Bevy Quest System Agent Skill

Bevy quest system

When to use

Load this when objectives advance. Dialogue that grants them stays on bevy-dialogue-system.

ECS shape

• Components: quest id and stage you define.
• 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.

Where a quest lives

| Fact | Owner |

| --- | --- |

| Definition | Id plus objectives in QuestCatalog. |

| Objective count | A number incremented from a Message. |

| Complete | A stage change on QuestProgress. Rewards go through inventory or economy skills. |

Scope

• A quest definition is a Resource table. Runtime progress is a Component on the party entity.
• One system writes stage changes. Completion grants stay as data another skill reads.

Examples

[dependencies]
bevy = "0.20"

Definitions live in one resource. The party entity stores which quests are active and how far each objective has counted.

#[derive(Clone, Copy, PartialEq, Eq, Hash)]
struct QuestId(u32);

#[derive(Clone, Copy, PartialEq, Eq)]
enum QuestStage {
    Inactive,
    Active,
    Complete,
}

#[derive(Clone, Copy)]
struct ObjectiveDef {
    id: u32,
    required: u32,
}

struct QuestDef {
    id: QuestId,
    objectives: Vec<ObjectiveDef>,
}

#[derive(Resource)]
struct QuestCatalog {
    quests: Vec<QuestDef>,
}

#[derive(Clone, Copy)]
struct ObjectiveProgress {
    id: u32,
    count: u32,
}

#[derive(Component)]
struct QuestProgress {
    quest: QuestId,
    stage: QuestStage,
    objectives: Vec<ObjectiveProgress>,
}

fn accept(catalog: Res<QuestCatalog>, mut commands: Commands, party: Query<Entity, Without<QuestProgress>>) {
    let Some(def) = catalog.quests.first() else {
        return;
    };
    for entity in &party {
        commands.entity(entity).insert(QuestProgress {
            quest: def.id,
            stage: QuestStage::Active,
            objectives: def
                .objectives
                .iter()
                .map(|objective| ObjectiveProgress {
                    id: objective.id,
                    count: 0,
                })
                .collect(),
        });
    }
}

A pickup or kill is a message. The quest system is the only writer of QuestProgress.

#[derive(Clone, Copy, PartialEq, Eq)]
enum QuestStage {
    Active,
    Complete,
}

#[derive(Component)]
struct QuestProgress {
    stage: QuestStage,
    objective: u32,
    count: u32,
    required: u32,
}

#[derive(Message)]
struct ObjectiveFact {
    objective: u32,
}

fn apply_facts(mut facts: MessageReader<ObjectiveFact>, mut quests: Query<&mut QuestProgress>) {
    for fact in facts.read() {
        for mut quest in &mut quests {
            if quest.stage != QuestStage::Active || quest.objective != fact.objective {
                continue;
            }
            quest.count = (quest.count + 1).min(quest.required);
            if quest.count >= quest.required {
                quest.stage = QuestStage::Complete;
            }
        }
    }
}

Register the message, accept on startup, then apply facts. One system writes QuestProgress. A completed stage is the reward payload other skills read.

#[derive(Clone, Copy, PartialEq, Eq)]
enum QuestStage {
    Active,
    Complete,
}

#[derive(Component)]
struct QuestProgress {
    stage: QuestStage,
    objective: u32,
    count: u32,
    required: u32,
}

#[derive(Message)]
struct ObjectiveFact {
    objective: u32,
}

#[derive(Resource, Default)]
struct QuestCatalog;

fn accept(mut commands: Commands) {
    commands.spawn(QuestProgress {
        stage: QuestStage::Active,
        objective: 1,
        count: 0,
        required: 3,
    });
}

fn apply_facts(mut facts: MessageReader<ObjectiveFact>, mut quests: Query<&mut QuestProgress>) {
    for fact in facts.read() {
        for mut quest in &mut quests {
            if quest.stage != QuestStage::Active || quest.objective != fact.objective {
                continue;
            }
            quest.count = (quest.count + 1).min(quest.required);
            if quest.count >= quest.required {
                quest.stage = QuestStage::Complete;
            }
        }
    }
}

pub struct QuestPlugin;

impl Plugin for QuestPlugin {
    fn build(&self, app: &mut App) {
        app.init_resource::<QuestCatalog>()
            .add_message::<ObjectiveFact>()
            .add_systems(Startup, accept)
            .add_systems(Update, apply_facts);
    }
}

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 field stored 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 |

| Quest definitions | QuestCatalog | Copying progress counts back into the catalog |

| Stage change | The one Update system that reads MessageReader<ObjectiveFact> | A dialogue system writing QuestProgress |

| 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 documentation | 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.

• Do not keep a second writer of QuestProgress. WHY: two mutable queries of the same component race unless .chain() orders them.
• Do not import another engine's node or signal types. WHY: quest stage is a component and a message, not a foreign object.
• No invented APIs or unnamed third-party crates as the chosen solver. WHY: this skill only names documented Bevy crates and modules.

Reference

Official links stay on unversioned latest pages. 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-dialogue-system - Lines that grant quests.
• bevy-data-resources - Tables and definitions.
• bevy-events-messaging - One-shot facts.

Complements

• bevy-state-machines - Mode flags.
• godot-quest-system - Godot package for quest tracking.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.