PrefixFingerprint::compute is called once per turn by the turn loop
prefix-stability check. The tool-side work serializes every tool to the
chat-API JSON shape, sorts the resulting strings, joins with newlines,
and SHA-256s the result. For a 60-tool catalog that is ~25-40 KB of
allocation plus a sort, all of which produces a byte-identical output
once the tool set is stable across turns (the common case after the
first turn of a session).
Introduce a process-local ToolCatalogCache that stores the joined+sorted
catalog under a content-derived u64 identity (length + per-tool name +
description + serialized input_schema). On a hit, the per-tool JSON
serialization, sort, and join are skipped entirely — the pre-computed
SHA-256 hex digest is returned directly.
The cache lives on PrefixStabilityManager (per-session ownership) and
backs a new PrefixFingerprint::compute_with_tool_cache entry point.
check_and_update, PrefixStabilityManager::new, and pin() all use the
cached path. The original compute() is kept as a fallback for callers
that do not have a cache in hand (e.g. CLI tools that build a one-shot
fingerprint).
The cache is bounded (default capacity = 8) and uses insertion-order
eviction, matching the eviction strategy already in
transcript_cache.rs. invalidate() is exposed for tool-registry hot-reload
and MCP attach paths.
Tests: 8 new unit tests cover the miss/hit path (pointer-equal Arc on
hit), identity collisions, schema change detection, capacity eviction,
invalidate, empty slice, and the equivalence between cached and uncached
fingerprints. The full 30-test prefix_cache suite passes; the wider
prefix-cache contract tests in settings, prompts, and
core::engine::tests continue to pass.
HUQIANTAO
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