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.cursor/rules/project.mdc

Cursor rules for Bevy

Cursor rules are a markdown file with a short frontmatter header. Cursor attaches them to the agent so it stops guessing how this repo builds, tests, and styles code. This copy is written for a Bevy project. This repository is a Bevy game. Data lives in components, behavior lives in systems, and schedules stay explicit. Do not reach for a God object.

The example uses a balanced, readable style, pragmatic tests, and comments only where the intent is not obvious. Open the generator to make the agent stricter, switch to test-first, quiet the comments, or pick a different file.

The example assumes Bevy. It does not assume a particular physics crate or UI crate unless Cargo.toml already depends on it.

Customize in the generatorBrowse the library

Example file

Balanced strictness, readable style, pragmatic tests. Download name: project.mdc. Legacy path: .cursorrules

---
description: "Working rules for a Bevy repository"
globs: "**/*"
alwaysApply: true
---

# Project rules

Follow these rules on every task in this Bevy repository.
If the project still uses a root `.cursorrules` file, keep the same rules there.

## Stack

This repository is a Bevy game. Data lives in components, behavior lives in systems, and schedules stay explicit. Do not reach for a God object.

## Layout

- Plugins group related systems. The app binary adds plugins. It does not contain the rules.
- Components are plain data. Systems are functions.
- Assets use the project's asset path and handles, not filesystem reads scattered through systems.

## Commands

- Run: `cargo run`
- Test: `cargo test`
- Check: `cargo clippy --all-targets -- -D warnings`

Use the script the repo already defines when it differs from the command above.

## Conventions

- Query the smallest component set that proves the rule.
- Use commands to spawn and despawn. Do not mutate the world unsafely.
- Exclusive systems are rare. Prefer a normal system with the right system param.
- Put one-shot work in `OnEnter` or an event. Do not gate it with a boolean checked every frame.
- Do not `unwrap` on a missing asset handle.
- Run `cargo fmt` and clippy on gameplay changes.

## Testing

Add or update tests when behavior changes.

Test a pure system with a minimal App or a plain function. Do not boot the full game.

## Style

- Optimize for the next reader over the shortest diff.
- Comment only when the why is non-obvious.
- Name things the way neighboring files already do.
- Do not reformat code you did not change.

## Errors

- Fail at the boundary. Do not swallow errors.
- Log a missing asset and skip the spawn. A missing optional component is absence, not a panic.

## Scope

- Do the requested task. Do not refactor unrelated code.
- Match the rigor already in the repo. Fix what you touch.
- Ask one clarifying question only when the task cannot be finished without it. Otherwise state the assumption and continue.

## Avoid

- Do not store an Entity and assume it still exists next frame without a check.
- Do not allocate a new collection inside a hot system every frame.
- Do not use thread-local hacks to smuggle state. Use a resource.
- Do not add a second ECS mindset. Bevy's world is the world.

## Done when

- The requested behavior works.
- `cargo test` passes and `cargo clippy --all-targets -- -D warnings` is clean for non-trivial changes.
- The diff stays inside the task.
- You list anything you could not run.

Add it to the project

  1. 01

    Copy a starting file

    Copy the example Cursor rules on this page, or open the generator if you want different testing, strictness, or comment rules.

  2. 02

    Save it at .cursor/rules/project.mdc

    Save it at `.cursor/rules/project.mdc` in the Bevy repo and create the folders if they are missing. If the project still uses one root rules file, save the body as `.cursorrules`.

  3. 03

    Start a fresh agent session

    Rules files are read when a session starts. A chat that is already open may still be using the old instructions.

  4. 04

    Give it one small task

    Ask for one change a Bevy agent often gets wrong, such as a system that queries the components it needs and does not spawn entities every frame by accident. If it ignores the file, confirm the path and start a new session.

Questions about this file

Where do I put Cursor rules in a Bevy repo?

Save it at `.cursor/rules/project.mdc` in the Bevy repo and create the folders if they are missing. If the project still uses one root rules file, save the body as `.cursorrules`.

Which tools read this Cursor rules?

Cursor reads `.cursor/rules/*.mdc` and, on older setups, a root `.cursorrules` file. Other tools ignore the frontmatter, so use AGENTS.md when more than one agent edits the repo.

What does the Bevy example assume?

The example assumes Bevy. It does not assume a particular physics crate or UI crate unless Cargo.toml already depends on it.

Does VibeSpell upload my repository?

No. Recipe pages are static, and the generator builds the file in your browser from the options you pick. Your source code never leaves the machine.

Other files for Bevy

Cursor rules for other stacks