diff --git a/MIGRATION.md b/MIGRATION.md
index edbe6d99..3fa864c3 100644
--- a/MIGRATION.md
+++ b/MIGRATION.md
@@ -227,7 +227,10 @@ What breaks:
 - **Typoed attributes raise `AttributeError`.** In v1, reading an unknown attribute silently returned (and inserted) an empty mapping, so typos went unnoticed and were truthy-checked as empty dicts. In v2 they fail loudly — code that probed for optional fields via bare attribute access should use `.get("field")` or `hasattr`.
 - **Undocumented nested fields are stripped.** API fields not (yet) in the SDK's generated types are dropped during hydration instead of being passed through. If you depend on a field the SDK does not model, upgrade the SDK to a version that includes it.
 
-Free-form record properties, such as `custom_metadata`, remain plain mappings and are not affected.
+Free-form record properties, such as `custom_metadata`, remain mappings with
+attribute access, and reading a missing key from them fails loudly the same
+way: indexing raises `KeyError` and attribute access raises `AttributeError`.
+Probe for optional keys with `.get("key")` or `"key" in mapping`.
 
 ## `SeamMultiWorkspace` is renamed to `SeamWithoutWorkspace`
 
diff --git a/seam/deep_attr_dict.py b/seam/deep_attr_dict.py
index 62aef3c8..5d8318fb 100644
--- a/seam/deep_attr_dict.py
+++ b/seam/deep_attr_dict.py
@@ -1,6 +1,10 @@
-# https://stackoverflow.com/a/3031270/559475
 class DeepAttrDict(dict):
-    MARKER = object()
+    """A dict whose keys are also readable as attributes, nested dicts included.
+
+    Reading a missing key raises like a plain dict: KeyError when indexing,
+    AttributeError for attribute access. Probe for optional keys with
+    ``.get()``, ``in``, or ``hasattr``.
+    """
 
     def __init__(self, value=None):
         if value is None:
@@ -16,11 +20,12 @@ def __setitem__(self, key, value):
             value = DeepAttrDict(value)
         super().__setitem__(key, value)
 
-    def __getitem__(self, key):
-        found = self.get(key, DeepAttrDict.MARKER)
-        if found is DeepAttrDict.MARKER:
-            found = DeepAttrDict()
-            super().__setitem__(key, found)
-        return found
+    __setattr__ = __setitem__
 
-    __setattr__, __getattr__ = __setitem__, __getitem__
+    def __getattr__(self, key):
+        try:
+            return self[key]
+        except KeyError:
+            # Raise AttributeError so hasattr, getattr defaults, and
+            # copy/pickle protocol probes behave like any other object.
+            raise AttributeError(key) from None
diff --git a/test/deep_attr_dict_test.py b/test/deep_attr_dict_test.py
index ab067800..4702ffbb 100644
--- a/test/deep_attr_dict_test.py
+++ b/test/deep_attr_dict_test.py
@@ -1,7 +1,81 @@
+import pytest
+
+from seam import Seam
 from seam.deep_attr_dict import DeepAttrDict
+from seam.resources import seam_event_from_dict
 
 
 def test_deep_attr_dict():
     attrdict = DeepAttrDict({"a": {"b": {"c": 5}}})
 
-    assert attrdict.a.b.c == 5
+    assert attrdict.a.b.c == 5  # pylint: disable=no-member
+
+
+def test_nested_dicts_keep_attribute_access():
+    attrdict = DeepAttrDict()
+    attrdict.a = {"b": {"c": 5}}
+
+    assert attrdict.a.b.c == 5  # pylint: disable=no-member
+    assert attrdict["a"]["b"]["c"] == 5
+
+
+def test_reading_a_missing_key_raises_key_error():
+    attrdict = DeepAttrDict({"reservation_id": "abc"})
+
+    with pytest.raises(KeyError):
+        attrdict["reservaton_id"]  # pylint: disable=pointless-statement
+
+
+def test_reading_a_missing_attribute_raises_attribute_error():
+    attrdict = DeepAttrDict({"reservation_id": "abc"})
+
+    with pytest.raises(AttributeError):
+        attrdict.reservaton_id  # pylint: disable=pointless-statement
+
+
+def test_reading_a_missing_key_does_not_insert_it():
+    attrdict = DeepAttrDict({"reservation_id": "abc"})
+
+    with pytest.raises(AttributeError):
+        attrdict.reservaton_id  # pylint: disable=pointless-statement
+
+    assert "reservaton_id" not in attrdict
+    assert len(attrdict) == 1
+    assert dict(attrdict) == {"reservation_id": "abc"}
+
+
+def test_missing_keys_work_with_standard_probes():
+    attrdict = DeepAttrDict({"reservation_id": "abc"})
+
+    assert not hasattr(attrdict, "reservaton_id")
+    assert getattr(attrdict, "reservaton_id", None) is None
+    assert attrdict.get("reservaton_id") is None
+    assert "reservaton_id" not in attrdict
+
+
+def test_custom_metadata_reads_do_not_mutate_the_device(recording_server):
+    device_payload = {
+        "device": {
+            "device_id": "44444444-4444-4444-4444-444444444444",
+            "custom_metadata": {"reservation_id": "abc"},
+        }
+    }
+
+    with recording_server([(200, device_payload)]) as (endpoint, _):
+        seam = Seam.from_api_key("seam_apikey_token", endpoint=endpoint)
+        device = seam.devices.get(device_id="44444444-4444-4444-4444-444444444444")
+
+        with pytest.raises(AttributeError):
+            device.custom_metadata.reservaton_id  # pylint: disable=pointless-statement
+
+        # The typo'd read leaves no key behind to re-serialize to the API.
+        assert dict(device.custom_metadata) == {"reservation_id": "abc"}
+
+
+def test_unknown_event_fallback_fields_stay_readable():
+    event = seam_event_from_dict(
+        {"event_id": "e", "event_type": "unknown.event", "foo": {"bar": 1}}
+    )
+
+    assert event.event_id == "e"
+    assert event.foo.bar == 1