1. A system comprising: one or more processors; and one or more computer-readable storage media storing instructions that are executable by the one or more processors to perform operations including: receiving an indication that an extension calls an application programming interface (API) to functionally extend an application; identifying a permission that specifies one or more of an allowed interaction for the extension and the application, or a disallowed interaction for the extension and the application; and causing the application to be functionally extended by the extension subject to the permission.