Build Mode is the editor for Mirror Engine, similar to when you open another game engine like Unreal, Unity, or Godot/Redot.
Play Mode is what the player plays, similar to when you open a game on Steam. When you publish your game, it'll run in Play Mode with the first scene you've selected.

Simply press B to instantaneously toggle between Build Mode and Play Mode. You'll see text on the bottom-left corner of the screen showing what mode you're in.
Why not P like Unity? So you can toggle between Build/Play with one hand on the keyboard and another on your mouse. It's a bit cumbersome to constantly move your hand back and forth, even though we know some prefer P. Let us know what you think!
Everything you build is in real-time with mirrored multiplayer. Similar to Minecraft, when an item is in-world for you, it's instantly available for everyone else.
To invite a contributor, click the bottom-left Mirror icon for settings and click "Invite". Send the link to anyone you wish.
W A S D: Move camera forward, left, backward, and right.Q: Move camera up.E: Move camera down.Scroll Wheel: Move camera forward and backward.Left-Click: Select an entity.Right-Click: Open radial menu, showing options to create an entity, delete an entity, and more.Right-Click + Hold: Rotate camera as you move your mouse.1: Use position gizmo2: Use rotate gizmo3: Use scale gizmoLeft-Click: Select an entity.Right-Click: Open radial menu, showing options to create an entity, delete an entity, and more.Games in Mirror Engine are live, including Build Mode.
Two players can build the same scene at the same time, or they can be in different scenes in the same game.
If you're in the same scene as another player, you can see their changes in real-time.
In Build Mode, click any entity and you'll see a gizmo attach it. These can be used to change the position, rotation, and scale of the entity.
Click the bottom-right "Transform" button to open the transform menu. When you change the entity's position, rotation, or scale via a gizmo, you'll see these values update in real-time.
Don't confuse "transform" with "translation": Translation only refers to position. We'll use "position" for simplicity.
For example, setting the scale to 0.5, 0.5, 0.5 (XYZ) will make the entity half the size of the original, and setting it to 2, 2, 2 (XYZ) will make it twice the size.
"Size" is a planned feature on the roadmap, allowing you to specify the exact size of an entity and know exactly how that will translate to the 3D world.