Prompt-injection outcomes often form after the agent reads
Original title: The Window Is in the Reasoning: When LLM Agents Decide to Follow Prompt Injections
Thinking cuts Qwen3 variance fixed at read time from 0.91 to 0.62, and single-run labels disagree up to 35%.
The paper tests whether an LLM agent’s decision to follow a prompt injection is fixed when the injection is read. Branching rollouts replay episodes to that step and resample the remainder across four AgentDojo suites. For a Qwen3 agent without thinking, 0.91 of outcome variance is between states at read time; with thinking, that share falls to 0.62. Thinking lowers Qwen3 attack success but raises it for two other models. Single-run labels for thinking agents disagree with a rerun up to 35% of the time.
Unconfirmed: The two other models whose attack success rises with thinking are not named in the abstract.
- The drop in read-time variance has a 95% CI of [-0.38, -0.22].
- The direction holds for three attack templates and two success labels.
- Resampling from reasoning prefixes shows the decision forms gradually.
- Stronger and lookahead monitors approach what the prefix already determines.
From the original article
1 min read at arxiv.orgLLM agents that read untrusted content can be hijacked by prompt injections. Runtime monitors that try to catch a hijack before the agent acts, and benchmarks that score each task with a single run, share an assumption: whether an agent follows an injection is settled when the injection is read.
By Haoxuan Jia · arxiv.org
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