bevyengine/bevy · error · ResourceFetchError::Immutable

Tried to mutably fetch resource with ID {0:?}, which is immu

Error message

Tried to mutably fetch resource with ID {0:?}, which is immutable

What it means

ResourceFetchError::ImmutableResource: a mutable fetch (get_mut/resource_mut style access by id) was attempted on a resource declared with Resource<Mutability = Immutable>, so mutable access cannot be granted.

Source

Thrown at crates/bevy_ecs/src/world/error.rs:77

    /// The entity with the given ID was requested mutably more than once.
    #[error("The entity with ID {0} was requested mutably more than once")]
    AliasedMutability(Entity),
}

/// An error that occurs when getting a resource of a given type in a world.
#[derive(thiserror::Error, Debug, Clone, Copy, PartialEq, Eq)]
pub enum ResourceFetchError {
    /// The resource has never been initialized or registered with the world.
    #[error("The resource has never been initialized or registered with the world. Did you forget to add it using `app.insert_resource` / `app.init_resource`?")]
    NotRegistered,
    /// The resource with the given [`ComponentId`] does not currently exist in the world.
    #[error("The resource with ID {0:?} does not currently exist in the world.")]
    DoesNotExist(ComponentId),
    /// Cannot get access to the resource with the given [`ComponentId`] in the world as it conflicts with an on going operation.
    #[error("Cannot get access to the resource with ID {0:?} in the world as it conflicts with an on going operation.")]
    NoResourceAccess(ComponentId),
    /// Tried to mutably fetch an immutable resource.
    #[error("Tried to mutably fetch resource with ID {0:?}, which is immutable")]
    Immutable(ComponentId),
}

#[cfg(test)]
mod tests {
    use crate::{
        prelude::*,
        system::{command::trigger, RunSystemOnce},
    };

    // Inspired by https://github.com/bevyengine/bevy/issues/19623
    #[test]
    fn fixing_panicking_entity_commands() {
        #[derive(EntityEvent)]
        struct Kill(Entity);

        #[derive(EntityEvent)]
        struct FollowupEvent(Entity);

View on GitHub (pinned to 396ca72708)

Solutions

1. Fetch the resource immutably instead
2. Declare the resource as mutable (remove the Immutable mutability) if mutation is required
3. Replace the value wholesale via insert_resource rather than mutating it

When it happens

Trigger: Thrown at crates/bevy_ecs/src/world/error.rs:77 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of bevyengine/bevy@396ca72708 (2026-08-20). Data as JSON: /api/errors/d37c5c94783d8498. Report an issue: GitHub.