diff --git a/src/levanter/data/text.py b/src/levanter/data/text.py
index 7e92d200b..4cc000e59 100644
--- a/src/levanter/data/text.py
+++ b/src/levanter/data/text.py
@@ -708,7 +708,9 @@ def _prepare_supervised_example(ex: dict, tokenizer: PreTrainedTokenizerBase, Po
             max_length=Pos.size,
         )
         ex = {k: v[0] for k, v in ex.items()}
-        input_ids = hax.named(ex["input_ids"], Pos)
+        # padding doesn't do truncation, so we have to do it ourselves.
+        # Truncate from the left since we want to predict the last tokens
+        input_ids = hax.named(ex["input_ids"][-Pos.size :], Pos)
         # mask out padding and anything before the start of the target
         loss_mask = hax.arange(Pos) >= ex["sources_len"] - 1
 
diff --git a/src/levanter/doremi.py b/src/levanter/doremi.py
index 9d048b24f..6d9165cfc 100644
--- a/src/levanter/doremi.py
+++ b/src/levanter/doremi.py
@@ -1,6 +1,6 @@
 import dataclasses
 import logging
-from typing import Optional, Tuple, TypeVar
+from typing import Mapping, Optional, Tuple, TypeVar
 
 import equinox as eqx
 import jax.numpy as jnp
@@ -56,7 +56,7 @@ def estimate_mixture_weights(
     loss_fn: ComputeLossFunction[M, T],
     initial_proxy: M,
     ref: M,
-    data_sources: dict[str, AsyncDataset[T]],
+    data_sources: Mapping[str, AsyncDataset[T]],
     sampling_weights: Optional[dict[str, float]] = None,
     *,
     validation_sets: Optional[dict[str, AsyncDataset[T]]] = None,
@@ -184,7 +184,9 @@ def doremi_step(state: DoremiState, ref, batch, domains):
 
     # we're not actually going to use the trainer for very much but it holds hooks and sets up contexts
     with trainer:
-        tagged_mixture = domain_tagged_mixture(data_sources, sampling_weights, domain_to_index, key=data_key)
+        tagged_mixture: MixtureDataset = domain_tagged_mixture(
+            data_sources, sampling_weights, domain_to_index, key=data_key
+        )
         state = load_checkpoint_or_initialize(
             DoremiState.init,
             trainer.checkpoint_path,
@@ -263,7 +265,7 @@ def _prepare_ref_model(ref, trainer):
 
 
 def domain_tagged_mixture(
-    data_sources: dict[str, AsyncDataset[T]],
+    data_sources: Mapping[str, AsyncDataset[T]],
     weights: dict[str, float],
     domain_to_index: dict[str, int],
     *,
diff --git a/src/levanter/main/doremi_lm.py b/src/levanter/main/doremi_lm.py
index 12b3e6ae0..742c3229c 100644
--- a/src/levanter/main/doremi_lm.py
+++ b/src/levanter/main/doremi_lm.py
@@ -109,7 +109,7 @@ def init_proxy_model():
         train_datasets = config.data.training_sets(ref_model.Pos.size)
         valid_datasets = config.data.validation_sets(ref_model.Pos.size)
 
-        train_datasets = {
+        causal_train_datasets = {
             k: CausalLmDataset(v, config.model.Pos, config.model.KeyPos, ignore_index=config.data.ignore_token_id)
             for k, v in train_datasets.items()
         }
@@ -122,7 +122,7 @@ def init_proxy_model():
             loss_function,
             proxy_model,
             ref=ref_model,
-            data_sources=train_datasets,
+            data_sources=causal_train_datasets,
             trainer_config=config.trainer,
             optimizer=optimizer,
             domain_weight_step_size=config.doremi.domain_weight_step_size,
diff --git a/src/levanter/main/train_lm.py b/src/levanter/main/train_lm.py
index b411bd59e..f2ad3e7ce 100644
--- a/src/levanter/main/train_lm.py
+++ b/src/levanter/main/train_lm.py
@@ -268,6 +268,9 @@ def compute_log_probs(model, example):
             checkpointer = trainer.config.checkpointer.create(trainer.run_id)
             checkpointer.wait_until_finished()
 
+    # This isn't necessary except when Levanter is run in a subprocess (as happens w/ ray)
+    trainer.tracker.finish()
+
 
 if __name__ == "__main__":
     levanter.config.main(main)()
diff --git a/src/levanter/models/lm_model.py b/src/levanter/models/lm_model.py
index 911e74b09..1a82aa7be 100644
--- a/src/levanter/models/lm_model.py
+++ b/src/levanter/models/lm_model.py
@@ -1,4 +1,5 @@
 import abc
+from dataclasses import dataclass
 from typing import Generic, Optional, Type, TypeVar
 
 import draccus
@@ -48,6 +49,7 @@ def causal(
 
 
 # TODO: for some reason, mypy doesn't like the discover_packages_path argument?
+@dataclass(frozen=True)
 class LmConfig(draccus.PluginRegistry, abc.ABC, Generic[LmT], discover_packages_path="levanter.models"):  # type: ignore
     @property
     @abc.abstractmethod
@@ -69,7 +71,7 @@ def Pos(self) -> Axis:
     def Embed(self) -> Axis:
         pass
 
-    cross_entropy_block_size: Optional[int] = 64000
+    cross_entropy_block_size: Optional[int] = None
     """
     The block size for computing cross-entropy loss. This is the number of tokens that are processed together
     in a single block. This can be adjusted to fit within memory constraints. It's deliberately set to a large
diff --git a/src/levanter/models/loss.py b/src/levanter/models/loss.py
index 154fc66ac..d705eda4d 100644
--- a/src/levanter/models/loss.py
+++ b/src/levanter/models/loss.py
@@ -58,7 +58,9 @@ def next_token_loss(
 
     if block_size is None:
         # Full softmax computation
-        logits = hax.dot(pred_embeddings, pred_lm_head, axis=Embed, preferred_element_type=dtype)
+        logits = hax.dot(pred_embeddings, pred_lm_head, axis=Embed)
+        if dtype is not None:
+            logits = logits.astype(dtype)
         target_y_full = hax.nn.one_hot(target_y, Vocab, dtype=pred_embeddings.dtype)
         return cross_entropy_and_logsumexp_penalty(
             logits,
@@ -261,9 +263,10 @@ def process_block(block_idx, acc, current_block_size):
 
         # Materialize the logits for the current block
         lm_head_b = pred_lm_head[Label, hax.dslice(start, Block)]  # [Contract, Block]
-        logits_b = hax.dot(
-            pred_embeddings, lm_head_b, axis=Contract, preferred_element_type=dtype
-        )  # [Batch, Seq, Block]
+        logits_b = hax.dot(pred_embeddings, lm_head_b, axis=Contract)  # [Batch, Seq, Block]
+
+        if dtype is not None:
+            logits_b = logits_b.astype(dtype)
 
         # Update max and logsumexp
         max_logit = hax.maximum(max_logit_prev, hax.max(logits_b, axis=Block))  # [Batch, Seq]
@@ -278,7 +281,7 @@ def process_block(block_idx, acc, current_block_size):
         # Update sumV. This is actually unnecessary if we're using one-hot targets
         # sV = sV_prev + hax.sum(target_y_b, axis=Label.name)
 
-        loss += hax.dot(logits_b, target_y_b, axis=Block, preferred_element_type=dtype)  # [Batch, Seq]
+        loss += hax.dot(logits_b, target_y_b, axis=Block)  # [Batch, Seq]
 
         return loss, logsumexp, max_logit  # , sV
 
@@ -351,7 +354,7 @@ def _block_cross_entropy_backward(
     num_blocks = vocab_size // block_size
 
     grad_embeddings = hax.zeros(pred_embeddings.axes, dtype=pred_embeddings.dtype)
-    grad_lm_head = hax.zeros(pred_lm_head.axes, dtype=pred_embeddings.dtype)
+    grad_lm_head = hax.zeros(pred_lm_head.axes, dtype=pred_lm_head.dtype)
 
     def process_block(block_idx, acc, current_block_size):
         """
@@ -372,14 +375,15 @@ def process_block(block_idx, acc, current_block_size):
 
         # Materialize the logits for the current block
         lm_head_b = pred_lm_head[Label, hax.dslice(start, Block)]  # [Contract, Block]
-        logits_b = hax.dot(
-            pred_embeddings, lm_head_b, axis=Contract, preferred_element_type=dtype
-        )  # [Batch, Seq, Block]
+        logits_b = hax.dot(pred_embeddings, lm_head_b, axis=Contract)  # [Batch, Seq, Block]
 
         # Materialize the target for the current block (one-hot)
         target_y_block = _block_one_hot(Block, start, labels_y, logits_b.dtype)  # [Batch, Seq, Block]
 
         # materialize the softmax for the current block
+        if dtype is not None:
+            logits_b = logits_b.astype(dtype)
+
         p_b = hax.exp(logits_b - log_z)  # [Batch, Seq, Block]
 
         delta_b = p_b - target_y_block
diff --git a/src/levanter/models/mpt.py b/src/levanter/models/mpt.py
index 0809d9d23..97b61f1dc 100644
--- a/src/levanter/models/mpt.py
+++ b/src/levanter/models/mpt.py
@@ -107,7 +107,7 @@ def from_hf(config: HfMptAttentionConfig):
 
 
 @LmConfig.register_subclass("mpt")
-@dataclass
+@dataclass(frozen=True)
 class MptConfig(HFCompatConfig):
     d_model: int = 768
     n_heads: int = 12
diff --git a/src/levanter/tracker/tensorboard.py b/src/levanter/tracker/tensorboard.py
index 360c32171..e819d6459 100644
--- a/src/levanter/tracker/tensorboard.py
+++ b/src/levanter/tracker/tensorboard.py
@@ -43,6 +43,9 @@ def log_artifact(self, artifact_path, *, name: Optional[str] = None, type: Optio
             pylogger.exception(f"Error logging artifact {artifact_path} to {log_path}")
             return
 
+    def finish(self):
+        self.writer.close()
+
 
 @TrackerConfig.register_subclass("tensorboard")
 @dataclass
diff --git a/src/levanter/tracker/tracker.py b/src/levanter/tracker/tracker.py
index 8b6816f17..99fd217e5 100644
--- a/src/levanter/tracker/tracker.py
+++ b/src/levanter/tracker/tracker.py
@@ -46,6 +46,14 @@ def log_summary(self, metrics: dict[str, Any]):
     def log_artifact(self, artifact_path, *, name: Optional[str] = None, type: Optional[str] = None):
         pass
 
+    @abc.abstractmethod
+    def finish(self):
+        """
+        Finish the tracker. This is called when the tracker is no longer needed. This can, e.g.,
+        force a commit of all metrics.
+        """
+        pass
+
     def __enter__(self):
         import levanter.tracker.tracker_fns as tracker_fns
 
@@ -81,6 +89,17 @@ def log_artifact(self, artifact_path, *, name: Optional[str] = None, type: Optio
         for tracker in self.loggers:
             tracker.log_artifact(artifact_path, name=name, type=type)
 
+    def finish(self):
+        excs = []
+        for tracker in self.loggers:
+            try:
+                tracker.finish()
+            except Exception as e:
+                excs.append(e)
+
+        if excs:
+            raise RuntimeError("Errors occurred when finishing trackers") from excs[0]
+
 
 class TrackerConfig(draccus.PluginRegistry, abc.ABC):
     discover_packages_path = "levanter.tracker"
@@ -109,6 +128,9 @@ def log_summary(self, metrics: dict[str, Any]):
     def log_artifact(self, artifact_path, *, name: Optional[str] = None, type: Optional[str] = None):
         pass
 
+    def finish(self):
+        pass
+
 
 @TrackerConfig.register_subclass("noop")
 @dataclasses.dataclass
diff --git a/src/levanter/tracker/wandb.py b/src/levanter/tracker/wandb.py
index 18f0251ec..981bebf83 100644
--- a/src/levanter/tracker/wandb.py
+++ b/src/levanter/tracker/wandb.py
@@ -72,6 +72,10 @@ def log_summary(self, metrics: dict[str, Any]):
     def log_artifact(self, artifact_path, *, name: Optional[str] = None, type: Optional[str] = None):
         self.run.log_artifact(artifact_path, name=name, type=type)
 
+    def finish(self):
+        logger.info("Finishing wandb run...")
+        self.run.finish()
+
 
 def is_wandb_available():
     try: