A service provider system may provide virtualized computing services to clients, including encapsulating services that call other (encapsulated) services. Once configured to delegate parameters to encapsulated services, an encapsulating service (and its API) may not need to be modified when an encapsulated service (or its API) is modified to support more, fewer, or different features or corresponding arguments. For example, an encapsulating service may receive a service request call from a client that includes arguments for an encapsulated service (e.g., as header information, or as an opaque block or uninterpreted key-value dictionary) and may call the encapsulated service, blindly passing along those arguments. An encapsulating service may access stored policy-based arguments or obtain arguments from a central service. Arguments passed to an encapsulated service may override default parameters of the service. An encapsulating service may blindly proxy responses received from an encapsulated service to a client.