Root cause from train-v8ztm 9-epoch HEALTH_DIAG aux next_bar_mse trajectory:
- Ep 0: 0.352 (learnable signal — below random baseline ln(2)≈0.693)
- Ep 9: 0.717 (above random baseline — aux is now WORSE than random)
- aux_dir_acc_long stuck at 0.19 (anti-correlated with truth)
Aux head's backward gradient was flowing back to shared trunk activation
h_s2 via dh_s2_out write at aux_heads_kernel.cu:599-613. Q-loss
gradient on h_s2 dominates (larger magnitude, structurally different
objective: cumulative discounted reward vs next-bar direction). h_s2
evolves to support Q's task; aux's CE loss climbs as h_s2 features
become anti-aligned with direction prediction.
Fix: stop-gradient. Aux reads h_s2 via forward, trains its own w1/b1/w2/b2
from CE loss, but does NOT propagate to h_s2. Q-loss is the sole shaping
force on h_s2. Aux must adapt to whatever h_s2 happens to be — if the
representation has direction signal, aux's params will extract it; if
not, aux can't learn (separate-trunk Option 2 deferred for that case).
This was the SEVENTH fix in today's chain (after 6 SP14 EGF cadence/
gate/saturation fixes). The EGF was a scaffold over a broken aux head;
fixing aux first is the architectural prerequisite for EGF to route
useful signal.
Verification: cargo check clean; sp14_oracle_tests 7/7 pass.
Validation: aux next_bar_mse should now DECREASE during training in
the next L40S smoke (vs the rising-from-0.35-to-0.72 pattern in v8ztm).
Deferred follow-up: aux_regime_backward has the same architecture
(propagates dh_s2 to trunk). Same fix is a candidate once next_bar
result validates the approach.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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