diff --git a/api/features/multivariate/serializers.py b/api/features/multivariate/serializers.py
index 6c04cb5ac68b..399d48ece8e6 100644
--- a/api/features/multivariate/serializers.py
+++ b/api/features/multivariate/serializers.py
@@ -55,17 +55,54 @@ def get_control_value(self, obj: dict[str, typing.Any]) -> str | int | bool | No
 class MultivariateFeatureOptionSerializer(NestedMultivariateFeatureOptionSerializer):
     class Meta(NestedMultivariateFeatureOptionSerializer.Meta):
         fields = NestedMultivariateFeatureOptionSerializer.Meta.fields + ("feature",)  # type: ignore[assignment]
+        extra_kwargs = {
+            "feature": {"required": False},
+        }
 
     def validate(self, attrs):  # type: ignore[no-untyped-def]
         attrs = super().validate(attrs)
+
+        # For partial updates or nested routes, use existing instance or URL value as fallback
+        feature = attrs.get("feature")
+        if feature is None and self.instance is not None:
+            feature = self.instance.feature
+
+        if feature is None:
+            view = self.context.get("view")
+            feature_pk = getattr(view, "kwargs", {}).get("feature_pk") if view else None
+            project_pk = getattr(view, "kwargs", {}).get("project_pk") if view else None
+            if feature_pk is not None:
+                qs = Feature.objects.filter(pk=feature_pk)
+                if project_pk is not None:
+                    qs = qs.filter(project__id=project_pk)
+                feature = qs.first()
+
+        if feature is None:
+            raise serializers.ValidationError({"feature": "Feature is required"})
+
+        # Ensure feature is always present in validated data (e.g. for creates via nested routes)
+        attrs["feature"] = feature
+
+        # Handle legacy records where default_percentage_allocation may be NULL in the database.
+        if self.instance and self.instance.default_percentage_allocation is not None:
+            instance_default_allocation = self.instance.default_percentage_allocation
+        else:
+            instance_default_allocation = 100
+
+        default_allocation = attrs.get(
+            "default_percentage_allocation",
+            instance_default_allocation,
+        )
+        attrs["default_percentage_allocation"] = default_allocation
+
         total_sibling_percentage_allocation = (
-            self._get_siblings(attrs["feature"]).aggregate(
+            self._get_siblings(feature).aggregate(
                 total_percentage_allocation=Sum("default_percentage_allocation")
             )["total_percentage_allocation"]
             or 0
         )
         total_percentage_allocation = (
-            total_sibling_percentage_allocation + attrs["default_percentage_allocation"]
+            total_sibling_percentage_allocation + default_allocation
         )
 
         if total_percentage_allocation > 100:
diff --git a/api/tests/integration/features/multivariate/test_integration_multivariate.py b/api/tests/integration/features/multivariate/test_integration_multivariate.py
index 49effb4c8ff9..864370233658 100644
--- a/api/tests/integration/features/multivariate/test_integration_multivariate.py
+++ b/api/tests/integration/features/multivariate/test_integration_multivariate.py
@@ -7,6 +7,8 @@
 from rest_framework.test import APIClient
 
 from features.models import Feature
+from features.multivariate.models import MultivariateFeatureOption
+from features.multivariate.serializers import MultivariateFeatureOptionSerializer
 from organisations.models import Organisation
 from projects.models import Project
 from users.models import FFAdminUser
@@ -40,6 +42,181 @@ def test_can_create_mv_option(client, project, mv_option_50_percent, feature):
     assert set(data.items()).issubset(set(response.json().items()))
 
 
+@pytest.mark.parametrize(
+    "client",
+    [lazy_fixture("admin_master_api_key_client"), lazy_fixture("admin_client")],
+)
+def test_create_mv_option__without_feature_in_payload__uses_feature_from_url(
+    client: APIClient,
+    project: int,
+    feature: int,
+) -> None:
+    # Given
+    url = reverse(
+        "api-v1:projects:feature-mv-options-list",
+        args=[project, feature],
+    )
+    data = {
+        "type": "unicode",
+        "string_value": "bigger",
+        "default_percentage_allocation": 50,
+    }
+    # When
+    response = client.post(
+        url,
+        data=json.dumps(data),
+        content_type="application/json",
+    )
+    # Then
+    assert response.status_code == status.HTTP_201_CREATED
+    body = response.json()
+    assert body["id"]
+    assert body["feature"] == feature
+    assert body["string_value"] == "bigger"
+    assert body["default_percentage_allocation"] == 50
+
+
+@pytest.mark.parametrize(
+    "client",
+    [lazy_fixture("admin_master_api_key_client"), lazy_fixture("admin_client")],
+)
+def test_create_mv_option__without_default_percentage_allocation__uses_default(
+    client: APIClient,
+    project: int,
+    feature: int,
+) -> None:
+    # Given
+    url = reverse(
+        "api-v1:projects:feature-mv-options-list",
+        args=[project, feature],
+    )
+    data = {
+        "type": "unicode",
+        "feature": feature,
+        "string_value": "test_value",
+    }
+
+    # When
+    response = client.post(
+        url,
+        data=json.dumps(data),
+        content_type="application/json",
+    )
+
+    # Then
+    assert response.status_code == status.HTTP_201_CREATED
+    assert response.json()["id"]
+    assert response.json()["default_percentage_allocation"] == 100
+
+
+@pytest.mark.parametrize(
+    "client",
+    [lazy_fixture("admin_master_api_key_client"), lazy_fixture("admin_client")],
+)
+def test_create_mv_option__without_default_percentage_allocation_with_existing_sibling_and_total_gt_100__returns_400(  # noqa: E501
+    client: APIClient,
+    project: int,
+    feature: int,
+    mv_option_50_percent: int,
+) -> None:
+    # Given
+    url = reverse(
+        "api-v1:projects:feature-mv-options-list",
+        args=[project, feature],
+    )
+    data = {
+        "type": "unicode",
+        "feature": feature,
+        "string_value": "another_value",
+    }
+
+    # When
+    response = client.post(
+        url,
+        data=json.dumps(data),
+        content_type="application/json",
+    )
+
+    # Then
+    assert response.status_code == status.HTTP_400_BAD_REQUEST
+    assert response.json()["default_percentage_allocation"] == [
+        "Invalid percentage allocation"
+    ]
+
+
+@pytest.mark.parametrize(
+    "client",
+    [lazy_fixture("admin_master_api_key_client"), lazy_fixture("admin_client")],
+)
+def test_partial_update_mv_option__without_feature_and_allocation__uses_existing_values(
+    client: APIClient,
+    project: int,
+    feature: int,
+    mv_option_50_percent: int,
+) -> None:
+    # Given
+    url = reverse(
+        "api-v1:projects:feature-mv-options-detail",
+        args=[project, feature, mv_option_50_percent],
+    )
+    data = {
+        "string_value": "updated_value",
+    }
+
+    # When
+    response = client.patch(
+        url,
+        data=json.dumps(data),
+        content_type="application/json",
+    )
+
+    # Then
+    assert response.status_code == status.HTTP_200_OK
+    assert response.json()["string_value"] == "updated_value"
+    assert response.json()["default_percentage_allocation"] == 50
+    assert response.json()["feature"] == feature
+
+
+@pytest.mark.parametrize(
+    "client",
+    [lazy_fixture("admin_master_api_key_client"), lazy_fixture("admin_client")],
+)
+def test_partial_update_mv_option__handles_legacy_null_default_percentage_allocation(  # noqa: E501
+    client: APIClient,
+    project: int,
+    feature: int,
+    mv_option_50_percent: int,
+) -> None:
+    # Given
+    # Simulate a legacy instance in memory where default_percentage_allocation is NULL.
+    mv_option = MultivariateFeatureOption.objects.get(id=mv_option_50_percent)
+    mv_option.default_percentage_allocation = None
+
+    serializer = MultivariateFeatureOptionSerializer(
+        instance=mv_option,
+        data={"string_value": "updated_legacy_value"},
+        partial=True,
+        context={
+            "view": type(
+                "View",
+                (),
+                {
+                    "kwargs": {
+                        "project_pk": project,
+                        "feature_pk": feature,
+                    },
+                },
+            )(),
+        },
+    )
+    assert serializer.is_valid(), serializer.errors
+    instance = serializer.save()
+
+    assert instance.string_value == "updated_legacy_value"
+    assert instance.default_percentage_allocation == 100
+    assert instance.feature_id == feature
+
+
 @pytest.mark.parametrize(
     "client, feature_id",
     [
@@ -183,6 +360,56 @@ def test_can_update_default_percentage_allocation(  # type: ignore[no-untyped-de
     assert set(data.items()).issubset(set(response.json().items()))
 
 
+@pytest.mark.parametrize(
+    "client",
+    [lazy_fixture("admin_master_api_key_client"), lazy_fixture("admin_client")],
+)
+def test_partial_update_default_percentage_allocation__sibling_total_gt_100__returns_400(  # noqa: E501
+    project: int,
+    mv_option_50_percent: int,
+    client: APIClient,
+    feature: int,
+) -> None:
+    # First let's create another mv_option with 30 percent allocation
+    url = reverse(
+        "api-v1:projects:feature-mv-options-list",
+        args=[project, feature],
+    )
+    data = {
+        "type": "unicode",
+        "feature": feature,
+        "string_value": "bigger",
+        "default_percentage_allocation": 30,
+    }
+    response = client.post(
+        url,
+        data=json.dumps(data),
+        content_type="application/json",
+    )
+    assert response.status_code == status.HTTP_201_CREATED
+    mv_option_30_percent = response.json()["id"]
+
+    # Next, let's partially update the 30 percent mv option to 51 percent
+    url = reverse(
+        "api-v1:projects:feature-mv-options-detail",
+        args=[project, feature, mv_option_30_percent],
+    )
+    data = {
+        "default_percentage_allocation": 51,
+    }
+    # When
+    response = client.patch(
+        url,
+        data=json.dumps(data),
+        content_type="application/json",
+    )
+    # Then
+    assert response.status_code == status.HTTP_400_BAD_REQUEST
+    assert response.json()["default_percentage_allocation"] == [
+        "Invalid percentage allocation"
+    ]
+
+
 @pytest.mark.parametrize(
     "client",
     [lazy_fixture("admin_master_api_key_client"), lazy_fixture("admin_client")],