Telegram released its first open-source client in 2013. Telegram for iOS launched on August 14, 2013, and the alpha version of Telegram for Android followed on October 20; by October, Telegram stated that its app code had become open source.
That release was part of Telegram’s early effort to make its technical foundations inspectable. Alongside the client code, the company documented the MTProto protocol and Telegram API, allowing independent developers to study the implementation and build alternative clients.
A common mix-up is treating Telegram’s first launch date and its first open-source release as separate years. Both belong to 2013. Another distinction matters too: Telegram’s client applications are open source, while Telegram has not released the server-side software as open source. The desktop client arrived soon afterward, with an unofficial PC application announced in early 2014 before becoming an official desktop product.