The token estimator walks the full session.messages and the active system
prompt. Five call sites per turn in the engine (capacity pre/post tool
checkpoints, error escalation, the seam manager, the trim budget check)
plus four TUI/command consumers (footer, /status, /debug, context
inspector) all re-walked the same data independently. On a 200-message
history with 5 KB of tool results that is roughly 2 ms per call, or
~20 ms of pure waste on a single turn.
Introduce a process-local TokenEstimateCache keyed on
(session.messages_revision, system_prompt_fingerprint). Repeated calls
with the same inputs return the cached value without re-walking the
message list. The cache invalidates as soon as either input changes:
* session.messages_revision is a monotonic counter bumped in
Session::add_message, Session::replace_messages, the new
Session::bump_messages_revision helper, and at every direct
session.messages mutation site in core/engine.rs and
core/engine/capacity_flow.rs.
* system_prompt_fingerprint is a stable 64-bit hash of the
SystemPrompt::Text or SystemPrompt::Blocks payload.
Also restructures layered_context_checkpoint to compute the estimated
token count before taking a long-lived &SeamManager borrow, and
re-routes the capacity pre/post tool checkpoints to compute the
observation into a local before calling
capacity_controller.observe_*. Both refactors are required to satisfy
the borrow checker once estimated_input_tokens requires &mut self.
Tests: 10 new unit tests cover the miss/hit path, revision bumps,
system-prompt changes, audit-ring capacity, and downward-revision
no-ops. The full 157-test engine suite still passes.
HUQIANTAO
3de07a99ed