The premise holds at realistic context: at ~8k tokens + high cache-hit the KV
transfer is ~30% of TTFT (712ms), sequential after the forward. Lever A was scoped
to prefix==0 so it never activated there. Broaden it to cover the FULL sequence
(cached prefix + new tokens): the per-layer backup hook fires after each layer's
full processing, so the HiCache-loaded prefix KV and forward-written new KV are both
final — transferring all of layer L's pages then overlaps the dominant prefix
transfer with the forward. Add a sonnet-bench verify mode for output-equality.
Gated by the flag; correctness verified next.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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