diff --git a/dascore/core/inventory.py b/dascore/core/inventory.py
index ca53d7cda..ebc99797a 100644
--- a/dascore/core/inventory.py
+++ b/dascore/core/inventory.py
@@ -665,6 +665,14 @@ class CouplingCondition(_IntervalModel):
     )
 
 
+def _wanted(field: str, info) -> bool:
+    """Whether a serialization's own include/exclude asked for a field."""
+    if (exclude := getattr(info, "exclude", None)) and field in exclude:
+        return False
+    include = getattr(info, "include", None)
+    return not include or field in include
+
+
 class OpticalPathAnnotation(_IntervalModel):
     """
     Key/value annotation attached to an interval of an optical path.
@@ -697,6 +705,29 @@ def _reject_empty_string(cls, value):
             raise ValueError(msg)
         return value
 
+    def _write_object_type(self, handler, info):
+        """
+        Tag the document as every model does, and keep the value with it.
+
+        Overridden rather than added beside: pydantic runs one model
+        serializer per class, so a second one here would take the base's
+        place and drop the ``object_type`` every document is dispatched by.
+
+        The value itself needs putting back because ``exclude_defaults``
+        compares with ``==`` and ``1 == True``, the default. A group
+        numbered from one would otherwise lose every ``1`` on the way out
+        and reload holding a boolean, which then mixes kinds with the
+        numbers beside it and is refused. Identity is what "still its
+        default" means for a field admitting both. A caller who asked for
+        the value to be left out is obeyed: this restores what
+        exclude_defaults dropped, not what anyone chose to filter.
+        """
+        out = super()._write_object_type(handler, info)
+        if "value" in out or self.value is True or not _wanted("value", info):
+            return out
+        out["value"] = self.value
+        return out
+
 
 # The coordinates a DistanceMap may be written in, in preference order.
 DISTANCE_MAP_AXES = ("channel", "instrument_distance")
diff --git a/tests/test_core/test_inventory.py b/tests/test_core/test_inventory.py
index bdc6378a5..e2ed18b93 100644
--- a/tests/test_core/test_inventory.py
+++ b/tests/test_core/test_inventory.py
@@ -2073,6 +2073,65 @@ def test_round_trip_equals(self, name):
         inventory = SAMPLE_INVENTORIES[name]
         assert dc.inventory(inventory.to_yaml()) == inventory
 
+    def test_an_annotation_value_of_one_survives(self):
+        """`1 == True`, and the value's default is True, so it was dropped."""
+        pytest.importorskip("yaml")
+        path = inv.OpticalPath(
+            optical_components=(inv.FiberSegment(optical_length=100.0),),
+            annotations=(
+                inv.OpticalPathAnnotation(
+                    start_distance=0.0, end_distance=10.0, group="hole", value=1
+                ),
+                inv.OpticalPathAnnotation(
+                    start_distance=20.0, end_distance=30.0, group="hole", value=2
+                ),
+            ),
+        )
+        array = inv.FiberArray(code="L001", optical_paths=(path,))
+        inventory = inv.Inventory(
+            networks=(inv.Network(code="XX", fiber_arrays=(array,)),)
+        )
+        text = inventory.to_yaml()
+        assert "value: 1" in text
+        # Without the value, the group reloads holding a boolean beside a
+        # number and is refused as mixing two kinds.
+        assert dc.inventory(text) == inventory
+
+    def test_an_annotation_still_names_its_class(self):
+        """Restoring the value must not displace the document's tag."""
+        annotation = inv.OpticalPathAnnotation(
+            start_distance=0.0, end_distance=1.0, group="hole", value=2
+        )
+        dumped = annotation.model_dump(mode="json")
+        assert dumped["object_type"] == "OpticalPathAnnotation"
+
+    def test_a_deliberately_excluded_value_stays_out(self):
+        """What a caller filtered is not what exclude_defaults dropped."""
+        annotation = inv.OpticalPathAnnotation(
+            start_distance=0.0, end_distance=1.0, group="hole", value=2
+        )
+        assert "value" not in annotation.model_dump(mode="json", exclude={"value"})
+        assert "value" not in annotation.model_dump(mode="json", include={"group"})
+
+    def test_a_flag_annotation_stays_terse(self):
+        """A value which really is the default is still left out."""
+        pytest.importorskip("yaml")
+        path = inv.OpticalPath(
+            optical_components=(inv.FiberSegment(optical_length=100.0),),
+            annotations=(
+                inv.OpticalPathAnnotation(
+                    start_distance=0.0, end_distance=10.0, group="noisy"
+                ),
+            ),
+        )
+        array = inv.FiberArray(code="L001", optical_paths=(path,))
+        inventory = inv.Inventory(
+            networks=(inv.Network(code="XX", fiber_arrays=(array,)),)
+        )
+        text = inventory.to_yaml()
+        assert "value:" not in text
+        assert dc.inventory(text) == inventory
+
     def test_round_trip_through_file(self, tmp_path):
         """The writer taking a path writes what the text form holds."""
         pytest.importorskip("yaml")