{"record":{"id":"6f201b62faeac15d","repo":"bevyengine/bevy","slug":"paramset-parameter-validation-failed-err","errorCode":null,"errorMessage":"ParamSet parameter validation failed: {err}","messagePattern":"ParamSet parameter validation failed: (.+?)","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"critical","filePath":"crates/bevy_ecs/src/system/system_param.rs","lineNumber":658,"sourceCode":"                })\n            }\n        }\n\n        impl<'w, 's, $($param: SystemParam,)*> ParamSet<'w, 's, ($($param,)*)>\n        {\n            $(\n                /// Gets exclusive access to the parameter at index\n                #[doc = stringify!($index)]\n                /// in this [`ParamSet`].\n                /// No other parameters may be accessed while this one is active.\n                pub fn $fn_name<'a>(&'a mut self) -> SystemParamItem<'a, 'a, $param> {\n                    // SAFETY: systems run without conflicts with other systems.\n                    // Conflicting params in ParamSet are not accessible at the same time\n                    // ParamSets are guaranteed to not conflict with other SystemParams\n                    unsafe {\n                        $param::get_param(&mut self.param_states.$index, &self.system_meta, self.world, self.change_tick)\n                    }\n                    .unwrap_or_else(|err| panic!(\"ParamSet parameter validation failed: {err}\"))\n                }\n            )*\n        }\n    }\n}\n\nall_tuples_enumerated!(impl_param_set, 1, 8, P, p);\n\n// SAFETY: Res only reads a single World resource\nunsafe impl<'a, T: Resource> ReadOnlySystemParam for Res<'a, T> {}\n\n// SAFETY: Res ComponentId access is applied to SystemMeta. If this Res\n// conflicts with any prior access, a panic will occur.\nunsafe impl<'a, T: Resource> SystemParam for Res<'a, T> {\n    type State = ComponentId;\n    type Item<'w, 's> = Res<'w, T>;\n\n    fn init_state(world: &mut World) -> Self::State {","sourceCodeStart":640,"sourceCodeEnd":676,"githubUrl":"https://github.com/bevyengine/bevy/blob/396ca727080776bd313bb892423b7d94e03b81b4/crates/bevy_ecs/src/system/system_param.rs#L640-L676","documentation":"ParamSet exposes its members through p0()..p7(); each accessor builds the member param from stored state via get_param and panics if that returns an error. The wrapped {err} is the concrete parameter validation failure - most commonly a required Res<T>/ResMut<T> inside the ParamSet whose resource does not exist in the World at call time (or state built for a different world).","triggerScenarios":"Calling param_set.pN() where the member is Res/ResMut of a resource that was never inserted or was removed; resources inserted via Commands by an earlier system that have not been flushed (apply_deferred not yet run) when the ParamSet member is fetched.","commonSituations":"Systems scheduled before the system that inserts the resource; resources created lazily by other plugins; tests running param code against partially built worlds.","solutions":["Read the {err} text embedded in the panic - it names the exact failing param and reason.","Insert required resources at startup (app.init_resource / insert_resource) before any schedule using them runs.","If presence is genuinely optional, make that member Option<Res<T>>/Option<ResMut<T>> inside the ParamSet and handle None."],"exampleFix":"// before\nfn sys(mut p: ParamSet<(ResMut<Score>, Res<Config>)>) {\n    let cfg = *p.p1(); // panics if Config was never inserted\n}\n\n// after\nfn sys(mut p: ParamSet<(ResMut<Score>, Option<Res<Config>>)>) {\n    let cfg = p.p1().as_deref().copied().unwrap_or_default();\n}","handlingStrategy":"validation","validationCode":"if !world.contains_resource::<Config>() {\n    world.init_resource::<Config>();\n}\n// safe to fetch a ParamSet member that is Res<Config>","typeGuard":null,"tryCatchPattern":null,"preventionTips":["Use Option<Res<T>> inside ParamSets for any resource not guaranteed present.","init_resource/insert_resource at app setup for all required resources.","When Commands insert resources, apply deferred (or order systems) before dependent access."],"tags":["bevy","ecs","system-param","param-set","resource","validation","panic"],"backgroundTag":"parameter-validation-failed","analyzedSha":"396ca727080776bd313bb892423b7d94e03b81b4","analyzedAt":"2026-08-20T16:12:39.808Z","contentChangedAt":"2026-08-20T16:12:39.808Z","schemaVersion":2},"datasetVersion":"2026-09-08T15:18:49.778Z"}