Decision models — state plus typed questions in, a calibrated probability per
allowed answer out, no text generated — went from nonexistent to a category
between 2026-09-15 and 2026-10-01, converging on three primitives (`noul`,
`choice`, `score`) and one schema. Every quantity in Mesh is a token count, and
a decision emits no tokens, so the spec could neither price one, rate one, nor
report one. Three defects, each checked in the code rather than inferred:
- `MeshReceipt::uj_per_accepted_token`, documented as "the wedge metric", is
`checked_div` by `tokens_accepted`. A decision's is 0 — measured
`"output_tokens": 0` on every request — so the headline efficiency figure
returned `None` for the whole category. The receipt sealed with no number.
- `estimate()` charged `prompt_tokens + max_tokens`, so a caller that left
`max_tokens` at a generation's default was priced for an essay the backend
will never write, and refused at a ceiling that would have covered the work.
- Nothing could express the dimension a decision's cost actually follows: the
schema it scores. One question over 20 options is more work than one over 2
at identical token counts.
Added `DecisionShape` on the request and `DecisionOutcome` on the outcome
(questions, options scored, and how much of the input was schema rather than
state), `CostProfile::with_option_cost` + `estimate_decision_uj`, and
`uj_per_decision` / `uj_per_option_scored` on the receipt. SPEC §3.6 states the
three MUSTs; §4's estimate formula and §6's field table are updated.
Additive throughout, following the pattern `CostProfile::calibration` already
set here: the new fields are `Option`, skipped when absent, and a declared
option cost of zero reproduces the token-only model exactly. So an existing
request's `request_id` is unchanged, a generation's signed core gains no keys,
and **the sealed conformance vectors in `conformance/v1/reference` still verify
byte-identically** — which is the real test that this is compatible, and it
passes. 52 tests, 0 failures, 0 clippy findings.
Two of the four new tests are the defects stated as invariants: a decision
receipt now has an efficiency figure where the token metric has none, and a
decision is not priced for tokens it never emits. Both were falsified by
reverting the fix — removing the `estimate()` branch fails two router tests,
making `uj_per_decision` read `tokens_accepted` fails two receipt tests.
`tools/joules-per-decision/` is the evidence: a harness that meters a real
Apache-2.0 decision model (Laya, ModernBERT-large 421M, via MLX) per fabric on
Apple silicon, and the numbers in §3.6 come from it. On an M5 Max a
three-question decision costs **0.104 J** — 0.0966 J on the GPU rail plus
0.0075 J of process CPU energy — at an 11.85 ms p50, which is ~35 mJ per
answered question. Idle baselines bracket the run and correct it by 0.08%, so
the figure is measurement, not modelling. The energy counters are read
unprivileged; the CPU-rail/GPU-rail asymmetry on macOS is noted there and is
the subject of follow-up work on `jcp-energy`.
The harness also reproduced a live defect in the category it measures: the
published Laya checkpoint ships a fitted temperature of 0.1006 for choice
questions with 11+ options, which sharpens logits roughly tenfold and reports
near-certainty on what may be a coin flip. A third-party porter found it by
reproducing upstream numerically and clamps it with a warning; the publisher
ships it. A calibrated probability is a claim, and this category ships claims
with no receipt — which is the argument for §3.6 in one line.
Co-Authored-By: Claude Opus 5 <[email protected]>
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