diff --git a/ddw/utils/unet.py b/ddw/utils/unet.py
index 6ad1eb2..c1f5d59 100644
--- a/ddw/utils/unet.py
+++ b/ddw/utils/unet.py
@@ -171,6 +171,9 @@ def update_hparam(self, hparam, value):
         """
         Update a hyperparameter in the hparams.yaml file.
         """
+        # DDP-safe: only the global-zero rank writes hparams.yaml.
+        if getattr(self, "global_rank", 0) != 0:
+            return
         logger = self.trainer.logger
         logdir = f"{logger.save_dir}/{logger.name}/version_{logger.version}"
         hparams_file = f"{logdir}/hparams.yaml"