What the study did
At selected points in a reasoning trace, the study generated many continuations from exactly that prefix and measured which answers they produced. It compared correct and incorrect source traces, localized changes in token coordinates, and tested whether small corrective edits could reopen measured access to the target.
Published configuration: Qwen2.5-Math-7B-Instruct · floor-sum task family · paper and evidence archive v1.1-RC18 · July 2026. Across 18 localized wrong-source traces, the median onset bracket was 4.5 tokens; the final wrong answer appeared a median 170 tokens after observed target reach crossed the declared dead threshold.
Where the claim stops: “Dead” is an instrument-relative label under the declared model, prefix, sampler, budget, extraction rule, validity gate, and target definition. It is not a proof that no recovery is possible under different conditions. ReachScan v0.3.5 also requires token-level prefix access; closed hosted APIs and hidden reasoning need new adapters.