As Microsoft moves closer to its unveiling of Windows Phone 8.1 in April, the company has started to share copies of its software development kit (SDK) more broadly with developers. Sources familiar with Microsoft’s plans have revealed to The Verge that Windows Phone developers have started receiving invites to access the 8.1 SDK ahead of the Build developer conference in April. The early access allows app developers to create Windows Phone applications based on new API changes and features in the upcoming update. In the lead up to the release of Windows Phone 8, Microsoft was criticized for keeping its SDK secret from developers, despite it leaking months in advance.