Bevy Ability System Agent Skill

Bevy ability system

When to use

Load this when an actor grants, cools down, or spends an ability. Combat resolution stays on bevy-combat-system.

ECS shape

• Components: ability id, charges, and cooldown remaining on the caster.
• Resources: run-wide tables live in a resource (bevy-data-resources).
• Systems: mutate those components on Update or the fixed timestep. Use .chain() when damage must land before death.
• Messages: one-shot facts (hit, pickup, quest advance) go through bevy-events-messaging. This file does not add a new event type name.

Ability choices

| Need | Bevy idiom |

| --- | --- |

| Ability definition | A Resource catalog plus a component row on the caster. |

| Cooldown | remaining stepped with Time<Fixed>::delta_secs in FixedUpdate. |

| Cast | MessageWriter<CastAccepted> after the cooldown and charge gates pass. |

Scope

• Cooldowns, charges, and catalogs are components and resources. Combat damage stays on bevy-combat-system.
• Stacking status effects is a component list you tick on FixedUpdate.

Examples

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

[dependencies]
bevy = "0.20"

Cooldown and charges share the caster. Tick, then accept the cast, on FixedUpdate.

#[derive(Component)]
struct Cooldown {
    remaining: f32,
    duration: f32,
}

#[derive(Component)]
struct Charges {
    current: u32,
    max: u32,
}

#[derive(Message)]
struct CastAccepted {
    caster: Entity,
}

fn tick_cooldowns(time: Res<Time<Fixed>>, mut query: Query<&mut Cooldown>) {
    let dt = time.delta_secs();
    for mut cooldown in &mut query {
        cooldown.remaining = (cooldown.remaining - dt).max(0.0);
    }
}

fn try_cast(
    mut casters: Query<(Entity, &mut Cooldown, &mut Charges)>,
    mut casts: MessageWriter<CastAccepted>,
) {
    for (entity, mut cooldown, mut charges) in &mut casters {
        if cooldown.remaining > 0.0 || charges.current == 0 {
            continue;
        }
        charges.current -= 1;
        cooldown.remaining = cooldown.duration;
        casts.write(CastAccepted { caster: entity });
    }
}

fn recharge(time: Res<Time<Fixed>>, mut casters: Query<(&Cooldown, &mut Charges)>) {
    let dt = time.delta_secs();
    for (cooldown, mut charges) in &mut casters {
        if cooldown.remaining <= 0.0 && charges.current < charges.max && dt > 0.0 {
            charges.current += 1;
        }
    }
}

pub struct AbilityPlugin;

impl Plugin for AbilityPlugin {
    fn build(&self, app: &mut App) {
        app.add_message::<CastAccepted>()
            .add_systems(FixedUpdate, (tick_cooldowns, try_cast, recharge).chain());
    }
}

The catalog is one resource. Live cooldown stays on the entity.

#[derive(Clone, Copy)]
struct AbilityDef {
    id: u32,
    cooldown: f32,
    cost: u32,
}

#[derive(Resource)]
struct AbilityCatalog {
    rows: Vec<AbilityDef>,
}

fn lookup(catalog: &AbilityCatalog, id: u32) -> Option<&AbilityDef> {
    catalog.rows.iter().find(|row| row.id == id)
}

#[derive(Component)]
struct Cooldown {
    remaining: f32,
    duration: f32,
}

#[derive(Component)]
struct Charges {
    current: u32,
    max: u32,
}

fn spawn_caster(mut commands: Commands, catalog: Res<AbilityCatalog>) {
    let def = lookup(&catalog, 1);
    let duration = def.map(|row| row.cooldown).unwrap_or(0.5);
    commands.spawn((
        Cooldown {
            remaining: 0.0,
            duration,
        },
        Charges { current: 2, max: 2 },
    ));
}

A shared lock sits in a resource so one system owns it.

#[derive(Message)]
struct CastAccepted {
    caster: Entity,
}

#[derive(Resource)]
struct GlobalCooldown {
    remaining: f32,
}

fn tick_gcd(time: Res<Time<Fixed>>, mut gcd: ResMut<GlobalCooldown>) {
    gcd.remaining = (gcd.remaining - time.delta_secs()).max(0.0);
}

fn arm_gcd(mut gcd: ResMut<GlobalCooldown>, mut casts: MessageReader<CastAccepted>) {
    for _cast in casts.read() {
        gcd.remaining = gcd.remaining.max(0.4);
    }
}

Register CastAccepted with add_message in the same plugin that adds tick_gcd and arm_gcd.

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 value stored off 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 cooldown stored in a process-global static. WHY: the caster's Cooldown component and the catalog Resource are the owners.
• 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

The library pin on bevy-master stays 0.20.0. Permalinks stay unversioned.

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-combat-system - Hit resolution.
• bevy-input-handling - Cast input.
• bevy-data-resources - Tables and definitions.
• bevy-events-messaging - One-shot facts.

Complements

• bevy-state-machines - Mode flags.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.