Decoupled Contrastive Decoding via Expert-Aligned Drafting

Abstract:Contrastive Decoding (CD) improves generation quality, but its amateur-model pass makes decoding expensive. Accelerating CD with speculative decoding raises a proposal-alignment question: should the contrastive signal shape the drafter, or should it remain only in verification? We study this question in the lightweight feature-level drafter regime. Two controlled diagnostics, matched Cross-alpha training and an Approximate Dual-Drafter decomposition, give the same diagnosis: contrastive-aware drafting does not consistently improve over expert-aligned drafting because the contrastive correction is usually weaker than drafter error, and reconstruction can amplify that error. We introduce Decoupled Contrastive Decoding (DCD), which drafts with an expert-aligned lightweight proposer and applies the amateur only in unchanged CD verification. Standard speculative verification preserves the vanilla-CD output distribution. Across the main 8B settings, EAGLE3-based DCD achieves average greedy speedups of 1.65 to 1.95x over vanilla CD and reduces MMLU proposal-path latency by about 5 to 12x relative to amateur-coupled proposal paths.

28 pages, 11 figures, 20 tables. Code: this https URL Accepted to EMNLP 2026 (Main Conference)

Computation and Language (cs.CL)

arXiv:2608.12913 [cs.CL]

(or arXiv:2608.12913v2 [cs.CL] for this version)

https://doi.org/10.48550/arXiv.2608.12913 Focus to learn more arXiv-issued DOI via DataCite

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