diff --git a/megatron/training/global_vars.py b/megatron/training/global_vars.py
index a3d41f5e45a..9a9100e7b01 100644
--- a/megatron/training/global_vars.py
+++ b/megatron/training/global_vars.py
@@ -188,33 +188,55 @@ def _set_tensorboard_writer(args):
 
 
 def _set_wandb_writer(args):
+    """
+    Minimal-fix version: sanitize only the fields that are known to break JSON
+    (bytes, tensor, callable) before passing to wandb.
+    """
     global _GLOBAL_WANDB_WRITER
-    _ensure_var_is_not_initialized(_GLOBAL_WANDB_WRITER,
-                                   'wandb writer')
+    _ensure_var_is_not_initialized(_GLOBAL_WANDB_WRITER, 'wandb writer')
+
     if getattr(args, 'wandb_project', '') and args.rank == (args.world_size - 1):
         if args.wandb_exp_name == '':
             raise ValueError("Please specify the wandb experiment name!")
 
-        import wandb
-        if args.wandb_save_dir:
-            save_dir = args.wandb_save_dir
-        else:
-            # Defaults to the save dir.
-            save_dir = os.path.join(args.save, 'wandb')
-        wandb_config = vars(args)
+        import wandb, json, os
+        from argparse import Namespace
+
+        # --- minimal sanitizer ----------------------------------------------
+        def _clean(obj):
+            if isinstance(obj, Namespace):
+                obj = vars(obj)
+            if isinstance(obj, dict):
+                return {k: _clean(v) for k, v in obj.items()
+                        if not isinstance(v, (bytes, type, type(lambda: None)))}
+            if isinstance(obj, bytes):
+                return obj.decode('utf-8', errors='ignore')
+            if hasattr(obj, 'tolist'):          # torch.Tensor / numpy.ndarray
+                return obj.tolist()
+            try:                                # final JSON safety check
+                json.dumps(obj)
+                return obj
+            except (TypeError, ValueError):
+                return repr(obj)                # last resort: stringify
+        # ---------------------------------------------------------------------
+
+        wandb_config = _clean(args)
+
         if 'kitchen_config_file' in wandb_config and wandb_config['kitchen_config_file'] is not None:
-            # Log the contents of the config for discovery of what the quantization
-            # settings were.
             with open(wandb_config['kitchen_config_file'], "r") as f:
                 wandb_config['kitchen_config_file_contents'] = f.read()
+
+        save_dir = args.wandb_save_dir or os.path.join(args.save, 'wandb')
+        os.makedirs(save_dir, exist_ok=True)
+
         wandb_kwargs = {
             'dir': save_dir,
             'name': args.wandb_exp_name,
             'project': args.wandb_project,
             'config': wandb_config}
-        if args.wandb_entity:
+        if getattr(args, 'wandb_entity', None):
             wandb_kwargs['entity'] = args.wandb_entity
-        os.makedirs(wandb_kwargs['dir'], exist_ok=True)
+
         wandb.init(**wandb_kwargs)
         _GLOBAL_WANDB_WRITER = wandb