1. A method performed by data processing apparatus, the method comprising: receiving, by a processing device, a request to classify an object, the request including at least one attribute value representing a characteristic of the object; determining, by a processing device, that there are two or more different models that are each configured to classify the object based on the at least one attribute value; providing, by a processing device, the at least one attribute value to each of the two or more different models; identifying, for a particular model among the two or more different models, one or more additional attributes and corresponding additional attribute values; providing the identified additional attribute values to the particular model; receiving classification data from each of the two or more different models, the classification data received from one of the models specifying, for the object, a different presentation availability than the classification data received from at least one other model among the two or more different models; and assigning a final classification to the object based on the classification data received from each of the two or more different models, the final classification specifying a presentation availability of the object.