record() called readLastEvidenceReceiptHash on every append, which re-read
and re-hashed the entire receipts.jsonl just to get the last hash and verify
the whole chain — O(N^2) over a flow's receipts, on the tool-call hot path
under the per-path lock (M2).
Read only the last line instead (windowed ReadAt from the end), failing
closed if that line is missing, wrong-schema, or hash-mismatched. This is
O(1) per append and stays correct across the multiple recorder instances
that write one path (L6): the file stays the single source of truth, so
there is no per-instance cache to go stale. Full-chain verification belongs
in a separate read-time verify tool, not on every write.
Off by default (EVIDENCE_RECEIPTS_ENABLED=false). Measured: 1024 concurrent
appends under -race dropped from ~49s to ~7.5s.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
