Bevy 0.19 — How to write idiomatic Bevy code Agent Skill

Provides authoritative idioms for Bevy 0.19 game projects in Rust.

Context cost
87.3k tokensestimated from the bundle, loaded when it triggers
Bundle
15 files · 349.3 kBtext throughout, nothing executable
Licence
not declarednone named by its author
Last change
—no release on file
Servers it uses
Noneruns standalone

What it does

Provides authoritative idioms for Bevy 0.20 game projects in Rust. Covers ECS data design (components, required components, queries, change detection, relationships, resources-as-components), communication (Event vs Message vs Observer, EntityEvent, lifecycle patterns), plugin organization, scheduling (run conditions, weak ordering, states, fixed timestep), assets, UI (text, widgets, text input), BSN scenes, rendering and WESL shaders, error handling, testing, performance tuning, and common pitfalls.

What this skill changes about the agent has not been set out here as a list. Its SKILL.md, shown below, says it in the author’s own words.

gamedevrust

The skill itself

A skill is instructions the model reads, so you can see what you are getting: below is the start of its SKILL.md. The front matter loads with every session, the body loads when the skill triggers, and the whole file comes with the install.

SKILL.md45.9 kB · preview
--- name: bevy ---
6# Bevy 0.20 — How to write idiomatic Bevy code
7
8Bevy moves fast. Idioms that were correct in 0.16 are broken in 0.17, 0.17 idioms shifted in 0.18, 0.18 shifted again in 0.19, and 0.19 shifted again in 0.20. The most consequential shifts you must internalize:
9
10- **0.17 split Event from Message.** Observers ↔ Event; EventReader/EventWriter was renamed to MessageReader/MessageWriter and lives on a distinct Message trait. Mixing them is the #1 source of confusion.
11- **0.17 renamed Trigger<E> to On<E>** and renamed lifecycle events: OnAdd → Add, OnInsert → Insert, OnRemove → Remove, OnDespawn → Despawn, OnReplace → Replace. **In 0.19 Replace was renamed again to Discard** (and on_replace → on_discard). The current set is Add, Insert, Discard, Remove, Despawn, observed as On<Add<MyComponent>> (0.20 moved the component generic into the event; 0.17–0.19 wrote On<Add, MyComponent>).
12- **0.17 made #[derive(Reflect)] auto-register** (via the inventory crate). register_type::<T>() is now needed only for concrete instantiations of generic types.
13- **0.17 introduced required components** (#[require(Other)]) that effectively replaced bundles for "always together" composition. Bundles still exist as tuples, but new code should declare requirements on the component itself.
14- **0.18 made EntityEvent immutable by default** (mutation moved to SetEntityEventTarget); moved RenderTarget off Camera; split AmbientLight/GlobalAmbientLight; renamed clear_children/remove_child to detach_all_children/detach_child; made same-value next_state.set(X) re-fire OnEnter/OnExit (use set_if_different — named set_if_neq in 0.18–0.19 — for the old behavior); and added cargo feature collections.
15- **0.19 made resources components.** Resource is now a subtrait of Component, and #[derive(Resource)] *also* implements Component. You can no longer #[derive(Component, Resource)] on one type (duplicate Component impl). Resources gain hooks, observers, relationships, and can be made immutable. See references/ecs.md.
16- **0.19 overhauled text** (migrated to parley): TextFont.font is now a FontSource (not Handle<Font>), font_size is a FontSize enum (not f32), with new weight/width/style fields and a LetterSpacing component.
17- **0.19 promoted UI out of experiment.** experimental_bevy_ui_widgets → bevy_ui_widgets (in the ui/default features) and experimental_bevy_feathers → bevy_feathers (still opt-in: features = ["bevy_feathers"]); UiWidgetsPlugins + InputDispatchPlugin are in DefaultPlugins. A first-class text input landed (0.19 EditableText; split into TextInput + EditableText in 0.20).
18- **0.19 renamed the old scene system to bevy_world_serialization.** SceneRoot(...) → WorldAssetRoot(...) for glTF scene spawning; DynamicScene → DynamicWorld. The bevy_scene/bevy::scene name now hosts the new **BSN** scene system (bsn!). See references/assets.md and the BSN section below.
19- **0.19 renamed light shadow fields** (shadows_enabled → shadow_maps_enabled, plus new contact_shadows_enabled), made Atmosphere a standalone entity (Atmosphere::earth(medium)), and replaced the render-graph with **render-graph-as-systems**. See references/rendering.md.
20- **0.20 moved the observer component generic onto the event.** On<Add, T> → On<Add<T>> (same for Insert/Discard/Remove/Despawn); On now takes one E: EventPattern. On<Add<(A, B)>> fires when *either* is added. Bare On<Add> is gone — dynamic-component observers use On<Add<()>> + .with_component(id). add.entity and plain On<MyEvent> observers are unchanged.
21- **0.20 flattened pointer events.** On<Pointer<Press>> → On<PointerPress>, Pointer<Click> → PointerClick, and so on for Over/Out/Enter/Leave/Move/Release/Cancel/Scroll/Drag*. Pointer is now a plain field: ev.pointer.id, ev.pointer.position (a Vec2), ev.button/ev.hit/ev.count directly on the event. Generic handlers: fn f<E: PointerEvent>(e: On<E>).
22- **0.20 tightened BSN syntax.** Scene includes need @ (@button("Ok"), @scene_var, @{expr}); entities inside Children [ … ] / bsn_list! { … } are separated by --; template_value(..) is deprecated (write the value bare); bsn!/bsn_list! want brace delimiters. A new Ready event fires bottom-up once an entity *and its whole subtree* exist. See references/bsn.md.
23- **0.20 reshaped UI.** bevy_ui::Button and Interaction are deprecated — import bevy::ui_widgets::Button and read Hovered + Has<Pressed>. A text field is now TextInput (behavior) + EditableText (state); EditableText alone renders but ignores input. Val gained Em/Rem (em(1.5), rem(2)), TextFont::default() sizes text as FontSize::Rem(1.), and FontSource::SansSerif became FontSource::sans_serif().
24- **0.20 moved shaders to WESL.** naga_oil is gone: custom shaders are .wesl with import a::b::c;, @if(DEF)/@else, and constants::NAME; module paths follow the crate file path (bevy_pbr::forward_io → bevy_pbr::render::forward_io). #[derive(ExtractComponent)] now requires #[extract_app(RenderApp)]. Tonemapping::None became a true passthrough (use the new Tonemapping::Linear), and ScreenSpaceTransmission is opt-in on Camera3d. See references/rendering.md.
This is a preview of SKILL.md. Download or install the skill to get the whole thing.
In the file
SKILL.md5,798 words
Files15
Licencenot declared
Why you can read it

Nothing in a skill executes. The client loads the text and the model follows it, so a skill can be audited the way a runbook is — by reading it.

What it costs in context

Skills are not billed by the call. They are paid for in context: every token the instructions occupy is a token your code, your diff and your conversation cannot use. Here is what this one takes and when it takes it.

≈230
always loaded
The name and description, so the model knows the skill exists and when to reach for it.
87,095
on trigger
The instruction body and 14 supporting files, read only when the skill fires.
43.7%
of a 200k window
Ten skills this size would not fit in the window together.
050k100k150k200k context window

87.3k tokens, estimated from the bundle at four bytes to the token, held for the rest of the session once it triggers. Heavy. Teams tend to install this one per project rather than globally, and load it only when the job comes up.

Servers bill, skills cost

A server charges by the month. A skill charges once per session, in context, and then keeps charging it for as long as the session lives.

Before and after

The same question, put to the same model twice: once as it comes, and once with these instructions loaded.

No worked example has been published for this skill yet.

The procedure it runs

The procedure has not been set out here step by step. It is in the body of its SKILL.md: the start of it is shown above, and the whole file comes with the install.

Prose, not code

These steps are written for a model to follow, not executed by a runtime. It can still be told to skip one, and it will say so when it does.

Servers it uses

None. This skill calls no MCP servers at all.

Everything it needs is in the instructions, so it works in a project with nothing connected — the model reads the file and changes how it works with what it can already reach.

Its SKILL.md does not declare which tools it asks the agent for, so what it touches is whatever the instructions themselves ask for — read them before you install it.

What it asks for
Writes filesnot declared
Network accessnot declared

This SKILL.md carries no allowed-tools line, so neither is declared. A skill grants no permissions of its own — it can only ask for tools your client already has.

What it will not do

Every skill is narrow, and the useful ones say where they stop. These are the jobs this one is the wrong tool for.

What this skill is not for has not been published here. Nothing is implied by that: it is a section the author has not filled in.

What is in the bundle

15 files, 349.3 kB on disk. A bundle is text throughout: the instructions the model reads, plus the templates it fills in.

  • SKILL.md45.9 kB
  • evals/evals.json15.8 kB
  • references/api-cheatsheet.md34.4 kB
  • references/assets.md23.5 kB
  • references/bsn.md14.9 kB
  • references/communication.md15.3 kB
  • references/ecs.md20.5 kB
  • references/errors.md15.6 kB
  • references/performance.md19.3 kB
  • references/pitfalls.md36.5 kB
  • references/plugins.md11.5 kB
  • references/rendering.md29.2 kB
  • references/scheduling.md19.5 kB
  • references/testing.md16.1 kB
  • references/ui.md31.3 kB
What is not in it

No dependencies and nothing executable: a skill is text the agent reads, so the bundle is 15 files you can review in full before installing.

Install

Installing copies the bundle into your project. Nothing runs at install time — the files sit on disk until the model reads them.

# Bevy 0.19 — How to write idiomatic Bevy code · 87.3k tokens when loaded · macOS, Linux mkdir -p .claude/skills && t=$(mktemp -d) && curl -fsSL -o "$t/bundle.tar.gz" https://mcprush.com/api/skills/bevy-0-19-how-to-write-idiomatic-bevy-code/bundle.tar.gz && tar -xzf "$t/bundle.tar.gz" -C "$t" && [ -d "$t/bevy-0-19-how-to-write-idiomatic-bevy-code" ] && { [ ! -e .claude/skills/bevy-0-19-how-to-write-idiomatic-bevy-code ] || { mv .claude/skills/bevy-0-19-how-to-write-idiomatic-bevy-code "$t/.previous" && echo "Previous copy moved to $t/.previous"; }; } && mv "$t/bevy-0-19-how-to-write-idiomatic-bevy-code" .claude/skills/ && echo "Installed .claude/skills/bevy-0-19-how-to-write-idiomatic-bevy-code"
On Windows, or any OS with Node 20+
npx mcprush@latest skill add pavlov-net/bevy-0-19-how-to-write-idiomatic-bevy-code

Windows PowerShell says 'running scripts is disabled on this system'? Run Set-ExecutionPolicy -Scope CurrentUser RemoteSigned once, or type npx.cmd instead of npx (codex.cmd, gemini.cmd, grok.cmd likewise). PowerShell 7 needs neither. Writes to .claude/skills/bevy-0-19-how-to-write-idiomatic-bevy-code/ in the current project. For every project instead, run mkdir -p ~/.claude/skills && t=$(mktemp -d) && curl -fsSL -o "$t/bundle.tar.gz" https://mcprush.com/api/skills/bevy-0-19-how-to-write-idiomatic-bevy-code/bundle.tar.gz && tar -xzf "$t/bundle.tar.gz" -C "$t" && [ -d "$t/bevy-0-19-how-to-write-idiomatic-bevy-code" ] && { [ ! -e ~/.claude/skills/bevy-0-19-how-to-write-idiomatic-bevy-code ] || { mv ~/.claude/skills/bevy-0-19-how-to-write-idiomatic-bevy-code "$t/.previous" && echo "Previous copy moved to $t/.previous"; }; } && mv "$t/bevy-0-19-how-to-write-idiomatic-bevy-code" ~/.claude/skills/ && echo "Installed ~/.claude/skills/bevy-0-19-how-to-write-idiomatic-bevy-code" (macOS, Linux) — or add --global to the npx line. The curl line replaces .claude/skills/bevy-0-19-how-to-write-idiomatic-bevy-code/ only after the download succeeds, so files left there by another skill of the same name, or by an older release, do not mix in. A copy already there is moved aside, not deleted, and the line prints where. It appears as bevy in Gemini CLI, Codex, Copilot and Grok (its own name); Claude Code lists it by the folder name, bevy-0-19-how-to-write-idiomatic-bevy-code.

Which clients pick it up on their own

A skill is a folder of text. Most of these read a skills folder and pick it up without being told; the rest take the same folder another way — an upload in the app, a parameter on an API call — and then choose it by its description just the same.

Claude Code.claude/skills/
Claude DesktopCustomize › Skills
ChatGPTSkills › Create › Upload
Cursor.cursor/skills/
VS Code.github/skills/
Codex CLI.agents/skills/
Gemini CLI.gemini/skills/
Grok Build.grok/skills/
Microsoft CopilotSkills › Upload a skill
PerplexityComputer › Skills › Upload a skill
DeepSeek Harness.dsh/skills/
Zed.agents/skills/
Devin Desktop.devin/skills/
Windsurf (legacy).windsurf/skills/
OpenAI Agents SDKInstructions or sandbox Skills
HTTP / APIa request parameter
This release
Versionnot versioned
Publishedno release date on file
PriceFree
Referencepavlov-net/bevy-0-19-how-to-write-idiomatic-bevy-code

Versions

Its author publishes no version number, so there is nothing here to pin to: what you install is the folder as it stands today. Instructions change more often than APIs do — a skill can be rewritten entirely without anything it depends on moving.

Pinning

Nothing to pin to: this skill carries no version number of its own. What you install is what the folder holds on the day you install it.

Reviews

—no reviews yet

Nobody has reviewed this skill yet. The rating is the mean of the reviews written here, so there is none until somebody writes the first.

Sign in to review it

A review is tied to an account so it can be answered, edited and taken down by the person who wrote it.

Sign in
Who can post

Anybody with an account, one review each. It carries the version that was current when you wrote it, and the publisher may reply once.

Who wrote it

PN
pavlov-net

Publishes on mcprush.

0 servers listed1 skill listednot claimed
Profile
Publisher
Servers0
Claim this skill