Bevy AI navigation

When to use

Load this when an actor must chase, patrol, or wait. This skill owns the intent and the desired velocity. Paths stay on bevy-navigation. Transform integration stays on bevy-character-movement. Line-of-sight sensors stay on bevy-spatial-queries.

ECS shape

• Components: a mode flag can be an enum Component (see bevy-state-machines for the pattern). Goal position can be your own component. No agent, radius, or layer type was opened.
• Resources: a shared patrol table can be a Resource you define. No navigation resource was opened.
• Systems: read the mode and write a desired velocity or waypoint on Update or Fixed. Use .chain() so policy runs before movement. Do not call an agent callback that writes Transform.
• Schedule: Fixed / FixedMain when the result feeds the movement step.

Policy map

Policy writes DesiredVelocity. It does not move Transform and it does not query a navmesh.

[dependencies]
bevy = "0.20"

#[derive(Clone, Copy, PartialEq, Eq, Component)]
enum Brain {
    Wait,
    Patrol,
    Chase,
}

#[derive(Component)]
struct WaypointIndex(usize);

#[derive(Component)]
struct Goal(Vec3);

#[derive(Component)]
struct DesiredVelocity(Vec3);

#[derive(Component)]
struct Player;

#[derive(Resource)]
struct PatrolRoute {
    points: Vec<Vec3>,
}

#[derive(Resource)]
struct RetargetClock(Timer);

fn tick_policy(
    time: Res<Time>,
    mut clock: ResMut<RetargetClock>,
    route: Res<PatrolRoute>,
    players: Query<&Transform, With<Player>>,
    mut actors: Query<(&Brain, &Transform, &mut WaypointIndex, &mut Goal)>,
) {
    clock.0.tick(time.delta());
    if !clock.0.just_finished() {
        return;
    }
    let player_pos = players.iter().next().map(|transform| transform.translation);
    for (brain, transform, mut cursor, mut goal) in &mut actors {
        match *brain {
            Brain::Wait => goal.0 = transform.translation,
            Brain::Patrol => {
                if route.points.is_empty() {
                    continue;
                }
                cursor.0 = (cursor.0 + 1) % route.points.len();
                goal.0 = route.points[cursor.0];
            }
            Brain::Chase => {
                if let Some(pos) = player_pos {
                    goal.0 = pos;
                }
            }
        }
    }
}

fn steer(mut actors: Query<(&Transform, &Goal, &mut DesiredVelocity), With<Brain>>) {
    for (transform, goal, mut velocity) in &mut actors {
        let offset = goal.0 - transform.translation;
        let flat = Vec3::new(offset.x, 0.0, offset.z);
        velocity.0 = if flat.length_squared() < 0.05 {
            Vec3::ZERO
        } else {
            flat.normalize() * 3.0
        };
    }
}

pub struct NavPolicyPlugin;

impl Plugin for NavPolicyPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(PatrolRoute {
            points: vec![Vec3::new(-6.0, 0.0, 0.0), Vec3::new(6.0, 0.0, 4.0)],
        })
        .insert_resource(RetargetClock(Timer::from_seconds(
            0.4,
            TimerMode::Repeating,
        )))
        .add_systems(FixedUpdate, (tick_policy, steer).chain());
    }
}

#[derive(Component)]
struct PatrolAgent;

fn spawn_patrol(mut commands: Commands) {
    commands.spawn((
        PatrolAgent,
        Transform::from_xyz(0.0, 0.0, 0.0),
        DesiredDir(Vec3::X),
    ));
}

#[derive(Component)]
struct DesiredDir(Vec3);

fn hold_when_close(mut agents: Query<(&Transform, &mut DesiredDir), With<PatrolAgent>>) {
    for (transform, mut desired) in &mut agents {
        if transform.translation.distance(Vec3::ZERO) < 0.5 {
            desired.0 = Vec3::ZERO;
        }
    }
}

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

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

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 navigation-agent or nav-server types from another engine. WHY: this skill writes Goal and DesiredVelocity; a foreign agent type would move Transform and skip the chain into movement.

• No named third-party navigation or physics crate. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No invented path, bake, or avoidance APIs. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.0. No AI navigation module was opened. Load references/ecs-ownership.md when splitting goal writes from velocity writes. Do not load it to bake a mesh.

Official Documentation

• Assets - Official asset catalog. No AI plugin was selected.
• Examples - Example tree on the latest branch.
• ECS - Component, Resource pairing via bevy-data-resources, systems, and .chain().
• App states are not per-entity patrol modes. Per-entity flags stay components (bevy-state-machines).

Related Skills

Prerequisites

• bevy-master - ECS ownership.
• bevy-navigation - Paths and navigation meshes.
• bevy-state-machines - Enum components for a mode flag.

Complements

• bevy-character-movement - Integrates a waypoint or velocity.
• bevy-spatial-queries - Sensors and spatial queries.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.