Windows 10 build 21277

10.0.21277.1000.rs_prerelease.201207-1443
	Build of Cobalt
OS family	Windows 11 (NT 10.0)
Version number	10.0
Build number	21277
Build revision	1000
Architecture	x86, x64, ARM64
Build lab	rs_prerelease
Compiled on	2020-12-07
Expiration date
Timebomb	2021-10-31 (+328 days)
SKUs
	Home (China, N, Single Language); Pro (China Only, N, Single Language); Pro Education (N); Pro (N) for Workstations; Education (N); Enterprise (N, (N) Evaluation, G (N), LTSC); Enterprise multi-session; IoT Enterprise; Team
Product key
	Use a Windows 10 Retail serial
About dialog

Windows 10 build 21277 is the first Insider Preview build of Cobalt, the 21H2 development semester of Windows as a service. It was released to the Windows Insiders in the Dev Channel on 10 December 2020 along with Iron build 20277. The Enterprise LTSC variant was publicly shared on 21 November 2024.

At the time of this build's release, the Dev Channel went under a temporary splitting of released builds. While the channel still received mandatory updates from the Iron semester, builds from this version would be released at the same time as optional updates, giving Insiders the ability to choose to keep receiving Iron builds or move to the new semester.

New features and changes[edit | edit source]

Shell[edit | edit source]

Many features such as the redesigned touch keyboard, emoji picker, voice typing, and theme-aware splash screens that were previously removed in Iron build 20246 have been reintroduced in this build and later made available to a subset of users. In addition, the Meet Now integration on the Windows taskbar introduced in Iron build 20221 has been added to Enterprise SKUs and Azure Active Directory (AAD) user accounts by default.

User interface[edit | edit source]

The animations for opening and closing windows have been changed to a simpler effect. Additionally, Aero Shake is also disabled by default.

Applications[edit | edit source]

Snip & Sketch can be used to create a screen snip and paste it directly into a File Explorer folder to save the screenshot there. This change was initially added to the Beta Channel in Feature Experience Pack build 120.2212.1070.0.
The legacy Snipping Tool application is now an optional feature and can be manually uninstalled via the Settings application.
Mixed Reality Portal's setup screens now features a video showing a headset and some experiences.
The scaling logic of the graphs in Task Manager’s Performance tab has been adjusted.

Bugs and quirks[edit | edit source]

The update process to this build has a chance of hanging for extended periods of time.
Some users might see a popup that says “Your PC ran into a problem and will restart in 1 min” after logging in to Windows with either an Azure Active Directory or Active Directory account. If encountered, save any data immediately and let the PC restart.
File Explorer might hang or crash in this build.
Live preview doesn't work for pinned site tabs on some machines, a gray window is seen instead of the intended content.
The modern Disk Management page located at Manage Disks and Volumes is completely blank, and as a result it will not display any drives present on the system. To workaround this issue, use the legacy Disk Management tool (diskmgmt.msc) to manage disk drives.

Gallery[edit | edit source]

Setup[edit | edit source]

Boot screen
Setup initial page
EULA
Copying files
Setting up devices
OOBE

General[edit | edit source]

The emoji picker showing some of the new and gender-neutralized emojis
Notepad demo showing some of the new emojis
Optional Features Settings showing Snipping Tool
New window animations
Windows Update Settings showing Iron build 20277 downloading and this build offered at the same time