Techniques are disclosed relating to testing application code. A computer system, in various embodiments, receives application code to be tested by the computer system and separate information defining actions to be performed at specified locations within the application code. In various embodiments, the computer system executes the application code in a test environment in which the actions defined by the separate information are retrieved and performed by a plurality of threads of the application code at the specified locations to control flow of the plurality of threads through the application code. In some embodiments, a first one of the plurality of threads is operable to perform at least one of the actions to control the flow of a second one of the plurality of threads.