jcp-trace: an undefined wedge metric is not zero #292
2026-10-17
jcp-trace: an undefined wedge metric is not zero
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The sibling of the jcp-efficiency fix, in the `eval/1` receipt:
t.uj_per_accepted_token = t.uj_total.checked_div(t.output_tokens).unwrap_or(0);
`uj_per_accepted_token` is a cost-per-result figure, so 0 is its
optimum. A run that burned prompt tokens and accepted no output has no
such figure — the division is undefined — and reporting 0 ranked it as
the cheapest entry in the ledger. Like the efficiency receipt, this one
seals the figure into the signed core (`eval_wire`) and reads it
straight back out in `verify()` rather than recomputing it, so the
conflation was sealed and consumed as the headline.
`mesh/SPEC.md` §6 states the family convention — the figure is
"omitted when no tokens were accepted" — and `MeshReceipt` already
implements it. This brings `eval/1` onto the same shape:
- `Totals::uj_per_accepted_token` and `EvalReceipt::uj_per_accepted_token`
become `Option<u64>`; `Default` is `None`.
- `eval_wire` omits the key when the figure is undefined instead of
sealing 0. Canonical CBOR sorts keys, so the conditional insert does
not change the wire of a receipt that has one.
- `verify()` reads it as optional, so an older receipt that carries the
figure still round-trips.
- `examples/ingest.rs` prints "—" rather than a number.
`a_run_that_accepted_nothing_seals_no_wedge_metric` builds a trace with
800 prompt tokens and no accepted output: 8 000 µJ of real spend, no
figure, the key absent from the signed core, and the envelope still
verifying with the spend on it.
Verified with `--features server` as well as the default build. The
`server` module is behind a non-default feature, so
`cargo test --workspace` never compiles it, and
`server::tests::http_roundtrip_returns_sealed_eval_receipt` asserts on
this exact wire field — it passes, but CI would not have told us: the
jcp row's `feature_tests` in standards-build.yml names only
`-p jcp-energy --features nvml`.
Co-Authored-By: Claude Opus 5 <[email protected]>