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.