diff --git a/tests/unit_tests/data/test_builder.py b/tests/unit_tests/data/test_builder.py
index da21b9fe735..236717f9012 100644
--- a/tests/unit_tests/data/test_builder.py
+++ b/tests/unit_tests/data/test_builder.py
@@ -40,6 +40,15 @@
 
 _MARGIN = 0.005
 
+# Pin the per-sample key sets that GPTDataset and BlendedDataset emit. These are the
+# upstream contract that drives:
+#   - BATCH_KEYS in pretrain_gpt.py / pretrain_hybrid.py (must remain a superset of
+#     these keys, modulo provenance fields).
+#   - `_add_provenance_keys` in tests/unit_tests/data/test_get_batch.py (must simulate
+#     the *exact* extras BlendedDataset adds on top of the inner dataset).
+_EXPECTED_GPT_DATASET_KEYS = {"tokens", "labels", "loss_mask", "position_ids"}
+_EXPECTED_BLENDED_DATASET_EXTRA_KEYS = {"dataset_id"}
+
 
 def create_file_prefixes(tokenizer, number_of_files, maximum_number_of_documents, dataset_dir):
     # Create dataset directory
@@ -483,6 +492,13 @@ def test_fast_builder(
                 ds_fast
             ), f"ds_slow: {len(ds_slow)}, ds_fast: {len(ds_fast)}, split_name: {split_name}"
             if isinstance(ds_slow, GPTDataset):
+                assert set(ds_slow[0].keys()) == _EXPECTED_GPT_DATASET_KEYS, (
+                    f"GPTDataset emits {set(ds_slow[0].keys())}; expected "
+                    f"{_EXPECTED_GPT_DATASET_KEYS}. If you changed the schema, "
+                    f"update _EXPECTED_GPT_DATASET_KEYS here, BATCH_KEYS in "
+                    f"pretrain_gpt.py / pretrain_hybrid.py, and `_add_provenance_keys` "
+                    f"in tests/unit_tests/data/test_get_batch.py."
+                )
                 assert torch.all(ds_slow[0]["tokens"] == ds_fast[0]["tokens"])
                 assert torch.all(ds_slow[-1]["tokens"] == ds_fast[-1]["tokens"])
                 numpy.testing.assert_array_equal(ds_slow.document_index, ds_fast.document_index)
@@ -498,6 +514,15 @@ def test_fast_builder(
                     ds_slow.dataset.index.sequence_pointers, ds_fast.dataset.index.sequence_pointers
                 )
             elif isinstance(ds_slow, BlendedDataset):
+                expected_blended_keys = (
+                    _EXPECTED_GPT_DATASET_KEYS | _EXPECTED_BLENDED_DATASET_EXTRA_KEYS
+                )
+                assert set(ds_slow[0].keys()) == expected_blended_keys, (
+                    f"BlendedDataset emits {set(ds_slow[0].keys())}; expected "
+                    f"{expected_blended_keys}. If you changed the schema, update "
+                    f"_EXPECTED_BLENDED_DATASET_EXTRA_KEYS here and the provenance "
+                    f"helper in tests/unit_tests/data/test_get_batch.py."
+                )
                 assert torch.all(ds_slow[0]["tokens"] == ds_fast[0]["tokens"])
                 assert torch.all(ds_slow[-1]["tokens"] == ds_fast[-1]["tokens"])
                 numpy.testing.assert_array_equal(ds_slow.dataset_index, ds_fast.dataset_index)
@@ -505,6 +530,9 @@ def test_fast_builder(
                     ds_slow.dataset_sample_index, ds_fast.dataset_sample_index
                 )
                 for ds_slow_i, ds_fast_i in zip(ds_slow.datasets, ds_fast.datasets):
+                    # Inner dataset under BlendedDataset is the un-wrapped GPTDataset:
+                    # contract checks the bare key set, not the BlendedDataset extras.
+                    assert set(ds_slow_i[0].keys()) == _EXPECTED_GPT_DATASET_KEYS
                     assert torch.all(ds_slow_i[0]["tokens"] == ds_fast_i[0]["tokens"])
                     assert torch.all(ds_slow_i[-1]["tokens"] == ds_fast_i[-1]["tokens"])
                     numpy.testing.assert_array_equal(
diff --git a/tests/unit_tests/data/test_get_batch.py b/tests/unit_tests/data/test_get_batch.py
index c136dcfa807..13c7c2dc672 100644
--- a/tests/unit_tests/data/test_get_batch.py
+++ b/tests/unit_tests/data/test_get_batch.py
@@ -65,6 +65,19 @@ def initialize_test_environment(
     return args
 
 
+def _add_provenance_keys(batch):
+    """Inject extra fields that real dataloader stacks add to per-sample dicts.
+
+    BlendedDataset.__getitem__ prepends a ``dataset_id`` provenance field; other
+    wrappers may add arbitrary metadata. get_batch must ignore these and return
+    exactly len(BATCH_KEYS) values — without this, the call-site unpacking fails
+    on tp_rank 0 (PR #4952). Applied inside every iterator builder so every
+    parametrized test case in this file exercises the cardinality contract.
+    """
+    batch["dataset_id"] = torch.tensor([0], dtype=torch.int64)
+    return batch
+
+
 def create_sft_data_iterator(max_seq_length: int = 1024):
     """Create a mock SFT data iterator matching the old SFTDataset output after DataLoader collation.
 
@@ -145,7 +158,7 @@ def create_sft_data_iterator(max_seq_length: int = 1024):
         "cu_seqlens": cu_seqlens.unsqueeze(0),
         "max_seqlen": max_seqlen,
     }
-    return iter([batch]), num_real_tokens
+    return iter([_add_provenance_keys(batch)]), num_real_tokens
 
 
 @pytest.mark.parametrize("tp_size", [1, 2, 4])
@@ -367,7 +380,7 @@ def create_pretrain_data_iterator(
             torch.ones((micro_batch_size, 1, seq_length, seq_length))
         ).bool()
 
-    return iter([batch])
+    return iter([_add_provenance_keys(batch)])
 
 
 @pytest.mark.parametrize("tp_size", [1, 2, 4])