HumanSignal/label-studio · error · ValidationError

Invalid or missing serializer class

Error message

Invalid or missing serializer class

What it means

validate_storage_instance raises ValidationError when the passed serializer_class is None/empty or lacks a Meta attribute, meaning the storage endpoint was wired with an unknown or wrong storage type before any DB lookup happens.

Source

Thrown at label_studio/io_storages/functions.py:38

    Preload and prepare a storage instance from request data.

    This function handles the common logic for loading existing storage instances
    or creating new ones from request data, including permission checks and
    serializer validation.

    Args:
        request: The HTTP request containing storage data
        serializer_class: The serializer class to use for validation

    Returns:
        The prepared storage instance

    Raises:
        PermissionDenied: If user doesn't have permission to access the storage
        ValidationError: If serializer validation fails
    """
    if not serializer_class or not hasattr(serializer_class, 'Meta'):
        raise ValidationError('Invalid or missing serializer class')

    storage_id = request.data.get('id')
    instance = None

    if storage_id:
        instance = get_object_or_404(serializer_class.Meta.model.objects.all(), pk=storage_id)
        if not instance.has_permission(request.user):
            raise PermissionDenied()

    # combine instance fields with request.data
    serializer = serializer_class(data=request.data)
    serializer.is_valid(raise_exception=True)

    # if storage exists, we have to use instance from DB,
    # because instance from serializer won't have credentials, they were popped intentionally
    if instance:
        instance = serializer.update(instance, serializer.validated_data)
    else:

View on GitHub (pinned to 0b49e9b539)

Solutions

1. Ensure the storage type in the request maps to a registered serializer in io_storages/functions.py get_storage_list()
2. Verify the custom storage's serializer subclasses ImportStorageSerializer/ExportStorageSerializer and defines Meta with model
3. If you hit this from a custom view, pass the correct serializer_class to validate_storage_instance
4. After adding a new storage type, restart the server so the registry is rebuilt

Example fix

// before (custom storage not registered)
class MyStorageSerializer:  # no Meta
    class Meta: pass  # missing model
// after
from label_studio.io_storages.serializers import ImportStorageSerializer
class MyStorageSerializer(ImportStorageSerializer):
    class Meta:
        model = MyStorage
        fields = '__all__'

def serializer_is_usable(serializer_class):
    return bool(serializer_class) and hasattr(serializer_class, 'Meta') and getattr(serializer_class.Meta, 'model', None) is not None

def is_drf_serializer(cls):
    return isinstance(cls, type) and hasattr(cls, 'Meta') and hasattr(cls.Meta, 'model')

from rest_framework.exceptions import ValidationError
try:
    instance = validate_storage_instance(request, serializer_class)
except ValidationError as e:
    logger.error('Bad storage serializer wiring: %s', e.detail)

When it happens

Trigger: Calling create() (the storage API view) for a storage type whose serializer class could not be resolved (get_storage_list mismatch, unsupported/removed storage type in the URL path, a coding error passing the wrong class).

Common situations: Registering a custom storage but forgetting to add its serializer to get_storage_list(); typos in storage type in the API request; a plugin/older storage type removed after an upgrade.

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AI-assisted analysis of HumanSignal/label-studio@0b49e9b539 (2026-08-29). Data as JSON: /api/errors/e174dee7936128b5. Report an issue: GitHub.