Strong Drafts Need Compact Memories: Long-Context Speculative Decoding with Compressed KV Cache

Tong Yuan, Chengxi Liao, Zeyi Wen


Abstract
Long-context LLM applications such as document summarization and multi-turn agents require generation from prefixes spanning tens of thousands of tokens, making decoding latency a major bottleneck. Speculative decoding (SD) reduces latency without changing model outputs, but its speedup depends on both accepted draft tokens and draft-step latency: Lightweight drafts are fast but lack the capacity to capture long-range dependencies, whereas strong independent drafts recover acceptance but incur growing KV-access cost at long prefixes. We introduce memory-augmented drafting for long-context SD, equipping a strong independent draft with compressed draft-side KV memory: A lightweight adaptor constructs and incrementally updates this memory to retain distant information and exact recent context. The target verifier retains its full KV cache and applies the standard accept/reject rule, preserving SD’s lossless guarantee. Experiments on Llama 3.1-8B and 70B targets at prefix lengths up to 32K show that our method reduces draft-side memory by over 70%. It achieves speedups of up to 2.08× and 3.33×, respectively, over autoregressive decoding.
Anthology ID:
2026.findings-emnlp.306
Volume:
Findings of the Association for Computational Linguistics: EMNLP 2026
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Hung-Yi Lee, Sunipa Dev, Jessy Li, Desmond Elliott
Venue:
Findings
SIG:
SIGDAT
Publisher:
Association for Computational Linguistics
Note:
Pages:
6481–6494
Language:
URL:
https://aclanthology.org/2026.findings-emnlp.306/
DOI:
Bibkey:
Cite (ACL):
Tong Yuan, Chengxi Liao, and Zeyi Wen. 2026. Strong Drafts Need Compact Memories: Long-Context Speculative Decoding with Compressed KV Cache. In Findings of the Association for Computational Linguistics: EMNLP 2026, pages 6481–6494, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
Strong Drafts Need Compact Memories: Long-Context Speculative Decoding with Compressed KV Cache (Yuan et al., Findings 2026)
Copy Citation:
PDF:
https://aclanthology.org/2026.findings-emnlp.306.pdf