Val-Flat-collapse fix#4 (task #94, 2026-04-24). Research-agent audit
of train-bv2n5 identified a positive-feedback loop driving atom
utilisation to 1%:
util < 0.4 → γ -= 0.01 (toward 0.90 floor)
γ × delta_z shrinks → TD targets concentrate on fewer bins
→ more collapse → util lower → γ lower ...
The original controller had the right structure but the wrong
direction on the low-util branch. Flipping the sign (`-=` → `+=`)
breaks the loop: when atoms collapse, raising γ widens the effective
TD-target support `γ × atom_support`, pushing targets across more
bins of the C51 grid and giving the network more distributional
signal to learn from.
Evidence from train-bv2n5 (pre-fix): atom_util stuck at 0.01 for 16
epochs; γ slowly drifted toward 0.90. No mechanism recovered.
Change localised to a single `else if util < 0.4` branch at
`training_loop.rs:660-675`. Same step size (0.005) as the healthy
branch avoids overshoot on the recovery path. Clamps retained
[0.90, 0.95] at this layer; the fused trainer re-clamps to
[0.90, 0.995] after regime adjustment downstream.
Other pending atom-util fixes from the audit (deferred pending
validation):
- Atom-entropy regularisation term in C51 loss (medium risk)
- Absolute v_half floor in update_eval_v_range (low risk)
- TD-target dithering before Bellman projection (low risk)
This single-line fix addresses the dominant mechanism — research
agent labelled it "lowest risk, highest leverage". If util remains
low after this, the others stack additively.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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