Current criterion uses |q_mean| / |prev_q_mean| which explodes near
zero crossings (legitimate cold-start has |prev_q_mean| ≈ 0.01-0.1,
making any non-tiny current trigger the ratio threshold).
smoke-test-n9xzr fired with prev=−0.0795, curr=0.7647, ratio=9.62×
despite this being natural cold-start growth, not geometric runaway.
Replace with rolling 5-epoch window of q_means. Compute median +
MAD (Median Absolute Deviation, a 50%-breakdown estimator robust
to single outliers); kill condition becomes
|q_mean − median(window)| > 4.0 × max(MAD(window), 0.01)
AND the existing adaptive floor.
Constants are declared `const` near the kill block:
- Q_DRIFT_WINDOW_SIZE=5 (matches smoke fold length, gives MAD a
meaningful estimator without averaging across regime shifts)
- Q_DRIFT_WARMUP_SAMPLES=3 (skip until ≥ 3 priors — smaller window
degenerates to half-range MAD that trips on monotonic
trajectories)
- Q_DRIFT_DEVIATION_THRESHOLD=4.0 (4 MADs ≈ 2.7σ Gaussian-
equivalent; clear outlier without firing on every legitimate
dip; literal MAD count rather than σ because q_mean is non-
Gaussian during cold-start)
- Q_DRIFT_MIN_DEVIATION=0.01 (numerical-stability floor when
window is constant)
Window resets at fold boundary alongside prev_epoch_q_mean /
adaptive_tau (A.1 pattern — cross-fold q-stats are independent
training runs, mixing them would inflate MAD or shift the
median).
Predicted impact on smoke-test-n9xzr:
- Plan C smoke fold 0 ep2: window has only 2 priors, warmup gate
not yet satisfied → kill stays silent (was firing on ratio=9.62×)
- Genuine geometric runaway (q_mean → 62.5 from baseline 0.5 over
4 epochs): ep3 deviation = |62.5 − 2.5|/MAD=2.0 = 30 > 4 AND
|62.5| > floor=3×12=36 → kill fires correctly
Both conditions ANDed (floor + deviation), preserving the
production-safety semantics of the original criterion. The floor
check is unchanged; only the divergence detector is replaced.
Per feedback_no_quickfixes.md: this is a principled robust-
statistics replacement, not a threshold relaxation.
Per feedback_no_partial_refactor.md: window field, constants,
criterion site, and fold-boundary reset land in lockstep — the
kill criterion's contract is internally consistent across
trainer/mod.rs, constructor.rs, and training_loop.rs.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
cca9dd36ae