attacks plus injected reasoning jailbreak Gemini, DeepSeek, and Claude models at up to 99% success.
A paper isolates the scratchpad reasoning channel as an output-prefix attack on reasoning LLMs, comparing reasoning-only, prefix-only, and combined attacks. Across 1,800 AdvBench cases against Gemini 3 Flash Preview, DeepSeek V4 Flash, and Claude Haiku 4.5, injecting malicious reasoning alone had about 0% success. Combining that reasoning with a trivial output prefix raised attack success to as high as 99% on some models. Contextual prefixes worked better than static ones, and results depended on the model.
- Malicious reasoning alone produced about 0% attack success.
- Adding a trivial output prefix raised success to as high as 99%.
- The study used 1,800 AdvBench cases on three 2026 frontier models.
- Contextual prefixes outperformed static prefixes.
- Susceptibility differed between exposed-reasoning and hidden-reasoning models.
From the original article
1 min read at arxiv.orgLarge Language Models (LLMs) consume and produce a single sequence of text; hence, if text can be added to the beginning of the LLM's response, i. e. , an output prefix, then all subsequent tokens will be conditioned on it.
By Lukáš Brůna · arxiv.org
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