Bevy Save Load Agent Skill

Bevy save and load

When to use

Load this when a design asks to persist player progress. Route asset and scene spawning to bevy-asset-scenes. Keep save bytes as data you serialize yourself until a current docs page names a format.

ECS shape

• What exists: app-wide progress is a Resource you serialize yourself. Level content loads through AssetServer and scene spawn (bevy-asset-scenes). A scene spawn is not a player slot.
• Format: progress structs use serde and RON. The file holds primitives and a version field. Entity ids and asset handles stay out of the file.
• Schedule: write the slot from an explicit request system. Do not write the file from every Update frame.

serde and ron sit beside Bevy. The progress type is a normal resource.

[dependencies]
bevy = "0.20"
serde = { version = "1", features = ["derive"] }
ron = "0.10"

use serde::{Deserialize, Serialize};

#[derive(Resource, Clone, Serialize, Deserialize)]
struct Progress {
    version: u32,
    coins: u32,
    level: String,
}

fn fresh_progress() -> Progress {
    Progress {
        version: 2,
        coins: 0,
        level: "town".to_string(),
    }
}

Encode and decode are plain functions. Call them from the request system, not from a per-frame query.

use serde::{Deserialize, Serialize};

#[derive(Clone, Serialize, Deserialize)]
struct Progress {
    version: u32,
    coins: u32,
    level: String,
}

fn encode_slot(progress: &Progress) -> String {
    ron::ser::to_string_pretty(progress, ron::ser::PrettyConfig::default())
        .expect("progress encodes")
}

fn decode_slot(text: &str) -> Progress {
    ron::from_str(text).expect("progress decodes")
}

fn round_trip() {
    let saved = Progress {
        version: 2,
        coins: 4,
        level: "town".to_string(),
    };
    let loaded = decode_slot(&encode_slot(&saved));
    assert_eq!(loaded.coins, 4);
}

Older files omit version. Migration builds the current struct before anything inserts it.

use serde::{Deserialize, Serialize};

#[derive(Serialize, Deserialize)]
struct SaveV1 {
    coins: u32,
}

#[derive(Resource, Clone, Serialize, Deserialize)]
struct Progress {
    version: u32,
    coins: u32,
}

fn load_any(text: &str) -> Progress {
    if let Ok(current) = ron::from_str::<Progress>(text) {
        return current;
    }
    let legacy: SaveV1 = ron::from_str(text).expect("legacy slot");
    Progress {
        version: 2,
        coins: legacy.coins,
    }
}

The save system runs only while SaveRequest is present, then removes it. One system owns the resource and the disk write.

use serde::{Deserialize, Serialize};

#[derive(Resource, Clone, Serialize, Deserialize)]
struct Progress {
    version: u32,
    coins: u32,
}

#[derive(Resource)]
struct SaveRequest {
    path: String,
}

fn write_slot(progress: Res<Progress>, request: Res<SaveRequest>) -> std::io::Result<()> {
    let text = ron::ser::to_string(&*progress).expect("progress encodes");
    std::fs::write(&request.path, text)
}

fn save_when_asked(mut commands: Commands, progress: Res<Progress>, request: Option<Res<SaveRequest>>) {
    let Some(request) = request else {
        return;
    };
    let _ = write_slot(progress, request);
    commands.remove_resource::<SaveRequest>();
}

pub struct SavePlugin;

impl Plugin for SavePlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(Progress {
            version: 2,
            coins: 0,
        })
        .add_systems(Update, save_when_asked);
    }
}

What to persist

| Need | Bevy idiom |

| --- | --- |

| Designer content | An asset Handle, or a Resource you own in code |

| Level instance | Scene spawn on bevy-asset-scenes |

| Player save slot | A versioned Resource written as RON when a SaveRequest exists |

Scope

• The slot path, version field, and RON text live on Progress and SaveRequest above.
• Scene spawn is not a save game. Do not treat bsn! as a user save.
• A native std::fs write is the desktop slot. Web builds need a different storage story. Do not call std::fs and assume a browser has that path.

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 free-floating struct 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 |

| 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 engine config-file types in the slot. WHY: the slot is a versioned Resource encoded as RON.
• No DynamicScene or scene-file writer this file does not list. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.
• No third-party save crates. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20. Scene spawn is not a save format.

Official Documentation

• Resources - App-wide data the slot serializes.
• Examples - Example tree on the latest branch. Scene spawn lives with bevy-asset-scenes.

Related Skills

Prerequisites

• bevy-master - Who owns which piece of state.
• bevy-data-resources - Resource versus Component.
• bevy-asset-scenes - Loading content. Not a save slot.

Downstream / consumers

• None until a save format is opened.

Master

• bevy-master - Bevy router.