A socket service may be used to implement client-defined function mappings (e.g., custom protocols) for sockets hosted by the socket service so that when a particular socket is opened for a particular client device, the socket operates according to a particular client-defined function mapping requested by the particular client device. To establish a particular client-defined function mapping, a socket manager may receive request from a user that specifies a particular endpoint (e.g., a client-registered function). When a client device requests a socket to be opened, the client device requests the particular client-defined function mapping to be associated with the socket. When the socket is opened, the endpoint is invoked. The particular client-defined function mapping may also specify one or more receive criteria for data received at the socket. If the one or more receive criteria is met, the endpoint is invoked by sending the data to the endpoint.