What causes "Cannot cache Scene Components with props/fields" in bevyengine/bevy?

Remove the ':' prefix from the scene component: @MySceneComponent { field: value }

bevyengine/bevy · error · syn::Error

Cannot cache Scene Components with props/fields

Error message

Cannot cache Scene Components with props/fields

What it means

The @-prefixed scene-component form (@MySceneComponent) parses a BsnType; if it has fields/props, err_if_cached rejects it under a ':' cache prefix — only fieldless scene components are cacheable in principle. As with the expression case, the earlier 'caching is only supported for scene assets' check (parse.rs:229-231) intercepts ':' + non-literal first today, so this message is the guard kept for when asset-only caching is relaxed.

Source

Thrown at crates/bevy_scene/macros/src/bsn/parse.rs:249

        }

        Ok(if input.peek(LitStr) {
            let path = input.parse::<LitStr>()?;
            if cached.is_none() {
                return Err(syn::Error::new(
                    path.span(),
                    "Cannot use scene assets without caching, please add the ':' prefix.",
                ));
            }
            BsnScene::Asset(path)
        } else if input.peek(Brace) {
            err_if_cached("Cannot cache scene expressions")?;
            BsnScene::Expression(braced_tokens(input)?)
        } else if input.peek(At) {
            input.parse::<At>()?;
            let sc = input.parse::<BsnType>()?;
            if sc.fields.len() > 0 {
                err_if_cached("Cannot cache Scene Components with props/fields")?;
            }
            BsnScene::SceneComponent(sc)
        } else {
            // PERF: do we really need this fork here?
            let path = input.fork().parse::<Path>()?;
            match PathType::new(&path) {
                PathType::Type | PathType::Enum => {
                    // Scene components are parsed before this if an @ is found.
                    // If this path is hit, that means it wasn't prefixed by @
                    return Err(syn::Error::new(
                        path.span(),
                        format!(
                            "Scene component {} needs to be prefixed by '@'",
                            path_to_string(&path),
                        ),
                    ));
                }
                PathType::Function | PathType::TypeFunction => {

View on GitHub (pinned to 396ca72708)

Solutions

Remove the ':' prefix from the scene component: @MySceneComponent { field: value }
If you need caching, put the content in a scene asset and load it with :"path.scene" instead

Example fix

// before
bsn!(:@MySceneComponent { speed: 10.0 });

// after
bsn!(@MySceneComponent { speed: 10.0 });

Defensive patterns

Strategy: validation

Validate before calling

// Rule: @SceneComponent forms take no ':' prefix; if the component has
// fields, caching is not an option at all today.

Prevention

Write @SceneComponent entries without the ':' prefix
Fieldless or not, prefer scene assets (":"path") when caching behavior is required

When it happens

Trigger: Writing :@MySceneComponent { field: value } — a cached scene component that carries fields (currently surfaced as the 'only scene assets' error due to the earlier check).

Common situations: Marking data-rich scene components as cached; assuming ':' works uniformly across all BSN scene forms.

Related errors

AI-assisted analysis of bevyengine/bevy@396ca72708 (2026-08-20). Data as JSON: /api/errors/927f78966cc07cb6. Report an issue: GitHub.