A service provider system may include an application fulfillment platform that delivers desktop applications to desktops on physical computing devices or virtual desktop instances. A computing resource instance may be registered with the platform, which generates a unique identifier and a security token for the computing resource instance using multiple authentication mechanisms. An end user of a customer organization may be registered with the platform, which generates a unique identifier and a security token for the end user using multiple authentication mechanisms. An application delivery agent may submit service requests to the platform on behalf of itself or the given user. The identity and security credentials included in the requests may be dependent on the request type and the entities on whose behalf they are submitted. A proxy service on the platform may receive the requests and validate the credentials, then dispatch the requests to other services on the platform.