AI agent skill · by @thedivergentai

Bevy Project Foundations

Docs-first stub for a Bevy Rust project layout. Godot counterpart: godot-project-foundations. Not a full port. Keywords: Bevy, Cargo, features, project layout.

Archived 2026-10-23 · Source updated 2026-10-11 · License: LGPL-3.0. Check the original repository for the latest version.

SKILL.md instructions

# Bevy project foundations (docs-first stub)

Not a full port. No extra crate beyond `bevy` in this phase.

## When to use

Load this when a new Bevy app needs a crate, a pin, and a place for feature code before gameplay systems exist. Folder names below are layout habits. They are not engine types.

## ECS shape

- **Crate:** Getting Started says start with `cargo init` or `cargo new my_project`, then `cargo add bevy`. The same page's `Cargo.toml` sample still shows `bevy = "0.19"`. The library pin on bevy-master is **0.20.0**.
- **Features:** The `bevy` 0.20.0 crate page documents profiles (`default`, `2d`, `3d`, `ui`) and `default-features = false` when you want a smaller set. One sample on that page still shows `version = "0.18"`. That sample is not the pin.
- **App shell:** `App::new().run()` (Apps page). `use bevy::prelude::*` brings in the guide's essentials. Plugins, systems, and resources are added on that `App`.
- **Code layout:** Split by feature in ordinary Rust modules if the crate grows. The opened pages do not define a required directory tree.

## Godot idea to Bevy idiom

| Idea | Bevy idiom (idea only) |
| --- | --- |
| New project | `cargo new` plus the `bevy` dependency |
| Engine module toggles | Cargo features and profiles on the `bevy` crate |
| Feature folders | Rust modules you choose. Not a scene tree |
| Editor project file | Not opened. Do not invent a project manifest beside `Cargo.toml` |

## Gaps

- Getting Started still shows `bevy = "0.19"` and a `v0.19.0` checkout example. Pin **0.20.0** from crates.io and docs.rs, not from that snippet.
- OS window, audio, and input dependencies are mentioned as a Setup page. That page was not opened. Do not invent package names.
- Workspace members, feature-folder enforcement, and naming linters are not in the opened pages.


## Expert Knowledge Delta

Library target in prose: **Bevy 0.20.0**. Lattice URLs stay unversioned.

Opened canon only: `Component`, `Resource`, `Query`, `Commands`, `Startup`, `Update`, `Fixed` / `FixedMain` (`bevy::time`), `.chain()`, `Plugin`, `DefaultPlugins`, `MinimalPlugins`. Messages and observers are an ownership slot. This file does not invent their struct 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 this skill did not open | A Gaps row | A fabricated crate (solver, navmesh module, net, XR) |

Load `references/a-gate-notes.md` for the same table plus the gap rule.

## NEVER

- Do not put `/0.20.0/` in lattice URLs. WHY: permalinks stay on `latest`. The 0.20.0 pin belongs in prose once.
- 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: those rows are gaps, blocked, or revisit.

- No Godot project types, scenes, or autoloads in this crate. WHY: a Godot port, an unopened API, or a public lie fails the docs-first bar.
- No invented Cargo features beyond the profiles and collections opened on docs.rs. WHY: a Godot port, an unopened API, or a public lie fails the docs-first bar.
- No third-party crates. WHY: a Godot port, an unopened API, or a public lie fails the docs-first bar.
- No Creation Station content. WHY: a Godot port, an unopened API, or a public lie fails the docs-first bar.
- No umbrella skill over Godot and Bevy. WHY: a Godot port, an unopened API, or a public lie fails the docs-first bar.
- Do not mark this row `ported`. It is `docs_mapped`. WHY: a Godot port, an unopened API, or a public lie fails the docs-first bar.

## Reference

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

### Official Documentation
- [Migration guides](https://bevy.org/learn/migration-guides/introduction/)  -  Unversioned hop index.
- [bevy crate](https://docs.rs/bevy/latest/bevy/)  -  Profiles, collections, and Cargo features (0.20.0).
- [Getting started](https://bevy.org/learn/quick-start/getting-started/)  -  `cargo new` / `cargo add bevy`. The version snippet is stale.
- [Apps](https://bevy.org/learn/quick-start/getting-started/apps/)  -  `App::new()`, `World`, and `Schedule`.
- [Examples](https://github.com/bevyengine/bevy/tree/latest/examples#examples)  -  Example tree on the `latest` branch.

### Related Skills

#### Prerequisites
- [bevy-master](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/bevy-master/SKILL.md)  -  Library pin and which Bevy skill to load next.

#### Complements
- [bevy-ecs-composition](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/bevy-ecs-composition/SKILL.md)  -  Entities, components, and systems after the crate exists.
- [godot-project-foundations](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/godot-project-foundations/SKILL.md)  -  Godot counterpart. Source idea only. Do not import Godot types.

#### Downstream / consumers
- [bevy-app-resources](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/bevy-app-resources/SKILL.md)  -  App-wide resources once the `App` exists.

#### Master
- [bevy-master](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/bevy-master/SKILL.md)  -  Bevy router.