diff --git a/ai_models/checkpoint.py b/ai_models/checkpoint.py
index 9f8e7a7..eef0a56 100644
--- a/ai_models/checkpoint.py
+++ b/ai_models/checkpoint.py
@@ -5,11 +5,15 @@
 # granted to it by virtue of its status as an intergovernmental organisation
 # nor does it submit to any jurisdiction.
 
+import json
+import logging
 import os
 import pickle
 import zipfile
 from typing import Any
 
+LOG = logging.getLogger(__name__)
+
 
 class FakeStorage:
     def __init__(self):
@@ -46,6 +50,24 @@ def tidy(x):
     return x
 
 
+def checkpoint_metadata(path, name="ai-models.json"):
+    with zipfile.ZipFile(path, "r") as f:
+        metadata = None
+        for b in f.namelist():
+            if os.path.basename(b) == name:
+                if metadata is not None:
+                    raise Exception(
+                        f"Found two metadata.json files in {path}: {metadata} and {b}"
+                    )
+                metadata = b
+
+    if metadata is not None:
+        with zipfile.ZipFile(path, "r") as f:
+            return json.load(f.open(metadata, "r"))
+
+    return None
+
+
 def peek(path):
     with zipfile.ZipFile(path, "r") as f:
         data_pkl = None
@@ -57,6 +79,9 @@ def peek(path):
                     )
                 data_pkl = b
 
+    LOG.info(f"Found data.pkl at {data_pkl}")
+
+    with zipfile.ZipFile(path, "r") as f:
         unpickler = UnpicklerWrapper(f.open(data_pkl, "r"))
         x = tidy(unpickler.load())
         return tidy(x)
diff --git a/ai_models/model.py b/ai_models/model.py
index e52e2cc..5f89416 100644
--- a/ai_models/model.py
+++ b/ai_models/model.py
@@ -20,7 +20,7 @@
 from climetlab.utils.humanize import seconds
 from multiurl import download
 
-from .checkpoint import peek
+from .checkpoint import peek, checkpoint_metadata
 from .inputs import get_input
 from .outputs import get_output
 from .stepper import Stepper
@@ -396,6 +396,9 @@ def patch_retrieve_request(self, request):
     def peek_into_checkpoint(self, path):
         return peek(path)
 
+    def checkpoint_metadata(self, path):
+        return checkpoint_metadata(path)
+
     def parse_model_args(self, args):
         if args:
             raise NotImplementedError(f"This model does not accept arguments {args}")
diff --git a/ai_models/outputs/__init__.py b/ai_models/outputs/__init__.py
index eeaa79c..d9af007 100644
--- a/ai_models/outputs/__init__.py
+++ b/ai_models/outputs/__init__.py
@@ -38,8 +38,18 @@ def __init__(self, owner, path, metadata, **kwargs):
         )
 
     def write(self, data, *args, **kwargs):
-        assert not np.isnan(data).any(), (args, kwargs)
-        return self.output.write(data, *args, **kwargs)
+        try:
+            return self.output.write(data, *args, **kwargs)
+        except Exception:
+            if np.isnan(data).any():
+                raise ValueError(
+                    f"NaN values found in field. args={args} kwargs={kwargs}"
+                )
+            if np.isinf(data).any():
+                raise ValueError(
+                    f"Infinite values found in field. args={args} kwargs={kwargs}"
+                )
+            raise
 
 
 class HindcastReLabel: