diff --git a/crates/ml/src/cuda_pipeline/c51_loss_kernel.cu b/crates/ml/src/cuda_pipeline/c51_loss_kernel.cu
index 311fc5204..ac7713079 100644
--- a/crates/ml/src/cuda_pipeline/c51_loss_kernel.cu
+++ b/crates/ml/src/cuda_pipeline/c51_loss_kernel.cu
@@ -411,6 +411,9 @@ extern "C" __global__ void c51_loss_batched(
     if (tid == 0) {
         float clamped_ce = fminf(avg_ce, MAX_PER_SAMPLE_CE);
         float weighted_loss = clamped_ce * is_weight;
+        /* NaN guard: zero out poisoned samples (bf16 logit overflow from GemmEx) */
+        if (!fast_isfinite(weighted_loss)) weighted_loss = 0.0f;
+        if (!fast_isfinite(clamped_ce)) clamped_ce = 0.0f;
         per_sample_loss[sample_id] = bf16(weighted_loss);
         td_errors[sample_id]       = bf16(clamped_ce);
         atomicAdd(total_loss, weighted_loss / (float)batch_size);
diff --git a/crates/ml/src/cuda_pipeline/mse_loss_kernel.cu b/crates/ml/src/cuda_pipeline/mse_loss_kernel.cu
index d6cc3191c..460439c9f 100644
--- a/crates/ml/src/cuda_pipeline/mse_loss_kernel.cu
+++ b/crates/ml/src/cuda_pipeline/mse_loss_kernel.cu
@@ -355,6 +355,13 @@ extern "C" __global__ void mse_loss_batched(
 
     if (tid == 0) {
         float weighted_loss = avg_mse * is_weight;
+        /* NaN guard: if any per-sample computation produced NaN, zero it out.
+         * This prevents one poisoned sample from killing the entire batch.
+         * The NaN source is bf16 logit inputs from cuBLAS GemmEx — when the
+         * f32 accumulated dot product exceeds bf16 max, the bf16 C-matrix write
+         * produces Inf/NaN which propagates through softmax expected-Q. */
+        if (!fast_isfinite(weighted_loss)) weighted_loss = 0.0f;
+        if (!fast_isfinite(avg_td)) avg_td = 0.0f;
         per_sample_loss[sample_id] = bf16(weighted_loss);
         td_errors[sample_id]       = bf16(avg_td);
         atomicAdd(total_loss, weighted_loss / (float)batch_size);