Bevy 3d World Building Agent Skill

Bevy 3D world building

When to use

Load this when placing meshes, lights, and colliders into a level. Scene spawn mechanics stay on bevy-asset-scenes. Physics bodies stay on bevy-physics-3d.

ECS shape

• Spawn: entities with transforms and render components. Primitives (Cuboid, Sphere, Cylinder) convert into Mesh and ride on Mesh3d plus MeshMaterial3d.
• Scenes: bevy_scene and bevy_gltf are features. Spawn details are on bevy-asset-scenes (AssetServer, scene spawn).
• Lights: a DirectionalLight is an entity with a Transform. Fog is DistanceFog on the camera entity. Full lighting setups stay on bevy-3d-lighting.
• Systems: which cells are present is a Resource. Spawning a cell is Commands in Update.

fn spawn_level(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    let stone = materials.add(StandardMaterial {
        base_color: Color::srgb(0.45, 0.42, 0.38),
        ..default()
    });
    let floor = materials.add(StandardMaterial {
        base_color: Color::srgb(0.28, 0.30, 0.26),
        ..default()
    });
    commands.spawn((
        Mesh3d(meshes.add(Cuboid::new(24.0, 0.2, 24.0))),
        MeshMaterial3d(floor),
        Transform::from_xyz(0.0, -0.1, 0.0),
    ));
    commands.spawn((
        Mesh3d(meshes.add(Cylinder::new(0.4, 3.0))),
        MeshMaterial3d(stone.clone()),
        Transform::from_xyz(-4.0, 1.5, -2.0),
    ));
    commands.spawn((
        Mesh3d(meshes.add(Sphere::new(0.6))),
        MeshMaterial3d(stone),
        Transform::from_xyz(2.0, 0.6, 1.0),
    ));
    commands.spawn((
        DirectionalLight {
            illuminance: 12_000.0,
            shadow_maps_enabled: true,
            ..default()
        },
        Transform::from_xyz(6.0, 10.0, 4.0).looking_at(Vec3::ZERO, Vec3::Y),
    ));
    commands.spawn((
        Camera3d::default(),
        DistanceFog {
            color: Color::srgb(0.55, 0.62, 0.70),
            falloff: FogFalloff::Linear {
                start: 12.0,
                end: 40.0,
            },
            ..default()
        },
        Transform::from_xyz(0.0, 8.0, 14.0).looking_at(Vec3::ZERO, Vec3::Y),
    ));
}

Kitbash props are ordinary entities. A helper returns the entity id so later systems can parent markers to it.

fn spawn_prop(
    commands: &mut Commands,
    meshes: &mut Assets<Mesh>,
    materials: &mut Assets<StandardMaterial>,
    at: Vec3,
    size: Vec3,
    color: Color,
) -> Entity {
    commands
        .spawn((
            Mesh3d(meshes.add(Cuboid::from_size(size))),
            MeshMaterial3d(materials.add(StandardMaterial {
                base_color: color,
                ..default()
            })),
            Transform::from_translation(at),
        ))
        .id()
}

Cells already in the world live on one resource. The streamer inserts a marker component and a mesh only for cells that are not recorded yet.

#[derive(Resource, Default)]
struct LoadedCells {
    coords: Vec<(i32, i32)>,
}

#[derive(Component)]
struct CellMarker {
    x: i32,
    z: i32,
}

fn stream_cells(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
    mut loaded: ResMut<LoadedCells>,
    focus: Query<&Transform, With<Camera3d>>,
) {
    let Ok(focus) = focus.single() else {
        return;
    };
    let cx = (focus.translation.x / 8.0).floor() as i32;
    let cz = (focus.translation.z / 8.0).floor() as i32;
    for dz in -1..=1 {
        for dx in -1..=1 {
            let coord = (cx + dx, cz + dz);
            if loaded.coords.contains(&coord) {
                continue;
            }
            loaded.coords.push(coord);
            commands.spawn((
                CellMarker {
                    x: coord.0,
                    z: coord.1,
                },
                Mesh3d(meshes.add(Cuboid::new(8.0, 0.1, 8.0))),
                MeshMaterial3d(materials.add(StandardMaterial {
                    base_color: Color::srgb(0.3, 0.34, 0.28),
                    ..default()
                })),
                Transform::from_xyz(coord.0 as f32 * 8.0, 0.0, coord.1 as f32 * 8.0),
            ));
        }
    }
}

[dependencies]
bevy = "0.20"

Level choices

| Need | Bevy approach |

| --- | --- |

| Packed level | A scene asset spawned through bevy-asset-scenes |

| Kitbash props | Entities with Mesh3d and MeshMaterial3d |

| Atmosphere | DirectionalLight, DistanceFog, and the camera entity |

Scope

• Primitives above cover greybox blocks, columns, and spheres. Heightmaps are a Mesh you fill yourself, then the same Mesh3d spawn.
• Collider authoring belongs with bevy-physics-3d once you choose a physics solver.

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

| Piece | Owner | Who may mutate | Schedule |

| --- | --- | --- | --- |

| Data on one entity | Component | One system via Query | Update or FixedMain |

| One value for the whole app | Resource | One system | Startup to insert, then the schedule that reads it |

| Spawn | Commands | The spawning system | Startup for the initial world; later schedules for runtime spawn |

| Order that must not race | .chain() | The chained systems only | Same schedule |

| Plugin boundary | Plugin | build registers systems and resources | Before the app runs |

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

| If you need | Do this | Do not |

| --- | --- | --- |

| Per-entity data | Component + Query | A script field copied onto the entity |

| App-wide config or clock | Resource | Copy the value onto every entity |

| Stable simulation step | FixedMain and the Fixed clock | Tie simulation to raw frame Update |

| Startup spawn | Commands in Startup | Spawn from a render system |

| Cross-system notice that is not shared state | Message or observer slot in bevy-events-messaging | A new event type invented in this file |

| An API outside the documented set | Check the official docs first | A made-up crate or API |

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.
• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.
• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• No CSG, grid-map, or world-environment node types in Bevy code. WHY: greybox meshes, lights, and DistanceFog are the documented Bevy pieces.
• No invented APIs or unnamed third-party crates as the chosen solver. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

Official links stay on bevy.org. The library pin on bevy-master stays 0.20.0.

Official Documentation

• Assets - Official asset catalog. No third-party crate was selected.
• Examples - Example tree on the latest branch.
• ECS - Component, systems, Query, Startup, Update, and .chain().
• Plugins - Plugin, DefaultPlugins, MinimalPlugins, and add_plugins.

Related Skills

Prerequisites

• bevy-master - Hub and schedules.
• bevy-ecs-composition - Components and systems.
• bevy-asset-scenes - Scene spawn.
• bevy-3d-lighting - Lights.
• bevy-3d-materials - Surfaces.
• bevy-camera-systems - View.

Complements

• bevy-procedural-generation - Generated layouts.
• bevy-physics-3d - Simulated props. Solver still unpicked.
• godot-3d-world-building - Same topic on the Godot track.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.