diff --git a/src/axolotl/monkeypatch/flex_attn.py b/src/axolotl/monkeypatch/flex_attn.py
index 97db4d414..a157906d5 100644
--- a/src/axolotl/monkeypatch/flex_attn.py
+++ b/src/axolotl/monkeypatch/flex_attn.py
@@ -10,6 +10,45 @@ from torch.nn.attention.flex_attention import (
 
 _MaskType = Union[torch.Tensor, BlockMask]
 
+
+def create_block_causal_mask(seq_lens: list[torch.Tensor]) -> torch.Tensor:
+    """
+    Given a batch tensor of seq lens defining the lengths of samples in each pack,
+    Construct a 2D block causal mask for each pack in the batch. For example, if
+    a single sample's seq_lens is [3, 2, 1], the mask would be::
+
+        mask = [
+            [1, 0, 0, 0, 0, 0],
+            [1, 1, 0, 0, 0, 0],
+            [1, 1, 1, 0, 0, 0],
+            [0, 0, 0, 1, 0, 0],
+            [0, 0, 0, 1, 1, 0],
+            [0, 0, 0, 0, 0, 1],
+        ]
+
+    Args:
+        seq_lens (List[torch.Tensor]): Sequence lengths of samples in each pack in the batch,
+            shape (batch_size, n), where n is the max number of sequences in a pack and can vary
+            across packs.
+
+
+    Returns:
+        Tensor: Block causal mask of shape (batch_size, max_seq_len, max_seq_len).
+    """
+    batch_block_attn_masks = []
+    batch_size = len(seq_lens)
+    for sample_idx in range(batch_size):
+        block_attn_masks = [
+            torch.tril(
+                torch.ones(seq_len, seq_len, dtype=torch.bool, device=seq_len.device)
+            )
+            for i, seq_len in enumerate(seq_lens[sample_idx])
+        ]
+
+        batch_block_attn_masks.append(torch.block_diag(*block_attn_masks))
+    return torch.stack(batch_block_attn_masks)
+
+
 def _get_document_ids_from_seq_lens(
     seq_lens: list[torch.Tensor],
 ) -> torch.Tensor:
diff --git a/src/axolotl/utils/collators/batching.py b/src/axolotl/utils/collators/batching.py
index 6508602cf..d449b38ad 100644
--- a/src/axolotl/utils/collators/batching.py
+++ b/src/axolotl/utils/collators/batching.py
@@ -11,7 +11,7 @@ from transformers import PreTrainedTokenizerBase
 from transformers.utils import PaddingStrategy
 
 from axolotl.monkeypatch.utils import get_seqlens_from_pos_ids
-from axolotl.monkeypatch.flex_attn import packed_block_causal_mask
+from axolotl.monkeypatch.flex_attn import create_block_causal_mask, packed_block_causal_mask
 
 
 @dataclass
@@ -177,7 +177,8 @@ class FlexBatchSamplerDataCollatorForSeq2Seq(DataCollatorForSeq2Seq):
         out = super().__call__(out_features, return_tensors=return_tensors)
 
         collated_seq_lens = get_seqlens_from_pos_ids(out["position_ids"])
-        out["attention_mask"] = packed_block_causal_mask(collated_seq_lens)
+        # out["attention_mask"] = packed_block_causal_mask(collated_seq_lens)
+        out["attention_mask"] = create_block_causal_mask(collated_seq_lens)
 
         return out