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refactor(token-meter): fold the surface once for the meter and the projection

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`contextBreakdown.messageTokens` and `measure().surfaceTokens` answer the same
question in the same heuristic vocabulary, and the panel's composition rows are
only honest while they agree. Each owner carried its own copy of the positional
fold — same pricing, same `{seq, tokens}` node list, same replace-range lookup
and guard, differing only in mutable versus immutable application — so an edit
to either one would have moved the panel away from `measure()` with both sides
still green. The duplication gate caught the shared 62 tokens.

`src/surface-fold.ts` now owns `foldSurfaceTokens`: total, allocation-fresh,
returning the event's price, the next surface, and the signed total delta. The
service assigns that result where it used to prepare a commit closure, which
keeps its validate-before-mutate replay transaction intact — the fold throws
before any state is touched, so a malformed event still fails identically on
every retry. `_prepareSurfaceMutation` and `_estimateSurfaceEvent` go away with
it, and the projection's apply drops to one call.

Covers the identity with a session that appends and then compacts, asserting
the projection figure equals the service surface at each boundary; the test
fails when either side of the fold is perturbed.
This commit is contained in:
Yichen Jiang
2026-08-05 16:29:51 +08:00
parent 46562b2c30
commit e62cbe12e4
12 changed files with 138 additions and 100 deletions
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packages/llm/token-meter/README.md
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@@ -29,7 +29,7 @@ When the composition provides `ctx.sessionProjections`, token-meter registers th
`contextPressure` carries optional `pressureTokens` — the newest provider-reported prompt size, summing uncached input plus cache reads and writes — and optional `contextWindow` from the newest `request/context` record. Pressure stays absent until a provider reports usage; capacity stays absent for a route whose adapter advertises none. Output is excluded, so the numerator holds still while a turn streams and steps forward when the next request reports its usage.
`contextBreakdown` carries heuristic `systemTokens`, `toolsTokens`, and `messageTokens` — the context's composition rather than its provider-billed size. The envelope figures reprice last-wins on every `request/header`; the message figure folds surface appends and positional replacements, so compaction shrinks it the same way it shrinks the next request. All three figures use the measurement service's fixed heuristic and are estimates: they do not reconcile with the provider-exact `pressureTokens`, and a UI should present them as approximations.
`contextBreakdown` carries heuristic `systemTokens`, `toolsTokens`, and `messageTokens` — the context's composition rather than its provider-billed size. The envelope figures reprice last-wins on every `request/header`; the message figure replays `surface-fold.ts` — the same positional fold `measure()` runs — so it equals `measure().surfaceTokens` at every event boundary and compaction shrinks it the way it shrinks the next request. All three figures use the measurement service's fixed heuristic and are estimates: they do not reconcile with the provider-exact `pressureTokens`, and a UI should present them as approximations.
All three units use the standard projection baseline, live frame, higher-seq-wins store, and JSON checkpoint paths. Unloading token-meter removes all three keys. A headless or TUI composition without the projection seam keeps the measurement service's existing behavior.