bevy-ecs

What the source does

• FilesystemNone observedNo reads or writes in the source
• NetworkNone observedNo outbound calls in the source
• ShellNone observedNo commands run in the source
• DependenciesPinnedNothing floats
• SecretsNone foundNo keys in source

Five audit layers

Gates applied: no behavioural pass.

Audit 0.4.1 · source sha 4543b3777841 · 21 Oct 2026Report an issue or request a re-scan

• Publishergamedev-skills GitHub organizationOwner of the repository this skill is read from
• Source foldergamedev-skills/awesome-gamedev-agent-skills/skills/other-engines/bevy-ecsThe exact folder we audited
• LicenceApache-2.0As the repository states it
• Commit pinned for this verdict4543b3777841A new audit runs when the source changes
• Repository statusNot archivedLast push 12 days ago
• Sandboxed runNot runSkills are read, not executed; layer L4 is skipped

Its description: Structure a Bevy app around its Entity Component System: build the App with plugins, define Component/Resource types, write systems with Query/Res/Commands, filter and order systems, and use the Time resource for…

---
name: bevy-ecs
description: >
  Structure a Bevy app around its Entity Component System: build the App with
  plugins, define Component/Resource types, write systems with Query/Res/Commands,
  filter and order systems, and use the Time resource for frame-rate-independent
  motion. Use when building or debugging a Bevy game in Rust — when the user
  mentions Bevy, ECS, App::new, add_systems, Query, Commands, components/systems,
  or a Cargo.toml depending on bevy.
---

# Bevy ECS

Structure a Bevy game in Rust around the Entity Component System: the `App` and
plugins, components and resources, systems with queries, scheduling, and
frame-rate-independent updates. New examples target **Bevy 0.20**. If the project
already pins another release, keep that release and use its matching migration guide.

## When to use

- Use when wiring a Bevy `App`, defining `Component`/`Resource` types, writing
  systems that query entities, ordering/filtering systems, or fixing
  borrow-conflict panics and frame-dependent movement.
- Use when `Cargo.toml` depends on `bevy` and code calls `App::new()`,
  `add_systems`, `Query`, or `Commands`.

**When *not* to use:** this is the ECS core. Deep rendering, custom shaders/
pipelines, UI layout, and audio are separate concerns. For engine-agnostic AI or
procedural algorithms, pair with `game-ai` / `procedural-gen`.

## Core workflow

1. **Detect and pin the version.** Read `Cargo.toml` and `Cargo.lock` first. For a
   new project use `bevy = "0.20"`; never silently migrate an existing project
   across a Bevy minor release. Treat the matching docs and migration guides as truth.
2. **Build the `App`.** `App::new().add_plugins(DefaultPlugins)` gives windowing,
   input, rendering, time, etc. Register systems into schedules: `Startup` (once)
   and `Update` (every frame).
3. **Model data as components, globals as resources.** `#[derive(Component)]` for
   per-entity data; `#[derive(Resource)]` for one-of-a-kind data (score, settings,
   the `Time` clock). Since 0.19 `Resource` extends `Component` and each resource is
   stored on its own entity, so do not derive both.
4. **Write systems as plain functions.** Parameters declare data access: `Query<...>`
   for entities, `Res<T>`/`ResMut<T>` for resources, `Commands` for deferred
   spawn/despawn. Systems run in parallel when their accesses don't conflict.
5. **Drive motion by `time.delta_secs()`** so speed is frame-rate independent.
6. **Order only what must be ordered** with `.chain()` or explicit constraints;
   gate systems with `run_if`. Group related setup into `Plugin`s. Build with
   `cargo run` and read the panics — Bevy reports conflicting queries at startup.

## Patterns

### 1. Cargo.toml + minimal App

```toml
# Cargo.toml — pin the version; the API differs across minor releases.
[dependencies]
bevy = "0.20"
```

```rust
// main.rs
use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)            // window, input, render, time, ...
        .add_systems(Startup, setup)            // runs once at startup
        .add_systems(Update, move_players)      // runs every frame
        .run();
}
```

### 2. Components, resources, and spawning

```rust
#[derive(Component)]
struct Player;

#[derive(Component)]
struct Velocity(Vec2);

#[derive(Resource)]
struct Score(u32);

fn setup(mut commands: Commands) {
    commands.insert_resource(Score(0));

    // Camera2d is a component with required components (bundles removed in 0.16);
    // spawning it pulls in Transform, Camera, etc. automatically.
    commands.spawn(Camera2d);

    // Spawn an entity as a tuple of components.
    commands.spawn((
        Player,
        Velocity(Vec2::new(150.0, 0.0)),
        Transform::from_xyz(0.0, 0.0, 0.0),
    ));
}
```

### 3. A system with a query + the Time resource

```rust
// Iterate every entity that has BOTH Velocity and Transform; mutate Transform.
fn move_players(time: Res<Time>, mut query: Query<(&Velocity, &mut Transform)>) {
    for (velocity, mut transform) in &mut query {
        // delta_secs() is f32 seconds (renamed from delta_seconds() in 0.16).
        transform.translation += velocity.0.extend(0.0) * time.delta_secs();
    }
}
```

### 4. Query filters (With / Without / Changed)

```rust
// Only entities tagged Player (the Player component itself isn't read).
fn aim_player(mut q: Query<&mut Transform, With<Player>>) { /* ... */ }

// Disjoint two mutable Transform queries so they don't conflict at runtime.
fn separate(
    mut players: Query<&mut Transform, With<Player>>,
    mut enemies: Query<&mut Transform, Without<Player>>,
) { /* ... */ }

// React only when Health changed since last run (change detection).
fn on_health_change(q: Query<&Health, Changed<Health>>) {
    for health in &q { /* update the HUD, etc. */ }
}
```

### 5. Resources: read and write

```rust
fn add_points(mut score: ResMut<Score>) {
    score.0 += 10;                 // ResMut = write access
}

fn show_score(score: Res<Score>) {
    info!("score: {}", score.0);   // Res = read access
}
```

### 6. Ordering, run conditions, and plugins

```rust
fn main() {
    App::new()
        .add_plugins((DefaultPlugins, GameplayPlugin))
        // .chain() forces order: damage resolves before death is checked.
        .add_systems(Update, (apply_damage, check_deaths).chain())
        // run_if gates a system on a condition each frame.
        .add_systems(Update, spawn_wave.run_if(wave_timer_finished))
        .run();
}

struct GameplayPlugin;
impl Plugin for GameplayPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(Score(0))
           .add_systems(Startup, setup)
           .add_systems(Update, (move_players, add_points));
    }
}
```

## Pitfalls

- **`delta_seconds()` not found** → it was renamed to `time.delta_secs()` (and
  `elapsed_secs()`) in 0.16. Using the old name fails to compile.
- **Movement speed scales with frame rate** → multiply per-frame changes by
  `time.delta_secs()`.

Excerpt, first 6,000 characters of SKILL.md, verbatim from the audited commit. It is text from someone else’s repository, quoted so you can judge it before your agent reads it.

git clone https://github.com/gamedev-skills/awesome-gamedev-agent-skills
git -C awesome-gamedev-agent-skills checkout 4543b3777841

Works in

1. 2026-10-21
   ✓SAFE B 89 / 100

   first audit · source 4543b3777841

Questions about bevy-ecs

• github.com/gamedev-skills/awesome-gamedev-agent-skills/tree/main — the audited source at commit 4543b3777841 and every finding aboveObserved · 21 Oct 2026

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