1. A method of managing a set of artifacts for an extensible runtime environment, the method comprising: registering an extension handler to interface a tooling system with an extension artifact, the tooling system allowing a user to perform operations to create and manipulate program code of an application that executes in a runtime environment, the extension artifact comprising an artifact included in the application and requiring a runtime extension to execute in the runtime environment, the runtime extension comprising runtime program code extending capabilities of the runtime environment; processing a tooling request for the extension artifact using the extension handler for the extension artifact, the tooling request comprising an operation directed at the extension artifact, the tooling request initiated by the user through the tooling system; and calling the extension handler to perform an artifact-specific function specified by the tooling request in response to the operation directed at the extension artifact, the extension handler performing the artifact-specific function according to interface characteristics of the extension artifact.