Crate bevy_fairy 

bevy_fairy

An agentic debugger for Bevy games. It has two halves, and a game uses one of them while an agent uses the other.

• The plugin (plugin) is what a game links. It registers BRP methods (bevy_fairy/status, commands, screenshot, capture, input, scene, guide, quit) and lets the game register its own typed commands.
• The server (server) is the bevy-fairy binary. It speaks the Model Context Protocol to a coding agent and the Bevy Remote Protocol to a running game.

The two halves do not depend on each other. A game that has never heard of this crate can still be inspected with Bevy’s own BRP methods, and a game that links the plugin can be driven by any BRP client, not only this one.

Which feature do I want?

you are	you want	your dependency
a game	the plugin	bevy_fairy = { version = "0.2", default-features = false, features = ["plugin"] }
an agent	the server	cargo install bevy_fairy

A game that takes the plugin as a dependency compiles none of the server’s dependencies. That is not a promise about the future; tests/plugin_leaf.rs fails the build if it stops being true.

Quick start: a game

[dependencies]
bevy = { version = "0.20", features = ["default"] }
bevy_fairy = { version = "0.2", default-features = false, features = ["plugin"] }

This example belongs to the game, not to this crate: it uses bevy, which this crate only depends on when the plugin feature is on, so it is shown rather than compiled.

use bevy::prelude::*;
use bevy::remote::http::RemoteHttpPlugin;
use bevy_fairy::plugin::DebuggerPlugin;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        // Owns the BRP RemotePlugin. Never add a second RemotePlugin next to it.
        .add_plugins(DebuggerPlugin::default())
        .add_plugins(RemoteHttpPlugin::default())
        .run();
}

DebuggerPlugin needs bevy::input::InputPlugin in the app. DefaultPlugins includes it; MinimalPlugins does not.

Quick start: an agent

cargo install bevy_fairy
bevy-fairy --help

Then point it at a running game. The game listens on 127.0.0.1:15702 by default; set BEVY_BRP_HOST and BEVY_BRP_PORT to change that.

The tool contract

Every tool’s advertised schema is generated from the same Rust type its handler parses, so the two cannot drift apart. Every result carries the frame it was observed on. A value that cannot be measured is refused with a reason, never answered with a plausible number.

Re-exports

pub use plugin::apply_pending_input;

pub use plugin::cursor_position;

pub use plugin::handle_input;

pub use plugin::ClockRequest;

pub use plugin::logical_for;

pub use plugin::typed;

pub use plugin::DebugCommandInfo;

pub use plugin::DebugCommands;

pub use plugin::DebugCommandsAppExt;

pub use plugin::DebugCursor;

pub use plugin::DebugReady;

pub use plugin::DebuggerPlugin;

pub use plugin::InputAction;

pub use plugin::InputCommand;

pub use plugin::InputKind;

pub use plugin::PendingInput;

pub use plugin::SceneLog;

pub use plugin::ScenePlayback;

pub use plugin::SceneStep;

pub use server::prelude;

pub use server::brp as brp_messages;

Modules

plugin

The Bevy plugin: what a game links. The game-side half of bevy_fairy: a Bevy plugin that lets a coding agent drive a running game over BRP (JSON-RPC over HTTP), without a shared filesystem.

server

The MCP server: what an agent talks to. The MCP server half of bevy_fairy.