One of the strangest part of the Dynamics CRM WebApi is the pluralisation of the entity names.
In the old OData endpoint, the entity set name was <EntityLogicalName>Set – however in the OData 4.0 endpoing, the Logical Name is pluralised by using a simplistic set of rules which often results in the incorrect plural name being picked.
The good news is that with version 9, the Xrm Api now supports:
This will return "contacts" and so we can safely use this without worrying if the plural name is correct or not or indeed if it changes in the future.
Hope this helps!