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Mixed Reality Extension SDK

The Mixed Reality Extension SDK lets developers and community members extend the AltspaceVR host app's worlds with multi-user games and other dynamic experiences.


  • Install Git
  • Install Node.js 8.12 or newer, which includes NPM 6.4.1 or newer, from

Get Started

The easiest way to start with the MRE SDK is to head over to the mixed-reality-extension-sdk-samples repo, and build a sample.

If you want to build the actual SDK itself, or want to build the functional test showcase MRE, jump to Build and Run section of this document

To see the APIs, jump to SDK documentation

If you have made a game or application in Unity3D, and you want it to support MREs, or you want to debug into the client runtime code itself, go to the Mixed Reality Extension Unity repository.

Other resources are on AltspaceVR's developer page


  • Written in TypeScript, and built on top of Node.js.
  • Utilizes a traditional game engine-style client-server model. All logic runs on the server, but the client performs the most CPU intensive and latency-sensitive tasks: animation, collision, rigid body physics simulation, rendering, and input handling.
  • Hides the complexity of multi-user synchronization and prediction, so developers focus on adding content, not debugging networking code.
  • Designed to be secure for users, tolerate high latency, minimize server activity, and seamlessly blend with the host app's native content.
  • Quick to start: Clone the samples repo, deploy an MRE locally, and see the extension running in AltspaceVR or other host apps within minutes.
  • We welcome contributions. Please see if you are interested in helping out.


The SDK enables you to create extensions that can

  • Modify the scene graph by loading glTF assets and scene files, instantiating primitives or the host app's built-in assets, or programmatically build meshes.
  • Create actors with 3d meshes (static or skinned), realtime lights and text objects
  • Create, load, and trigger keyframe animations, sounds, music, textures
  • Assign rigid body properties, physics forces, collision geometry, and have objects collide naturally with the host app world, or with other extensions.
  • Filter actors and behaviors to groups of users, or even have single-user actors
  • Apply click detection behaviors and register event handlers on the behaviors.
  • Attach actors to host app's avatars
  • Make actors grabbable and clickable when held

Current State

Developer Preview


The MRE SDK has been in developer preview in AltspaceVR since December 2018, and while the API surface is fairly stable, internals are regularly refactored. We strive to maintain backwards compatibility when possible, but until we ship the 1.0 release, there will be occasional breaking changes, which will require MRE redeploys.

The SDK does not have a feature rich set of APIs yet. We have focused on the networking and synchronization, rather than adding more APIs. Expect this to improve over time. Our

The SDK deploys anywhere Node.js works.


We want to deliver a feature-rich set of APIs, enabling creation of high quality, rich 3d experiences. There are many features we want to add, including

  • Rigid body constraints
  • Input latency improvements
  • Protocol optimization
  • 2D UI layout system with common UI controls

However, our highest priority is reliability, predictability, and ease of use, and as a consequence we spend much more time making sure we have a solid, flexible infrastructureare than adding shiny new features.

Major known Issues

  • Rigid body physics state syncronization is jittery.
  • Users can't reliably directly collide with rigid body objects, except by grabbing.
  • A number of client-side errors don't get send to the node log, which makes debugging hard. This includes glTF loading errors and using the wrong name when playing animations.


We are working towards 1.0 release. Please look at our project page for remaining work.

How to Build and Deploy the SDK and functional tests

From command prompt:

  • git clone
  • cd mixed-reality-extension-sdk
  • npm install -g lerna
  • npm install This will install all dependent packages. (and will do very little if there are no changes)
  • npm run build This should not report any errors.
  • npm start This should print "INF: Multi-peer Adapter listening on..."
  • See also: Using Visual Studio Code instead of command line

Testing an MRE In AltspaceVR

  • In AltspaceVR, go to your personal home
  • Make sure you are signed in properly, not a guest
  • Activate the World Editor
  • Click Basics group
  • Click on SDKApp
  • For the URL field, please enter the URL ws://localhost:3901
  • Click Confirm
  • After the app has been placed, you will see the MRE Anchor (the white box with red/green/blue spikes on it), and you can use it to move the MRE around, and you can see the status of the MRE connection by looking at the icon on the anchor. To hide the anchor, uncheck "Edit Mode".

You should now see a functional test load up inside AltspaceVR.

Pre-deployed MREs

We have deployed the hello world and functional test MREs to servers in the cloud. The URLs are

  • ws://
  • ws://
  • ws://
  • ws://

Using Visual Studio Code

We recommend Visual Studio Code, a lightweight code editor, which is easy to use and offers full debugging capabilities for Node.js servers.

  • Install from here: Visual Studio Code
  • You may want to add the TSLint extension to get style tips - use View->Extensions(ctrl+shift+X), search for TSLint, click Install.
  • To build: use Tasks->Run Build Task... (ctrl+shift+B), and you can select npm: Build for some or all packages.
  • To choose which MRE to launch: go to debugger sidebar: (ctrl+shift+D), and from the dropdown choose desired MRE.
  • To launch the MRE server: use Debug->Start Debugging (F5). To stop the server: user Debug->Stop Debugging (shift+F5)

Hosting and Multi-User Testing

To learn about additional deployment options and multi-user testing in AltspaceVR, see

Getting In Touch

To report issues and feature requests: Github issues page.

To chat with the team and other users: join the MRE SDK discord community.

Or attend the biweekly AltspaceVR developer meetups.

Reporting Security Issues

Security issues and bugs should be reported privately, via email, to the Microsoft Security Response Center (MSRC) at You should receive a response within 24 hours. If for some reason you do not, please follow up via email to ensure we received your original message. Further information, including the MSRC PGP key, can be found in the Security TechCenter.


Code licensed under the MIT License.

Code of Conduct

This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact with any additional questions or comments.