Speculative decoding changes the numerical history of a quantized runtime as well as its execution cost. When an application requires a fixed ordinary greedy output, the runtime must establish where constraints are needed and whether the qualified design saves time. We connect local state/path diagnosis, phase-scoped repair and measured comparisons with ordinary and matched target-only execution. Natural donor-state crosses on CUDA and Vulkan distinguish current-path, retained-state and interaction effects; a separate B570 fork study shows that immediate agreement can coexist with later divergence. On RTX, a query-layout transformation retains batched projection and feed-forward work. Target prefill and continuation form an inclusion-minimal passing set within a tested four-switch family, and the frozen design qualifies full responses on 24 project-unused inputs. Resident-request speed is 1.742 times ordinary execution; the matched draft-free path already achieves 1.683 times, leaving a 1.035-times speculative increment. Seven tasks regress against matched request time, and one round reverses direction. Wider fixed-capacity drafting and a B570 endpoint repair qualify while losing performance. Auxiliary M4 and 358H cases bound the value of added control and draft-only optimization. These results support a configuration-specific decision process from diagnosis through qualification to cost comparison; repair evidence remains restricted to the tested Qwen3 family.

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