Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, provide a technique for supporting native program code extensions for managed code environments. In one aspect, a method includes the actions of: obtaining a package of information defining an extension to an application program interface of a runtime, the package of information including native program code and descriptors that identify different computing platforms supported by the extension; obtaining a declaration of a target computing platform for an application program that employs the extension; comparing the declaration of the target computing platform for the application program with the descriptors that identify the different computing platforms supported by the extension; if the target computing platform is supported by the extension, generating an installable package for the application program; and if the target computing platform is not supported by the extension, providing a notification corresponding thereto.