Technologies are described herein for executing business logic extensions on a client computing system. A server platform exposes server-hosted application services to client applications. Calls to the server-hosted application services may be executed along with custom business logic extensions. A client platform is also provided that exposes client-hosted application services to a client application. The same business logic extensions that execute on the server platform can be utilized by the client platform. When a client application is online and connected to the server platform, the client application utilizes the server-hosted application services in the manner described above. When the client application is offline, the client application utilizes the client-hosted application services. The server platform synchronizes the business logic extensions to the client platform.