Microsoft to Finally Improve and then Open Source the Windows App SDK

The Windows App SDK is the most recent evolution of the native Windows app development platform. But it’s been largely ignored by Microsoft for years with sporadic updates, missed deadlines, and no sense of leadership or direction whatsoever.

Today, finally, Microsoft addressed the many complaints it’s received from frustrated developers. And it promises to do better, while working toward a future milestone in which this framework becomes open sourced.

“We hear you loud and clear and are taking it seriously,” Microsoft software engineer Beth Pan writes in a new post to the WinUI 3 repository on GitHub. “We’re entering a new phase of focused improvements over the next 6 months, including product work and foundational changes to support a more open and collaborative future.”

Pan also says that Microsoft is “actively working” toward open sourcing the Windows App SDK, though that work will require a lot of time. “This isn’t a flip-the-switch moment,” she says. “It’s a deliberate process.”

So what’s going on here?

The Windows App SDK, previously codenamed Project Reunion, is the result of a years-long effort to unified Windows desktop app development using a set of technologies that date back to Windows 8 and the Windows Runtime. Previously versions of this technology, WinRT and the Universal Windows Platform (UWP), were mobile app platforms with versions tied to specific Windows versions. The Windows App SDK is decoupled from specific Windows versions and it now targets traditional desktop apps, not mobile apps. It also uses the latest Microsoft UI framework, called WinUI 3.

Microsoft released the version Windows App SDK version in late 2021, and it shipped a series of minor upgrades in the intervening years, albeit on an increasingly sporadic and unpredictable schedule. But it’s been greatly neglected in the past year or so, with Microsoft ignoring developer questions about bug fixes and functional updates.

Aside from the obvious, this is a problem because Microsoft releases new capabilities for developers targeting Windows apps almost exclusively through the Windows App SDK. At Build 2024, for example, Microsoft announced that developers would be able to utilize the on-device AI capabilities in their own apps using the Windows Copilot Runtime. But the first pre-release version of that set of APIs didn’t ship until the following February, 9 months later. And it was tied to an early pre-release Windows App SDK that developers couldn’t use in production code. At Build 2025, this past May, it replaced the Windows Copilot Runtime with Windows AI Foundry. But that, too, requires a pre-release experimental version of the Windows App SDK.

According to Pan, Microsoft’s new plan for the Windows App SDK is to release version 1.8 in August and then begin a series of more minor updates that can be released more quickly. It will make these releases available on GitHub, mirroring Microsoft’s internal repositories. And it will better document how to configure the SDK and its dependencies.

The team–if there is a team, it’s unclear–has set up a project board on GitHub so it can collaborate with outside developers on prioritizations.

“Our current focus is on foundational work that unlocks value for contributors and increase transparency,” Pan writes. “We are aligning this work with Microsoft’s broader business priorities to ensure long-term support and impact.”