# Fidelity-aware harness headroom audit

Status: **PROMISING PREMISE, NO CONTRIBUTION CLAIM**.

The audit answers whether engine instrumentation has enough incremental signal
to justify a prospective experiment.  It does not establish generalization.

## Simulator shortlist lower bound

On the frozen 12-cell SimFid task, the strongest calibrated SLO simulator
reading places TP2/MNS32 and TP2/MNS64 in the same first tie bucket.  Real-final
evaluation of that two-cell bucket selects TP2/MNS32 and has zero real regret.
A method requiring a real calibration probe plus final verification cannot beat
two real cell evaluations on this task.  Therefore “better initial selection”
is not a viable claim here; the remaining headroom is shorter real verification
inside the same shortlist.

## Five-second prefix result

The retrospective Phase-6 dataset contains 37 primary anchors across 12 cells.
Stable labels use the frozen same-placement 2-of-3 adjudication: 28 feasible and
9 infeasible.  Three TP4 primary measurements disagree with their repeated
labels, so single-run feasibility is not treated as ground truth.

Using leave-one-cell-out folds, identical L2 logistic models, and a 5-second
prefix:

| Metric | Outcome-only | Instrumentation-aware | Delta |
|---|---:|---:|---:|
| Accuracy | 78.38% | 89.19% | +10.81 pp |
| Balanced accuracy | 70.63% | 81.55% | +10.92 pp |
| Brier score | 0.1297 | 0.0901 | -0.0396 |
| Correct only in this model | 0 | 4 | +4 |
| McNemar exact two-sided p | — | 0.125 | not significant |

At the frozen conservative threshold 0.95, both policies make zero false
accepts and zero false rejects on this retrospective set.  Outcome-only safely
cuts 36.35% of measured primary-trial cost; instrumentation-aware safely cuts
61.10%, an additional 24.75 percentage points.  Regularization sensitivity for
accuracy delta is `[0.00, +10.81]` percentage points, so the sign is
non-negative but the magnitude is not stable.

Longer prefixes do not strengthen the case monotonically.  At 10 seconds,
headline accuracy is 91.89% outcome-only versus 89.19% instrumentation-aware;
at 15 seconds it is 88.89% versus 91.67%; at 20 seconds it is 86.11% versus
91.67%, but both 0.95 policies make one false reject.  Five seconds is therefore
a training-selected operating point, not a test result.

## Interpretation

There is enough headroom to run a held-out pilot, but not enough evidence to
claim the harness contribution:

- the 5-second cost gap is operationally large;
- only four paired classifications differ, so significance is absent;
- all examples share one workload/SLO/engine task;
- completion timestamps are reconstructed from arrival + TTFT + TPOT rather
  than recorded directly;
- three adjudication disagreements are concentrated in transient TP4 runs;
- outcome-only already recovers the simulator shortlist oracle with very few
  real cells.

The next experiment must therefore freeze the 5-second model and threshold,
record exact monotonic completions, use a held-out trace, and label each anchor
with three full repetitions.  The registered protocol is
`docs/fidelity-aware-harness-protocol-20260714.md`.

## Artifacts

- `runs/fidelity-headroom/analyze_existing.py`
- `runs/fidelity-headroom/metrics.json`
- `runs/fidelity-headroom/analyze_prefixes.py`
- `runs/fidelity-headroom/prefix-metrics.json`
- `runs/fidelity-headroom/test_analysis.py`
- `runs/fidelity-headroom/test_prefix_analysis.py`

## Sanity block

| Family | n | Min | Max | Distinct | Invariant/result |
|---|---:|---:|---:|---:|---|
| Real SimFid cell scores | 12 | 1.2833 | 3.2833 | 7 | Non-negative; not identical |
| Prefix examples at 5 s | 37 | 5 s | 5 s | 1 expected | All 12 cells represented |
| Adjudicated labels | 37 | 0 | 1 | 2 | 28 positive / 9 negative |
| Primary/adjudicated disagreement | 37 | 0 | 1 | 2 | 3 TP4 disagreements retained |
| Full primary elapsed time | 37 | 14.566 s | 62.064 s | 37 | Every 5 s prefix is in range |
| Outcome probability | 37 | in `[0,1]` | in `[0,1]` | >1 | Checked before metrics |
| Instrumentation probability | 37 | in `[0,1]` | in `[0,1]` | >1 | Checked before metrics |
| Layer-1 streams | 12 | 14,174 records | 58,725 records | 12 | Contiguous, zero drops |

Checked invariants: same folds/model family and cutoff; no full verdict in a
feature; prefix-only Layer-1 slicing; non-negative costs/counters; bounded
ratios/probabilities; both labels present; per-config results not identical;
tie expansion before top-k; no imputation of non-monotonic frontiers.  The main
limitation is reconstructed request completion time, explicitly marked on all
37 five-second examples.