The guard cleanup agent incorrectly removed the backward dX overflow clamp
from relu_mask_kernel. This clamp is a LEGITIMATE bf16 overflow prevention
at the type boundary — identical to the forward-pass bias kernel clamping.
The dX GemmEx writes bf16 output. When the f32 accumulated sum exceeds
bf16 max (~65504), it writes Inf. The relu_mask ±500 clamp converts Inf
to a finite value, preventing cascade through the backward chain.
Also reverted the outside-graph params cast (the in-graph capture is correct).
Remaining issue: grad_norm drops to 0 in epoch 2+ with f32 master weights.
The model learns in epoch 1 (grad_norm=0.004) but stagnates in epoch 2+.
Investigating CUDA graph replay of f32→bf16 params cast.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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