The exposure aux backward GEMM amplifies gradients through the hidden activation matrix (d_logits × h_b0), producing grad_norm=5000+. With max_grad_norm=10, this throttles ALL gradients by 500x (lr/500). Fix: reduce aux_weight 0.5→0.01 (50x) + clip_grad_buf_inplace after aux GEMM to cap combined gradient at max_grad_norm. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>