Bevy Character Movement Agent Skill

Bevy character movement

When to use

Load this when a Bevy app needs a player or actor that moves in discrete steps (walk, jump feel, ground check) and you need the ECS shape before any solver code exists. Coyote time and a jump buffer are design notes. They are not a node port.

ECS shape

• Components own pose and intent: position, velocity, and a jump-buffer or coyote timer as data. The Quick Start ECS page shows components as Rust structs that derive Component.
• Resources own clocks and tuning shared by every mover. bevy::time 0.20.0 documents Time, Fixed (fixed timestep clock), and Virtual.
• Systems read intent and write velocity or position. Register them on the fixed schedule, not on a per-node physics callback.
• Schedule: run_fixed_main_schedule runs FixedMain from virtual time (bevy::time 0.20.0). Order dependent systems with .chain() (ECS page). Do not paste a Godot physics-process function.

Engine counterparts

| Godot idea | Bevy idiom |

| --- | --- |

| Body that you move yourself | Entity with movement components and a system |

| Step that should stay stable when frame time changes | System on the fixed timestep (Fixed / FixedMain) |

| Coyote time, jump buffer | Timer data on the entity or a small resource, consumed by the movement system |

| Collision response | Collision response belongs on bevy-physics-3d. |

Scope

Locomotion here is kinematic data on FixedUpdate: intent, velocity, coyote time, and a jump buffer. Collision response belongs on bevy-physics-3d. Do not call move_and_slide, _physics_process, or CharacterBody2D.

[dependencies]
bevy = "0.20"

#[derive(Component, Default)]
struct PlanarIntent {
    axis: Vec2,
    jump_pressed: bool,
}

#[derive(Component)]
struct KinematicBody {
    velocity: Vec3,
    grounded: bool,
    coyote: f32,
    jump_buffer: f32,
}

#[derive(Resource)]
struct MoveTuning {
    speed: f32,
    jump_speed: f32,
    gravity: f32,
    coyote_time: f32,
    jump_buffer_time: f32,
}

impl Default for MoveTuning {
    fn default() -> Self {
        Self {
            speed: 6.0,
            jump_speed: 8.0,
            gravity: 20.0,
            coyote_time: 0.12,
            jump_buffer_time: 0.12,
        }
    }
}

fn read_move_intent(keys: Res<ButtonInput<KeyCode>>, mut bodies: Query<&mut PlanarIntent>) {
    let mut axis = Vec2::ZERO;
    if keys.pressed(KeyCode::KeyA) {
        axis.x -= 1.0;
    }
    if keys.pressed(KeyCode::KeyD) {
        axis.x += 1.0;
    }
    let jump_pressed = keys.just_pressed(KeyCode::Space);
    for mut intent in &mut bodies {
        intent.axis = axis;
        intent.jump_pressed = jump_pressed;
    }
}

fn step_kinematic(
    time: Res<Time<Fixed>>,
    tuning: Res<MoveTuning>,
    mut bodies: Query<(&mut PlanarIntent, &mut KinematicBody, &mut Transform)>,
) {
    let dt = time.delta_secs();
    for (mut intent, mut body, mut transform) in &mut bodies {
        body.velocity.x = intent.axis.x * tuning.speed;
        if body.grounded {
            body.coyote = tuning.coyote_time;
        } else {
            body.coyote = (body.coyote - dt).max(0.0);
            body.velocity.y -= tuning.gravity * dt;
        }
        if intent.jump_pressed {
            body.jump_buffer = tuning.jump_buffer_time;
            intent.jump_pressed = false;
        } else {
            body.jump_buffer = (body.jump_buffer - dt).max(0.0);
        }
        if body.jump_buffer > 0.0 && body.coyote > 0.0 {
            body.velocity.y = tuning.jump_speed;
            body.jump_buffer = 0.0;
            body.coyote = 0.0;
            body.grounded = false;
        }
        transform.translation += body.velocity * dt;
        if transform.translation.y <= 0.0 {
            transform.translation.y = 0.0;
            body.velocity.y = 0.0;
            body.grounded = true;
        }
    }
}

fn spawn_mover(mut commands: Commands) {
    commands.spawn((
        Transform::default(),
        PlanarIntent::default(),
        KinematicBody {
            velocity: Vec3::ZERO,
            grounded: true,
            coyote: 0.0,
            jump_buffer: 0.0,
        },
    ));
}

pub struct CharacterMovementPlugin;

impl Plugin for CharacterMovementPlugin {
    fn build(&self, app: &mut App) {
        app.init_resource::<MoveTuning>()
            .add_systems(Startup, spawn_mover)
            .add_systems(Update, read_move_intent)
            .add_systems(FixedUpdate, step_kinematic);
    }
}

Sample ButtonInput::just_pressed in Update. FixedUpdate can run zero times or several times in one frame, so a press read there is dropped or repeated. The intent component keeps the edge until FixedUpdate consumes it.

When a physics skill writes Grounded, chain the coyote refill after that write so the timer sees the contact from the same tick.

#[derive(Component)]
struct Grounded;

#[derive(Component)]
struct CoyoteTimer(f32);

#[derive(Resource)]
struct CoyoteWindow(f32);

fn refill_coyote(window: Res<CoyoteWindow>, mut movers: Query<&mut CoyoteTimer, With<Grounded>>) {
    for mut timer in &mut movers {
        timer.0 = window.0;
    }
}

fn tick_coyote(time: Res<Time<Fixed>>, mut movers: Query<&mut CoyoteTimer, Without<Grounded>>) {
    let dt = time.delta_secs();
    for mut timer in &mut movers {
        timer.0 = (timer.0 - dt).max(0.0);
    }
}

pub struct CoyotePlugin;

impl Plugin for CoyotePlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(CoyoteWindow(0.12)).add_systems(
            FixedUpdate,
            (refill_coyote, tick_coyote).chain(),
        );
    }
}

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 Godot node script |

| 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 this skill | Load the skill that owns it | 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 step locomotion on raw Update when the step must stay stable. WHY: bevy::time documents Fixed and run_fixed_main_schedule (FixedMain) for a fixed clock.
• Do not read ButtonInput::just_pressed inside FixedUpdate. WHY: that schedule can skip a frame or run more than once, so the press is lost or applied twice.
• Do not call move_and_slide, _physics_process, or CharacterBody2D. WHY: those are Godot types. This skill only owns ECS movement data.
• Do not invent a character-controller or shape-cast API. WHY: this skill owns kinematic intent and the fixed step; contact queries live on the physics skills.
• Do not mutably query the same movement component from two unordered systems. WHY: systems run in parallel unless .chain() orders them.

Reference

The library pin on bevy-master stays 0.20.0. Official pages stay on the unversioned site.

Official Documentation

• Quick Start: ECS - Entities, Component, systems, Query, Commands, and .chain().
• Fixed timestep - Fixed and FixedMain via run_fixed_main_schedule on the ECS schedule page.
• Examples - Example tree on the latest branch.

Related Skills

Prerequisites

• bevy-master - ECS ownership, schedules, and which Bevy skill to load.

Complements

• bevy-physics-3d - Collision and physics solvers. Player motion stays in this skill.
• godot-characterbody-2d - Equivalent Godot skill. Do not import Godot types.

Downstream / consumers

• Genre skills cover loop and encounter design.

Master

• bevy-master - Bevy router.