Beyond Parallel Blindness: Information Floors and Model Gaps in Block Drafting

TL;DR - This paper decomposes block-drafting rejection into an unavoidable “information floor” caused by missing within-block context and a “model gap” caused by imperfect proposals. The results show that revealing just one preceding token removes most of the information floor, while current drafters still have substantial room for improvement.

• On Qwen3-4B, the all-parallel information floor reaches 0.286 at the final block position, capping optimal per-slot acceptance at 71%.
• Realizing one earlier token removes 86–100% of the floor, indicating that the missing information is highly local.
• Mutual-information analysis independently supports this short-range conditioning effect.
• Proposal quality remains a major bottleneck: the final-slot model gap causes 43–64% of DFlash rejection and 85–92% of DSpark’s oracle-conditioned rejection.