The @ValidateOnly flag can be used when building a batch in order to provide the end-user with in-row validation, using the "Validate" button in the UI. However, this feature is not officially supported in batch extensions - they require creating custom SQL stored procedures used for validation, that only run during commit and are viewed by the end-user in the exception batch, if/when a validation error is caught. We now know that the @ValidateOnly flag can indeed be used in batch extensions, but it has to be a previously unloaded (new) to the database batch extension spec. We would like this functionality to be available for any batch extension, old or new, and to be fully supported as a feature just like for a regular batch.