fix: projection error

packages/llm/token-meter/src/surface-projection.ts

@@ -10,7 +10,9 @@
  * heuristic price of the exact replaced range, so the fold keeps a running
  * total plus at most one pending claim and never retains per-node prices.
  * The counts are exact by construction: producers derive them from the same
- * fixed estimator this module prices appends with.
+ * fixed estimator this module prices appends with. A replacement without an
+ * armed claim folds with zero delta because bounded state cannot reconstruct
+ * the replaced range; this preserves replay at the cost of possible drift.
  *
  * @module @deepseek-ai/dsh-token-meter/surface-projection
  */
@@ -47,16 +49,19 @@ export interface SurfaceTokensFold {
  * Fold one committed event onto a running surface-token total.
  *
  * A shadow-price event arms a claim; any other event expires it, and a
- * surface `replace` must consume a claim naming its exact range — the
+ * surface `replace` consumes the claim naming its exact range — the
  * producers append the metering event and the replacement synchronously
  * adjacent, so a surviving claim always prices the very next event.
+ * A replace with no claim folds with zero delta because the bounded state
+ * cannot reconstruct the replaced range. An armed claim for another range
+ * still fails because the adjacent events contradict each other.
  * @param claim - the claim armed by the immediately preceding event, if any.
  * @param event - the next committed session event.
  * @returns the signed token delta and the claim state after this event.
- * @throws when a replacement arrives without a claim for its exact range —
- *   every in-repo replace producer meters its replacement, so an unpriced
- *   replacement is a shadow-price contract violation and must fail loud
- *   rather than let the total drift.
+ * @throws when a replacement arrives with an armed claim for a different
+ *   range — the metering event was adjacent, so this is a live producer's
+ *   shadow-price contract violation, not historical data, and must fail
+ *   loud rather than let the total drift.
  */
 export function foldSurfaceProjection(
   claim: ShadowPriceClaim | undefined,
@@ -74,10 +79,15 @@ export function foldSurfaceProjection(
   const tokens = message === null ? 0 : estimateMessage(message)
   const op = event.surfaceOp
   if (op === 'append') return { deltaTokens: tokens, claim: undefined }
-  if (claim === undefined || claim.start !== op.start || claim.end !== op.end) {
+  // Sessions recorded before the shadow-price protocol log replacements with
+  // no adjacent metering event; the bounded state cannot reconstruct the
+  // replaced range's price, so fold those neutrally — historical replay
+  // degrades to drift instead of failing.
+  if (claim === undefined) return { deltaTokens: 0, claim: undefined }
+  if (claim.start !== op.start || claim.end !== op.end) {
     throw new Error(
       `token surface: replace at seq ${event.seq} over range ${op.start}-${op.end} has no adjacent shadow price`
-      + (claim === undefined ? '' : ` (armed claim covers ${claim.start}-${claim.end})`),
+      + ` (armed claim covers ${claim.start}-${claim.end})`,
     )
   }
   return { deltaTokens: tokens - claim.tokens, claim: undefined }