Modifying application behaviour

A data processing system comprising: an operating system providing an application programming interface; an application supported by the operating system and operable to make calls to the application programming interface; an intercept library configured to intercept calls of a predetermined set of call types made by the application to the application programming interface; and a configuration data structure defining at least one action to be performed for each of a plurality of sequences of one or more calls having predefined characteristics, the one or more calls being of the predetermined set of call types; wherein the intercept library is configured to, on intercepting a sequence of one or more calls defined in the configuration data structure, perform the corresponding action(s) defined by the configuration data structure.