Speculative Block Drafting Has an Information Floor: 0.286 Rejection on Qwen3-4B Caps Any Drafter at 71% Acceptance

arXiv 2608.27339·medium signalXBlueskyLinkedInCopy link

Block drafters propose several tokens per forward pass, and their rejections mix two different losses that accepted-length metrics cannot separate. The authors define an information floor, the minimum expected rejection at a given conditioning order, so rejection above it is the model gap. Across four domains, four open-weight targets and a frontier API target, the all-parallel floor hits 0.286 at the final slot on Qwen3-4B, one realized token removes 86-100% of that floor, and current drafters sit far above their floors, with the final-slot model gap accounting for 43-64% of DFlash rejection and 85-92% of DSpark's oracle-conditioned rejection.