Investigation confirms vLLM Mooncake connector DOES correctly register
externally-received KV blocks in the prefix cache. No bug exists.
Evidence from vLLM logs (per-instance):
inst_1: prefix_cache=14.7%, external_cache=72.1% <- high external hit
inst_4: prefix_cache=52.4%, external_cache=59.0%
The 0.5% aggregate APC from /metrics was a measurement artifact:
inst_0 received 718M query tokens (cold-start prefills) with 0% hit,
diluting the aggregate. D-instances have 20-72% external cache hit.
The /metrics endpoint's prefix_cache_hits_total counter does not include
external hits. The vLLM log's "External prefix cache hit rate" is the
correct metric for Mooncake-transferred KV reuse.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
