Apple's original plan with macOS was to require all developers to rewrite their software into the Cocoa APIs. This caused much outcry among existing Mac developers, who threatened to abandon the platform rather than invest in a costly rewrite, and the idea was shelved. To permit a smooth transition from Mac OS 9 to Mac OS X, the Carbon Application Programming Interface (API) was created. Applications written with Carbon were initially able to run natively on both classic Mac OS and Mac OS X, although this ability was later dropped as Mac OS X developed. Carbon was not included in the first product sold as Mac OS X: the little-used original release of Mac OS X Server 1.0, which also did not include the Aqua interface. Apple limited further development of Carbon from the release of Leopard onwards and announced that Carbon applications would not run at 64-bit. A number of macOS applications continued to use Carbon for some time afterwards, especially ones with heritage dating back to the classic Mac OS and for which updates would be difficult, uneconomic or not necessary. This included Microsoft Office up to Office 2016, and Photoshop up to CS5. Early versions of macOS could also run some classic Mac OS applications through the Classic Environment with performance limitations; this feature was removed from 10.5 onwards and all Macs using Intel processors.