Techniques for using a sandbox to test changes to the metadata of a metadata-driven application. In one set of embodiments, a mainline and a sandbox are created in a runtime environment of a metadata-driven application. The mainline is configured to include a mainline version of a metadata document of the application. The sandbox is configured to include a sandbox (i.e., test) version of the metadata document. At application runtime, requests to access and/or modify the metadata document are serviced either from the mainline or the sandbox based on a sandbox context of the current user session. Thus, changes to the metadata document may be tested on the sandbox version without affecting the mainline version, and vice versa. In various embodiments, multiple sandboxes may be supported in the runtime environment. In further embodiments, the sandbox version may be merged into the mainline version without bringing the runtime environment offline.