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The Mixed Reality Extension SDK enables developers to build 3D world extensions for AltspaceVR, using Node.JS.
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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, 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.


  • 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 AltspaceVR host app's native content.
  • Quick to start: Clone this repo (or the samples repo), deploy an MRE locally, and see the extension running in the host app 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 or load keyframe animations.
  • Assign rigid body properties, physics forces, collision geometry, and have objects collide naturally with the host app world, or with other extensions.
  • Apply input behaviors and register event handlers on the behaviors.

Current State

Developer Preview


This is an early developer preview, so there will be rough edges and bugs.

We would love for you to experiment with this SDK by deploying locally or using ngrok to invite friends to join while you test. However, during the developer preview phase we don't recommending deploying your app to cloud services. Until we reach the Feature Complete milestone, and in parallel with the AltspaceVR integration, there will be occasional breaking changes, which will require server redeploys.

The SDK also 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.

The SDK should deploy 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

  • User masking - hide actors and disable behaviors for a subset of users
  • streaming and single-shot sound playback
  • Additional input behaviors, such as grab&throw
  • 2D UI layout system with standard UI controls
  • Particle system
  • Realtime lighting
  • Rigid body constraints
  • Input latency improvements
  • Protocol optimization

Major known Issues

  • Rigid body physics state syncronization is somewhat jittery
  • Users can't reliably directly interact with rigid body objects.
  • Text alignment is backwards.
  • 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 prioritize quality over dates, but a reasonable guess would be

  • December 2018 - inital public preview released.
  • January/Febraury 2019 - regular beta updates. Occasional breaking changes.
  • March 2019 - 1.0 release. At this point we expect the protocol to be stable enough to not require any server redeploys.
  • Ongoing - We expect to continuously roll out new features, well beyond the 1.0 release.

How to Build and Deploy the SDK 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 Space 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://
  • 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 AltspaceVR SDK Slack channel.

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.

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