Tsinghua AI Open-Sources AKV, a Cache Engineering Server for LLM Agents on SGLang

Tsinghua University and CUHK researchers open-sourced AKV, a serving system for KV cache engineering in LLM agents. Instead of summarizing old history, agents drop selected KV cache entries and keep reusing the rest. Across 3 models and 4 agent benchmarks, average cost falls 27.7%. Average task score rises 1.7 points. The code is Apache-2.0 licensed and builds on SGLang.

TL;DR

• What it is: Serving system plus policy library, no weights. Tested on GPT-OSS-120B (5.1B active), MiniMax-M2.7 (230B total, 10B active) and MiroThinker-1.7-Mini (30B total, 3B active).
• Runs on: NVIDIA GPUs via SGLang. Paper used 8 A800-80GB GPUs in BF16. Quick start serves GPT-OSS-120B on 2 GPUs (tensor parallel 2).
• Performance: 27.7% lower average cost and +1.7 points average score versus threshold summarization.
• Best: GPT-OSS-120B on SWE-bench Verified: cost per task 20.0 to 9.7 cents, score 40.0 to 43.2.
• Worst: MiniMax-M2.7 on BrowseComp-Plus scores 58.9, 3.4 points below Text Engineering’s 62.3.

What is cache engineering, and how is it different from context engineering?

AKV is a Tsinghua serving system that lets LLM agents drop and reposition their own KV cache.

Context engineering trims or summarizes the text history. Editing that prefix invalidates every later KV state. The retained suffix must then be re-prefilled. Harnesses therefore summarize late and aggressively, which can lose details from middle rounds.

Cache engineering leaves the text trajectory untouched. It only changes which KV entries stay visible to attention. Retained entries are reused without recomputation. They can still carry information from dropped messages. In the research paper’s cost analysis, KV cache reads made up 82.4% of total API cost.

How does AKV work?

AKV defines 3 cache operations. Append is the standard update for new tokens. Drop removes selected entries from future attention without changing positions. Repos rewrites retained keys to positions 0 through r-1 using a RoPE rotation. Values stay unchanged, so no hidden states are recomputed. In ablations, adding Repos raised GPT-OSS-120B’s BrowseComp-Plus pass rate from 55% to 62%.

The hard part is serving. Identical tokens can produce different cache states under different operation histories. Plain prefix matching could reuse the wrong states. AKV extends SGLang’s radix tree with drop and reposition nodes. It frees dropped KV only when no active request still needs it. Segment-batch attention restores evicted states in 1 prefill pass.

What does the AKV API look like?

The interface is stateless and OpenAI-compatible. Clients send the full message history and two optional fields. drop_message maps a trigger message index to the messages to drop. reposition lists the indices after which positions get compacted.

A CachePolicy builds those fields from a trigger, a selector and a reposition rule. Built-in policies are turn-based, token-based or model-based. A CacheTool lets the model call drop_context itself.

How does AKV perform on agent benchmarks?

The main policy is a tool-response rolling drop that keeps the latest 12 tool responses. The Standard baseline summarizes at 96K tokens (192K for MiroThinker). Text Engineering summarizes at 48K. Benchmarks are BrowseComp-Plus, GAIA-Text, SWE-bench Verified and xbench-DeepSearch-2510.

Average pass rate (%) and cost (US cents per task) across 4 benchmarks. Source: Table 1 of the paper.

AKV also cuts cost 21.9% versus Text Engineering. The accuracy gap opens on long runs. On BrowseComp-Plus at 96K tokens and above, GPT-OSS-120B scores 37% with AKV. Standard scores 24% and Text Engineering 32%, per the project page.

Costs are API-equivalent estimates from published rates. Repositioned tokens are charged at the cache-hit rate, a deliberately conservative choice.