diff --git a/crates/ml/src/cuda_pipeline/gpu_dqn_trainer.rs b/crates/ml/src/cuda_pipeline/gpu_dqn_trainer.rs
index d84b541c7..6ac073cc6 100644
--- a/crates/ml/src/cuda_pipeline/gpu_dqn_trainer.rs
+++ b/crates/ml/src/cuda_pipeline/gpu_dqn_trainer.rs
@@ -2221,6 +2221,28 @@ impl GpuDqnTrainer {
         self.stream
             .memcpy_dtoh(&self.grad_buf.slice(..read_len), host_slice)
             .map_err(|e| MLError::ModelError(format!("per_branch_grad_norms dtoh: {e}")))?;
+        // `memcpy_dtoh` into PINNED host memory is asynchronous w.r.t. the
+        // host — cudarc calls `cuMemcpyDtoHAsync_v2` under the hood, which
+        // queues a DMA on `self.stream` and returns before the transfer
+        // completes. Without an explicit post-copy sync the host-side
+        // sum-of-squares loop below races the DMA and reads a mix of
+        // newly-transferred bytes and stale bytes left over from the
+        // previous epoch's readback. That DMA-in-progress race is the
+        // source of the residual `grad_abs[dir]` non-determinism: the
+        // direction gradient itself is bit-identical across runs (same
+        // CUDA kernels, same per-stream cuBLAS handles post-Option-C, same
+        // `CUBLAS_WORKSPACE_CONFIG`), but the aggregate L2 accumulator
+        // sums whatever bytes happen to already be in pinned memory when
+        // the host-side loop reaches each element. Magnitude tolerates the
+        // same race because mag gradients are ~300× larger than dir (the
+        // trunk-to-mag-branch path carries most of the Q-loss signal), so
+        // a few partially-updated bytes contribute a negligible fraction
+        // of the L2 sum; dir's small magnitude makes it proportionally
+        // more sensitive. Sync after the copy so both norms read the
+        // final transferred bytes.
+        self.stream.synchronize().map_err(|e| {
+            MLError::ModelError(format!("per_branch_grad_norms post-dtoh sync: {e}"))
+        })?;
 
         let param_sizes = compute_param_sizes(&self.config);
         let mut norms = [0.0_f32; 4];