Ilker ManapandClaude Opus 5
a075ff4120
Tier 1 of plan §7.5: a radix tree of request prefixes that estimates which replica has already prefilled one. Bytes, not tokens — needing the engine's exact tokenizer is what makes tier 2 a separate project. It went in inert, and the decision metric is what caught it: 144 lookups, zero hits. The cause was a rule I had written and tested for. A node's label is the first request that created it, so the second request for the same system prompt diverges in its last few bytes — a new question after the same preamble, the single most common shape there is. Requiring the whole label to match threw away the 700 bytes they shared. What matters is the shared head, because that is what the engine cached, and a partial label match now counts. Two unit tests asserted the wrong rule and have been inverted. With that fixed, measured end to end against engines whose caches are too small for the working set: 22.5% cache hits without, 81.7% with, and the breakdown shows the misses are exactly the first round, when the tree is empty. The scenario matters as much as the number. With a handful of prompts and a large cache, scattering is free — every replica ends up holding everything — and prefix routing can only add constraint. The first version of the test measured that case and showed a loss, which was a true statement about a situation the feature is not for. Two things keep it from being the disaster prefix routing usually is. The imbalance guard, which needs both an absolute and a relative gap before it fires, because either alone misfires — absolute on an idle cluster, relative on a busy one. And it is a scorer rather than a filter: affinity is a vote the queue and inflight scorers can outvote, not a rule. The tree records the replica the picker actually chose, not the one the scorer proposed, and a node prefers the replica it has seen most rather than most recently — a count is stable under a randomizing picker, where recency just chases whatever the picker last did. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>