2026-10-20 · for chapter 08 (prompt injection supply chain) · from the research reading The Window Is in the Reasoning: When LLM Agents Decide to Follow Prompt Injections
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This paper bears directly on the prompt-injection chapter's treatment of where a containment boundary can sit. It shows that in the native tool-calling layout the injection arrives at the end of the context, so the read-time boundary that most runtime monitors assume has no lead time, and that for reasoning agents part of the compliance decision only forms during the chain of thought. The chapter's argument that injection defence must be structural rather than detective gains support, but it should be revised to say that a pre-action window does exist inside the reasoning and that only strong monitors or simulated-action lookahead can use it. It should also record that enabling thinking raised attack success for two of seven models, which cuts against any framing of reasoning as a safety control. Finally, the result that single runs mislabel thinking agents up to a third of the time should be carried into the replay chapter: a replayed incident that fails to reproduce is not evidence the original run was wrong.
Claim. For agents with thinking enabled, part of the decision to follow an injection moves out of the context and into the reasoning, so it is not fixed when the injection is read.
Strongest counterclaim. The premise is model-dependent and sometimes absent: SmolLM3-3B has V of 0.25 in both modes and 46% mixed states, so there is no read-time decision for thinking to move; Gemini 2.5 Flash on slack shows V rising with thinking (+0.25, CI spanning zero).
Open question for the chapter. Does the read-time share drop with thinking on a non-AgentDojo benchmark such as InjecAgent or WASP, and for other families (Llama 4, DeepSeek R1, Gemma 3 thinking variants)?
Sources: https://arxiv.org/html/2610.24048v1, https://arxiv.org/abs/2610.24048v1