# Fidelity-aware harness headroom audit Status: **HISTORICAL PREMISE DID NOT PASS PROSPECTIVE P1; NO CONTRIBUTION CLAIM**. The audit answers whether engine instrumentation has enough incremental signal to justify a prospective experiment. It does not establish generalization. Post-run update: the exact-timestamp held-out P1 completed and failed the registered gate. Under the stronger simulator-aware `k=2` end-to-end replay, telemetry preserved zero regret but saved only 1.426% online H20-hours versus sim top-k + real final. The current route is closed; see `docs/fidelity-aware-harness-p1-report-20260714.md`. ## 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. ## Strong simulator-aware calibration baseline The original nested comparison used the same simulator shortlist but did not put Frontier's per-anchor prediction in either model. A stronger retrospective audit now gives both models frozen-calibrated simulated throughput, simulated SLO pass rate, and simulated feasibility. Under the same leave-one-cell-out folds, 5-second cutoff, L2 logistic family, regularization 1.0, and threshold 0.95: | Metric | Sim + outcome | Sim + outcome + instrumentation | Delta | |---|---:|---:|---:| | Accuracy | 81.08% | 89.19% | +8.11 pp | | Balanced accuracy | 72.42% | 81.55% | +9.13 pp | | Brier score | 0.1058 | 0.0957 | -0.0101 | | Safe early decisions | 20/37 | 25/37 | +5 | | Valid full-trial cost reduction | 50.89% | 68.98% | +18.09 pp | | Residual verification H20-hours | 0.5240 | 0.3310 | -36.84% | Both 0.95 policies have zero false accept and zero false reject on this retrospective task. Only three 0.5-threshold classifications differ in favor of instrumentation and none in favor of the strong baseline; McNemar's exact two-sided p-value is 0.25. The cell-bootstrap accuracy-delta interval is `[0.00,+18.18]` percentage points. The result is not robust to regularization: at 0.1 the strong baseline is more accurate and the instrumentation policy makes two unsafe decisions; at 10.0 the strong baseline is also more accurate. Thus the stronger comparison still has enough point-estimate headroom for a held-out test, but it materially weakens the evidence and makes a prospective task-level result mandatory. ## 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` - `runs/fidelity-headroom/analyze_strong_baseline.py` - `runs/fidelity-headroom/strong-baseline-metrics.json` - `runs/fidelity-headroom/test_strong_baseline.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 | | Matched frozen simulator anchors | 37 | pass rate 0.0688 | pass rate 1.0 | 12 pass-rate values | Every prefix matched exactly once | | Frozen simulator anchor corpus | 92 | positive throughput | positive throughput | >1 | No duplicate cell/anchor run | 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.