diff --git a/crates/ml/src/trainers/dqn/fused_training.rs b/crates/ml/src/trainers/dqn/fused_training.rs
index 2f65b682f..9fd52716f 100644
--- a/crates/ml/src/trainers/dqn/fused_training.rs
+++ b/crates/ml/src/trainers/dqn/fused_training.rs
@@ -1514,8 +1514,10 @@ impl FusedTrainingCtx {
         Ok(())
     }
 
-    /// Restore best-epoch params into online weights. Pure async DtoD + unflatten.
+    /// Restore best-epoch params into online weights. DtoD + unflatten + sync.
     /// Called before evaluation to ensure the evaluator sees the best model.
+    /// Syncs the stream to guarantee writes are globally visible before the
+    /// evaluator (which may use a different stream) reads them.
     pub(crate) fn restore_best_params(&mut self) -> Result<()> {
         let snapshot = self.best_params_snapshot.as_ref()
             .ok_or_else(|| anyhow::anyhow!("No best params snapshot saved"))?;
@@ -1530,6 +1532,9 @@ impl FusedTrainingCtx {
         // Unflatten: params_bf16 → individual weight tensors
         self.trainer.unflatten_online_weights(&self.online_dueling, &self.online_branching)
             .map_err(|e| anyhow::anyhow!("restore_best_params unflatten: {e}"))?;
+        // Sync: ensure DtoD + unflatten complete before evaluator reads on its stream
+        self.stream.synchronize()
+            .map_err(|e| anyhow::anyhow!("restore_best_params sync: {e}"))?;
         Ok(())
     }