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.

Developer preview release set for Build

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. Although Microsoft has not yet publicly acknowledged Windows Phone 8.1, apart from during a Skype meeting with The Verge, we understand the company has already sharing SDKs and builds of Windows Phone 8.1 with select partners and developers. Microsoft is planning to detail Windows Phone 8.1 fully at its Build developer conference, and we're told the company will release a developer preview version of the OS in early April. Thanks to a recent developer preview program, this will allow anyone with a Windows Phone to upgrade to 8.1 immediately following the preview program release.

While Microsoft is not planning to detail Windows Phone 8.1 fully until Build, the company is planning to share lots of details behind closed doors over the coming weeks. Microsoft is holding a confidential “meet the new Windows Phone” event for close partners in London on February 28th. The meeting is specifically targeted at business customers with roadmap updates and a technical briefing on Windows Phone 8.1. Microsoft previously promised an enterprise “feature pack” update for Windows Phone in the first half of 2014, and functions like VPN will be built into Windows Phone 8.1.

More Windows Phone details later this month

The software maker is also holding a press event at Mobile World Congress ahead of its confidential closed-door meeting. Microsoft press event will take place on February 23rd, just a day before Nokia unveils its Android phone. Sources familiar with Microsoft’s plans have revealed to The Verge that the event won’t include a full demonstration of Windows Phone 8.1 features, but the company will discuss how Windows is evolving and its plans to grow Windows Phone market share in 2014.

Screenshots of a Windows Phone 8.1 Notification Center leaked over the weekend, and Microsoft is also planning to add additional features alongside some major API changes. A voice-based “Cortana” personal assistant will ship with Windows Phone 8.1, alongside improvements to volume controls, a feature to mirror Windows Phone screens to TVs or monitors, and access to quick settings from the new Action Center. Microsoft is expected to release Windows Phone 8.1 to existing devices in the weeks following its Build developer preview.