1. A method, comprising: receiving, via a software development tool user interface, a definition of a custom application session parameter, wherein the custom application session parameter corresponds to an application session parameter that is developer-composed in connection with development of an application to run on a server, and wherein the definition of the custom application session parameter includes at least a field indicating a lifetime associated with the custom application session parameter; and generating programmatically, using a processor, based at least in part on the definition of the custom application session parameter, an application code configured to store and manage the custom application session parameter, including by making the custom application session parameter available from a first application user interface page to one or more other application user interface pages based at least in part on evaluating an expression associated with an event payload of the first application user interface page to determine a value of the custom application session parameter, wherein the application code configured to store and manage the custom application session parameter includes code configured to create, at a client system with which a remote user of the application being run on the server is associated, an instance of an application session parameter manager, wherein the instance of the application session parameter manager is configured to store and manage the custom application session parameter based at least in part on the field indicating the lifetime associated with the custom application session parameter.