SpecGrove: Reusable Tree Capacity for Concurrent Diffusion Speculation

Concurrent block-diffusion speculative decoding must share limited verification capacity across requests with heterogeneous returns from tree expansion. We introduce SpecGrove, which turns a single DFlash proposal and DDTree expansion per request into reusable, nested capacity tiers. SpecGrove scores tiers by covered draft-prefix mass and jointly selects heterogeneous tree sizes using measured packed-pass cost. An exact-row Bellman frontier optimizes the resulting finite-tier surrogate, and selected trees share one request-isolated target pass. We establish optimality for this finite-tier objective and characterize conditions under which packed verification preserves the joint autoregressive output law. Across Qwen3-4B and Qwen3-8B on four reasoning and code-generation benchmarks, SpecGrove improves complete wall-clock throughput by 13.0–16.8% over matched DFlash and 4.7–39.0% over capacity-feasible fixed-size DDTree at concurrency eight. Under Poisson arrivals, it reduces tail latency relative to matched allocation controls.