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

Need	Owner
Chase vs patrol vs wait	Enum Component plus a system that writes Goal
Retarget throttle	Timer resource ticked on FixedUpdate
Crowd and avoidance	bevy-navigation
Bake and obstacles	bevy-navigation
Line of sight	bevy-spatial-queries

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

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	One script that owns the whole actor
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 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.