diff --git a/crates/ml/src/cuda_pipeline/c51_grad_kernel.cu b/crates/ml/src/cuda_pipeline/c51_grad_kernel.cu
index ec4fa71b9..a76b9da56 100644
--- a/crates/ml/src/cuda_pipeline/c51_grad_kernel.cu
+++ b/crates/ml/src/cuda_pipeline/c51_grad_kernel.cu
@@ -78,9 +78,12 @@ extern "C" __global__ void c51_grad_kernel(
     int A_d = branch_sizes[d];
     float inv_A = 1.0f / (float)A_d;
 
-    int branch_offset = 0;
+    /* Branch-major base offset: all B samples for branches 0..d-1 precede branch d.
+     * Layout: [B*B0*NA | B*B1*NA | B*B2*NA | B*B3*NA] (contiguous per-branch blocks).
+     * Matches cuBLAS backward pointer arithmetic for per-branch dY matrices. */
+    int branch_base = 0;
     for (int dd = 0; dd < d; dd++)
-        branch_offset += branch_sizes[dd] * num_atoms;
+        branch_base += batch_size * branch_sizes[dd] * num_atoms;
 
     /* Per-branch loss weighting: magnitude (d==1) gets ZERO C51 gradient.
      * C51 cross-entropy inherently prefers low-variance actions (Small positions
@@ -97,7 +100,7 @@ extern "C" __global__ void c51_grad_kernel(
     for (int a = 0; a < A_d; a++) {
         float dueling_grad = (a == a_d) ? (1.0f - inv_A) : (-inv_A);
         float grad_val = branch_scale * d_combined * dueling_grad;
-        int adv_idx = b * total_branch_atoms + branch_offset + a * num_atoms + j;
+        int adv_idx = branch_base + b * (A_d * num_atoms) + a * num_atoms + j;
         atomicAdd(&d_adv_logits[adv_idx], grad_val);
     }
 }