diff --git a/examples/sft/sft.py b/examples/sft/sft.py
index 629b556c2..152781b0b 100644
--- a/examples/sft/sft.py
+++ b/examples/sft/sft.py
@@ -80,6 +80,7 @@ def train(config: SFTConfig):
         raise ValueError("Must specify either --initialize_from_hf or --initialize_from")
     else:
         converter = None
+        model_config = config.model
 
     levanter.initialize(config)
 
@@ -106,10 +107,10 @@ def train(config: SFTConfig):
             input_role=config.input_role,
             output_role=config.output_role,
         )
-        train_dataset = mk_chat_sft_dataset(chat_config, tokenizer)
+        train_dataset = mk_chat_sft_dataset(chat_config, tokenizer, model_config.Pos)
     else:
         assert config.supervised_data is not None
-        train_dataset = mk_supervised_dataset(config.supervised_data, tokenizer)
+        train_dataset = mk_supervised_dataset(config.supervised_data, tokenizer, model_config.Pos)
     logger.info("Supervised dataset created")
     train_dataset = PermutationDataset(train_dataset, data_key)
 
diff --git a/src/levanter/data/audio.py b/src/levanter/data/audio.py
index 12695a20b..fbb118cfe 100644
--- a/src/levanter/data/audio.py
+++ b/src/levanter/data/audio.py
@@ -270,6 +270,7 @@ class ProcessedAudioCache(AsyncDataset[AudioTextDict]):
     """
 
     def __init__(self, cache: TreeCache[AudioTextDict]):
+        super().__init__()
         self.cache = cache
 
     async def async_len(self) -> int:
diff --git a/src/levanter/data/dataset.py b/src/levanter/data/dataset.py
index def0c158a..4d71241d4 100644
--- a/src/levanter/data/dataset.py
+++ b/src/levanter/data/dataset.py
@@ -48,6 +48,9 @@ class AsyncDataset(DatasetBase[T_co]):
     * `current_len`: Returns the current length of the dataset. This may be None if no current length is known.
     """
 
+    def __init__(self):
+        self._min_known_len = 0
+
     @abc.abstractmethod
     async def async_len(self) -> int:
         raise NotImplementedError
@@ -95,7 +98,12 @@ async def wait_until_len_at_least(self, length: int) -> int:
         The default implementation is a naive busy-wait loop. You should override this method for more efficient
         implementations.
         """
-        return await naive_busy_wait_until_len_at_least(self, length)
+        if length <= self._min_known_len:
+            return self._min_known_len
+
+        res_len = await naive_busy_wait_until_len_at_least(self, length)
+        self._min_known_len = max(self._min_known_len, res_len)
+        return res_len
 
     def as_sync_dataset(self):
         return SyncifiedDataset(self)
@@ -206,6 +214,7 @@ def __getitem__(self, index: int) -> T_co:
 
 class AsyncifiedDataset(AsyncDataset[T_co]):
     def __init__(self, dataset: SyncDataset[T_co]):
+        super().__init__()
         self.dataset = dataset
 
     async def async_len(self) -> int:
@@ -239,6 +248,7 @@ class ListAsyncDataset(AsyncDataset[T]):
     """
 
     def __init__(self, data: list[T], is_complete: bool = False):
+        super().__init__()
         self.data = data
         self.is_complete = is_complete
         if not is_complete:
@@ -315,6 +325,7 @@ def __init__(
         *extra_args,
         **extra_kwargs,
     ):
+        super().__init__()
         self.dataset = dataset
         self.fn = fn
         self._extra_args = extra_args
diff --git a/src/levanter/data/mixture.py b/src/levanter/data/mixture.py
index eb1bdfaaf..63c623e4b 100644
--- a/src/levanter/data/mixture.py
+++ b/src/levanter/data/mixture.py
@@ -53,6 +53,7 @@ def __init__(
         key: PRNGKeyArray | int,
         stop_strategy: str = StopStrategy.RESTART_STRATEGY,
     ):
+        super().__init__()
         self.weights = MixtureDataset._normalize_weights(weights)
         self.datasets = {name: dataset for name, dataset in datasets.items() if self.weights.get(name, 0) > 0}
         self.dataset_index = Index(self.datasets.keys())
diff --git a/src/levanter/data/permutation.py b/src/levanter/data/permutation.py
index 6599d4974..66a1887fd 100644
--- a/src/levanter/data/permutation.py
+++ b/src/levanter/data/permutation.py
@@ -14,6 +14,7 @@ class PermutationDataset(AsyncDataset[T_co]):
     # TODO: add epoch reshuffling
 
     def __init__(self, dataset: AsyncDataset[T_co], key: jax.random.PRNGKey):
+        super().__init__()
         self.dataset = dataset
         self.key = key
         self._permutation: Optional[Permutation] = None
@@ -72,6 +73,7 @@ class EraShufflingDataset(AsyncDataset[T_co]):
     """
 
     def __init__(self, dataset: AsyncDataset[T_co], era_length: int, *, key: jax.random.PRNGKey):
+        super().__init__()
         self.dataset = dataset
         self.era_length = era_length
         self.key = key
diff --git a/src/levanter/data/text.py b/src/levanter/data/text.py
index 0654d1dfa..7e92d200b 100644
--- a/src/levanter/data/text.py
+++ b/src/levanter/data/text.py
@@ -75,6 +75,7 @@ class EpochDataset(AsyncDataset[T_co]):
     """
 
     def __init__(self, dataset: AsyncDataset[T_co], max_epochs: Optional[int] = None):
+        super().__init__()
         self.dataset = dataset
         self.max_epochs = max_epochs
 
@@ -154,6 +155,7 @@ class TokenSeqDataset(AsyncDataset[np.ndarray]):
     """
 
     def __init__(self, doc_cache: TreeCache[dict], seq_len: int):
+        super().__init__()
         self.doc_cache = doc_cache
         self.seq_len = seq_len
         self._store: Optional[TreeStore] = None
@@ -687,7 +689,7 @@ def preprocess_supervised_example(
     }
 
 
-def _prepare_supervised_example(ex: dict, tokenizer: PreTrainedTokenizerBase) -> LmExample:
+def _prepare_supervised_example(ex: dict, tokenizer: PreTrainedTokenizerBase, Pos: hax.Axis) -> LmExample:
     """
     Prepare an example for training. This function converts the (cached) batch encoding into an LmExample.
 
@@ -699,11 +701,15 @@ def _prepare_supervised_example(ex: dict, tokenizer: PreTrainedTokenizerBase) ->
     """
     with local_cpu_mesh():
         # annoyingly, pad expects things to be batched so we have to prepend a batch axis
-        ex = tokenizer.pad({k: np.expand_dims(v, 0) for k, v in ex.items()}, return_tensors="np", padding="max_length")
+        ex = tokenizer.pad(
+            {k: np.expand_dims(v, 0) for k, v in ex.items()},
+            return_tensors="np",
+            padding="max_length",
+            max_length=Pos.size,
+        )
         ex = {k: v[0] for k, v in ex.items()}
-        input_ids = hax.named(ex["input_ids"], "position")
+        input_ids = hax.named(ex["input_ids"], Pos)
         # mask out padding and anything before the start of the target
-        Pos = input_ids.resolve_axis("position")
         loss_mask = hax.arange(Pos) >= ex["sources_len"] - 1
 
         # don't predict the padding
@@ -714,7 +720,7 @@ def _prepare_supervised_example(ex: dict, tokenizer: PreTrainedTokenizerBase) ->
         return lm_ex
 
 
-def mk_supervised_dataset(config: LMSupervisedDatasetConfig, tokenizer: PreTrainedTokenizerBase):
+def mk_supervised_dataset(config: LMSupervisedDatasetConfig, tokenizer: PreTrainedTokenizerBase, Pos: hax.Axis):
     import levanter.data
 
     # Choose data source based on config
@@ -746,7 +752,7 @@ def mk_supervised_dataset(config: LMSupervisedDatasetConfig, tokenizer: PreTrain
     if tokenizer.pad_token is None:
         tokenizer.pad_token = tokenizer.eos_token
 
-    return cached_dataset.map(lambda ex: _prepare_supervised_example(ex, tokenizer))
+    return cached_dataset.map(lambda ex: _prepare_supervised_example(ex, tokenizer, Pos))
 
 
 @dataclass
@@ -799,7 +805,9 @@ def preprocess_chat_example(batch, tokenizer: PreTrainedTokenizerBase) -> dict:
     }
 
 
-def mk_chat_sft_dataset(config: ChatSFTDatasetConfig, tokenizer: PreTrainedTokenizerBase) -> AsyncDataset[LmExample]:
+def mk_chat_sft_dataset(
+    config: ChatSFTDatasetConfig, tokenizer: PreTrainedTokenizerBase, Pos: hax.Axis
+) -> AsyncDataset[LmExample]:
     """Creates a dataset from JSONL files containing chat format data for SFT."""
     source = config.get_shard_source("train")
     if source is None:
@@ -824,7 +832,7 @@ def mk_chat_sft_dataset(config: ChatSFTDatasetConfig, tokenizer: PreTrainedToken
         tokenizer.pad_token = tokenizer.eos_token
 
     # Reuse the supervised prepare function directly
-    return cached_dataset.map(lambda ex: _prepare_supervised_example(ex, tokenizer))
+    return cached_dataset.map(lambda ex: _prepare_supervised_example(ex, tokenizer, Pos))
 
 
 @dataclass
diff --git a/src/levanter/eval.py b/src/levanter/eval.py
index 99e132dc2..16342be4d 100644
--- a/src/levanter/eval.py
+++ b/src/levanter/eval.py
@@ -60,6 +60,7 @@ def tags(self):
     def __init__(
         self, datasets: Sequence[tuple[AsyncDataset[T], Sequence[str]]], max_examples_per_dataset: Optional[int] = None
     ):
+        super().__init__()
         self.datasets = []
         tag_index: dict[str, int] = {}
         for i, (dataset, tags) in enumerate(datasets):
diff --git a/src/levanter/main/train_lm.py b/src/levanter/main/train_lm.py
index b1b5d4aaa..b411bd59e 100644
--- a/src/levanter/main/train_lm.py
+++ b/src/levanter/main/train_lm.py
@@ -209,7 +209,7 @@ def main(config: TrainLmConfig):
 
         if config.supervised_data is not None:
             logger.info("Using supervised data")
-            supervised_eval = [(levanter.data.text.mk_supervised_dataset(config.supervised_data, tokenizer), "")]
+            supervised_eval = [(levanter.data.text.mk_supervised_dataset(config.supervised_data, tokenizer, Pos), "")]
             # TODO Add tags
             cb = levanter.eval.cb_tagged_lm_evaluate(
                 EvalBatch,
diff --git a/src/levanter/store/cache.py b/src/levanter/store/cache.py
index 558bbfceb..e7a5306d4 100644
--- a/src/levanter/store/cache.py
+++ b/src/levanter/store/cache.py
@@ -191,6 +191,7 @@ def __init__(
         ledger: Optional["CacheLedger"],
         _broker,  # handle of _TreeStoreCacheBuilder
     ):
+        super().__init__()
         self.cache_dir = cache_dir
         self.ledger = ledger
         self._was_already_finished = ledger is not None and ledger.is_finished
diff --git a/tests/test_doremi.py b/tests/test_doremi.py
index d2cf8b590..bbab04f52 100644
--- a/tests/test_doremi.py
+++ b/tests/test_doremi.py
@@ -38,6 +38,7 @@ def platform_of_array(x):
 
 class LogitDataset(AsyncDataset[Example]):
     def __init__(self, W, noise, x_mask, x_bias, *, key):
+        super().__init__()
         self.W = W
         self.noise = noise
         self.x_mask = x_mask
diff --git a/tests/test_new_loader.py b/tests/test_new_loader.py
index e6f9a3dd7..94b5238b2 100644
--- a/tests/test_new_loader.py
+++ b/tests/test_new_loader.py
@@ -64,6 +64,7 @@ def test_local_batched_data_loading_model_axis_1():
 
 class StructuredDataset(AsyncDataset):
     def __init__(self, seq_len):
+        super().__init__()
         self.seq_len = seq_len
         self.begin = 0
         self.end = 256
@@ -138,6 +139,7 @@ def test_structured_batches_model_axis_2():
 
 class StructuredDatasetWithNames(AsyncDataset):
     def __init__(self, Height: Axis, Width: Axis, begin, end, stride):
+        super().__init__()
         self.Height = Height
         self.Width = Width
         self.begin = begin