diff --git a/crates/ml/src/cuda_pipeline/c51_loss_kernel.cu b/crates/ml/src/cuda_pipeline/c51_loss_kernel.cu
index bd2f33ab1..21a55e4a2 100644
--- a/crates/ml/src/cuda_pipeline/c51_loss_kernel.cu
+++ b/crates/ml/src/cuda_pipeline/c51_loss_kernel.cu
@@ -708,6 +708,7 @@ extern "C" __global__ void c51_loss_reduce(
     float* __restrict__ total_loss,             /* [1] output */
     int batch_size
 ) {
+    if (threadIdx.x != 0) return;  /* single writer — total_loss[0] */
     float sum = 0.0f;
     for (int i = 0; i < batch_size; i++) {
         sum += per_sample_loss[i];