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 can be a Resource (see bevy-data-resources). Level content loads through AssetServer and may spawn with spawn_scene / queue_spawn_scene (bevy::scene, docs.rs latest, 0.20.0).
• What was checked and rejected: https://docs.rs/bevy/latest/bevy/scene/struct.DynamicScene.html returned 404 on 2026-10-11. The live scene index documents Scene and bsn!, not a save-file serializer. Do not call DynamicScene::serialize from this skill.
• Schedule: no save system is specified here. Do not invent a filesystem write inside Update.

Coming from Godot

Godot idea	Bevy idiom
Resource file for designer data	An asset Handle, or a Resource you own in code
Packed scene instance	Scene spawn on bevy-asset-scenes
Player save slot	Leave it as data you have not serialized yet. Check the docs before naming a slot format.

Scope

• Check docs.rs latest for a save path, slot index, version field, or RON/JSON writer before writing one.
• Scene spawn is not a save game. Do not treat bsn! as a user save.
• Wasm has no general filesystem story on the docs pages for this skill. Do not invent one.

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 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 Godot Resource, FileAccess, or config-file types. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• 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

docs.rs links use the unversioned latest permalink. The library pin on bevy-master stays 0.20.0.

Official Documentation

• Migration guides - Unversioned hop index.
• bevy crate - Unversioned API index.
• bevy::scene - Spawn API that is not a save format. DynamicScene struct URL 404.
• bevy::asset - Disk assets, not player slots.
• Resources - App-wide data that a future save would have to name.
• Examples - Example tree on the latest branch.

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.

Complements

• godot-save-load-systems - Equivalent Godot skill.

Downstream / consumers

• None until a save format is opened.

Master

• bevy-master - Bevy router.