Bevy Project Foundations Agent Skill

Bevy project foundations

When to use

Load this when a new Bevy app needs a crate, a pin, and a place for feature code before gameplay systems exist. Folder names below are layout habits. They are not engine types.

ECS shape

• Crate: cargo new plus a bevy dependency pinned at 0.20.
• Features: default pulls windowing, input, and rendering. Turn default-features off only when the binary should stay headless.
• App shell: App::new(), plugins, then systems and resources on that App.
• Code layout: Split by feature in ordinary Rust modules. The engine does not require a directory tree.

Pin the crate and keep dev builds usable. Bevy's default debug profile compiles dependencies unoptimized, so the dev profile lifts dependency opt-level while the game crate stays easy to step through.

[package]
name = "game"
version = "0.1.0"
edition = "2021"

[dependencies]
bevy = "0.20"

[profile.dev]
opt-level = 1

[profile.dev.package."*"]
opt-level = 3

A headless tool crate drops the default feature set and adds only what that binary calls.

[dependencies]
bevy = { version = "0.20", default-features = false }

The windowed game plugs DefaultPlugins and a feature plugin. GameConfig is app-wide state inserted before the first schedule runs.

#[derive(Resource)]
struct GameConfig {
    title: String,
}

fn boot(config: Res<GameConfig>) {
    let _title = config.title.as_str();
}

pub struct GamePlugin;

impl Plugin for GamePlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(GameConfig {
            title: "Game".to_string(),
        })
        .add_systems(Startup, boot);
    }
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_plugins(GamePlugin)
        .run();
}

A second module owns one feature. The plugin is the boundary. Systems stay in the module that declares their components.

#[derive(Component)]
struct Player;

fn spawn_player(mut commands: Commands) {
    commands.spawn(Player);
}

pub struct PlayerPlugin;

impl Plugin for PlayerPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(Startup, spawn_player);
    }
}

Layout choices

| Need | Bevy idiom |

| --- | --- |

| New crate | cargo new plus the bevy dependency |

| Window, input, rendering | Cargo features and profiles on the bevy crate |

| Feature split | Rust modules you choose |

| Project file | Cargo.toml plus Rust modules |

Scope

• Pin bevy = "0.20" in Cargo.toml. DefaultPlugins brings the window, input, and audio backends that crate feature enables.
• Workspace members and naming linters are ordinary Cargo. This skill does not invent a folder enforcer.

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 free-floating struct outside the World |

| 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 scene-tree project types or autoload singletons in this crate. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.
• No invented Cargo features beyond the profiles this skill names. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.
• No third-party crates. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.

Official Documentation

• Getting started - cargo new / cargo add bevy.
• Apps - App::new(), World, and Schedule.
• Examples - Example tree on the latest branch.

Related Skills

Prerequisites

• bevy-master - Library pin and which Bevy skill to load next.

Complements

• bevy-ecs-composition - Entities, components, and systems after the crate exists.

Downstream / consumers

• bevy-app-resources - App-wide resources once the App exists.

Master

• bevy-master - Bevy router.