Panic when attempting to spawn scene from component spawned by scene

[Semihazah12]

Bevy version and features

Bevy Main (0.19.dev, rev#83a067d4)
Features:

• "bevy_scene"

• "bevy_asset"

• "debug"

• the Rust version you're using (you can get this by running cargo --version)

  • cargo 1.96.0-nightly (a357df4c2 2026-04-03)

• the operating system or browser used, including its version

  • CachyOS

What you did

Used a bsn scene to spawn an entity with a component with a component hook that spawn another scene;

What went wrong

Expected: Both scenes to spawn successfully.
Result: Panic with the following message:

thread 'main' (113688) panicked at /.cargo/git/checkouts/bevy-50d7e162b728c6c6/83a067d/crates/bevy_ecs/src/error/handler.rs:130:1:
Encountered an error in observer `bevy_scene::spawn::on_add_scene_patch_instance`: Parameter `ResMut<'_, QueuedScenes>` failed validation: Resource does not exist
If this is an expected state, wrap the parameter in `Option<T>` and handle `None` when it happens, or wrap the parameter in `If<T>` to skip the system when it happens.

Additional information

Minimal Reproducible Sample:

use bevy::{
    ecs::{lifecycle::HookContext, world::DeferredWorld},
    prelude::*,
};
fn main() {
    let mut app = App::new();
    app.add_plugins(DefaultPlugins);
    app.add_systems(Startup, startup);
    app.run();
}

fn startup(mut commands: Commands) {
    commands.queue_spawn_scene(test_scene_1());
}

fn test_scene_1() -> impl Scene {
    bsn!(SpawnScene2OnInsert)
}

fn test_scene_2() -> impl Scene {
    bsn!(#Name)
}

#[derive(Component, Default, Clone)]
#[component(on_insert = Self::on_insert)]
struct SpawnScene2OnInsert;

impl SpawnScene2OnInsert {
    fn on_insert(mut world: DeferredWorld, _context: HookContext) {
        world.commands().queue_spawn_scene(test_scene_2());
    }
}

Potential Cause:
queue_spawn_scene inserts a ScenePatchInstance, which then adds the scene handle to QueuedScenes via an observer. When a queued scene is spawned, the world accesses QueuedScenes via resource_scope, taking it out of the world temporarily. When SpawnScene2OnInsert is inserted, it queues a command which is executed immediately, before QueuedScenes is return into the world, causing a panic.

[Semihazah12]

I have a "should work but at what cost?" solution. In on_add_scene_patch_instance, if QueuedScenes isn't present, we can init a new copy. Then in spawn_queued, if there is a QueuedScenes resource present, we merge the two together.

[cart]

I think the best we can do is:

1. Slim down QueuedScenes to the minimum by breaking out "waiting scenes" into its own resource
2. Store two copies (one in the system and one in the resource) and mem::swap them when the system run

I have the change queued up. Thanks for investigating / providing a repro!