diff --git a/gnm/shape/gnm_data_loader.py b/gnm/shape/gnm_data_loader.py
index b260c58f..6722fd94 100644
--- a/gnm/shape/gnm_data_loader.py
+++ b/gnm/shape/gnm_data_loader.py
@@ -110,8 +110,8 @@ def _validate_gnm_data(
     has exactly the expected fields, the missing fields and the extra fields.
   """
   expected_fields = gnm_data_schema.GNM_DATA_ATTRIBUTES
-  missing_fields = list(set(expected_fields) - set(data.keys()))
-  extra_fields = list(set(data.keys()) - set(expected_fields))
+  missing_fields = sorted(set(expected_fields) - set(data.keys()))
+  extra_fields = sorted(set(data.keys()) - set(expected_fields))
   return not missing_fields and not extra_fields, missing_fields, extra_fields
 
 
diff --git a/gnm/shape/gnm_data_loader_test.py b/gnm/shape/gnm_data_loader_test.py
index e51a3e8b..c6dd694b 100644
--- a/gnm/shape/gnm_data_loader_test.py
+++ b/gnm/shape/gnm_data_loader_test.py
@@ -14,12 +14,15 @@
 
 """Tests for gnm_data_loader."""
 
+# pylint: disable=protected-access
+
 from unittest import mock
 
 from absl.testing import absltest
 from absl.testing import parameterized
 from etils import epath
 from gnm.shape import gnm_data_loader
+from gnm.shape import gnm_data_schema
 from gnm.shape.data.versions import gnm_specs
 from gnm.shape.data.versions import gnm_test_catalog
 
@@ -45,6 +48,19 @@ def test_print_gnm_versions(self):
 class GNMModelLoadingTest(parameterized.TestCase):
   """Tests for loading GNM model files."""
 
+  def test_validate_gnm_data_returns_sorted_diagnostics(self):
+    data = {field: object() for field in gnm_data_schema.GNM_DATA_ATTRIBUTES}
+    del data['identity_names']
+    del data['joint_names']
+    data['zz_extra'] = object()
+    data['aa_extra'] = object()
+
+    valid, missing, extra = gnm_data_loader._validate_gnm_data(data)
+
+    self.assertFalse(valid)
+    self.assertEqual(missing, ['identity_names', 'joint_names'])
+    self.assertEqual(extra, ['aa_extra', 'zz_extra'])
+
   @parameterized.product(
       version=_MAINTAINED_MAJOR_GNM_VERSIONS,
       variant=gnm_test_catalog.ALL_VARIANTS,