Svelte 5 Runes Universal Reactivity & Lifecycle Boundary Isolation

--- description: Svelte 5 Runes Universal Reactivity & Lifecycle Boundary Isolation - Enforces strict lifecycle boundaries and explicit effect disposal when crossing component contexts with universal runes (.svelte.ts). Prevents memory leaks caused by unbounded $effect.root scopes, detached signal listeners, and stale closure traps when reactive state instances escape Svelte component lifecycle trees into global or shared singletons. globs: src/**/*.{svelte,svelte.ts,ts} alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/svelte5-runes-lifecycle-teardown-guardrail" --- # Svelte 5 Runes Universal Reactivity & Lifecycle Boundary Isolation > **Severity**: CRITICAL | **Category**: frontend > **Anti-Pattern Warning**: Leaking Unbounded Universal Runes and Stale Closures Across Component Mount Boundaries ## System Constraints You are a Principal Engineer enforcing Svelte 5 Runes Universal Reactivity & Lifecycle Boundary Isolation. Mandate strict adherence to reactive lifecycle domains: never initialize side-effecting universal runes outside $effect.root in shared modules, mandate deterministic teardown returns from $effect, and prevent stale closure retention in long-lived stateful classes across component unmounts. ## Strict Guidelines - Engineers MUST ALWAYS wrap reactive root subscriptions in `$effect.root()` when instantiating reactive runes inside standalone TypeScript modules (`.svelte.ts`) outside component component tree lifecycles. - Engineers MUST ALWAYS return an explicit teardown cleanup function from `$effect` and `$effect.pre` whenever registering event listeners, timers, DOM observers, or external stream subscriptions. - Engineers MUST NOT instantiate module-level `$state` objects that retain references to component-scoped callbacks or DOM nodes, avoiding garbage collection prevention. - Code SHOULD pass state explicitly using `getContext` and `setContext` rather than importing global mutable `$state` singletons across heterogeneous component routes. - Components NEVER capture un-tracked snapshot state into long-lived event handler closures; engineers MUST derive dynamic state via `$derived` or access the latest `$state` proxy directly at invocation time. ## Bad vs Good Implementations ### Cross-Boundary Module Subscription Teardown Contract #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Event listener is registered directly on the window without lifecycle binding or explicit teardown. When multiple consumers instantiate ViewportTracker across navigations, listeners accumulate indefinitely causing memory leaks. ```typescript // src/lib/stores/viewport.svelte.ts // ANTI-PATTERN: Leaks effect root and leaves window listener detached upon route navigation export class ViewportTracker { width = $state(typeof window !== 'undefined' ? window.innerWidth : 0); constructor() { if (typeof window !== 'undefined') { // $effect cannot be declared bare in a standard class constructor without lifecycle boundary window.addEventListener('resize', () => { this.width = window.innerWidth; }); } } } ``` #### Production Standard (Correct) > **Standard**: Encapsulates reactive observation within `$effect.root`, maintaining an explicit teardown handle. Consumers can invoke `destroy()` or bind it to `onDestroy`/unmount lifecycles to ensure total cleanup. ```typescript // src/lib/stores/viewport.svelte.ts // PRODUCTION FIX: Explicit effect lifecycle root isolation with symmetric disposal contract export class ViewportTracker { #width = $state(0); #cleanup: (() => void) | null = null; constructor() { if (typeof window === 'undefined') return; this.#width = window.innerWidth; this.#cleanup = $effect.root(() => { const onResize = () => { this.#width = window.innerWidth; }; window.addEventListener('resize', onResize, { passive: true }); return () => { window.removeEventListener('resize', onResize); }; }); } get width(): number { return this.#width; } destroy(): void { this.#cleanup?.(); this.#cleanup = null; } } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Runes Universal Reactivity & Lifecycle Boundary Isolation

Svelte 5 Runes Universal Reactivity & Lifecycle Boundary Isolation defines an industrial-grade behavioral standard for frontend engineering. Enforces strict lifecycle boundaries and explicit effect disposal when crossing component contexts with universal runes (.svelte.ts). Prevents memory leaks caused by unbounded $effect.root scopes, detached signal listeners, and stale closure traps when reactive state instances escape Svelte component lifecycle trees into global or shared singletons. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Leaking Unbounded Universal Runes and Stale Closures Across Component Mount Boundaries Without an explicit system instruction file, developers spend hours debugging silent failures.

Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Leaking Unbounded Universal Runes and Stale Closures Across Component Mount Boundaries	Critical	Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production. Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture Inconsistency	AI assistants produce mixed paradigms across files, creating fragmented codebase conventions.	Medium	Increases cognitive load during code reviews and leads to maintenance debt across engineering teams. Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.

Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

Engineers MUST ALWAYS wrap reactive root subscriptions in `$effect.root()` when instantiating reactive runes inside standalone TypeScript modules (`.svelte.ts`) outside component component tree lifecycles.

Engineers MUST ALWAYS wrap reactive root subscriptions in `$effect.root()` when instantiating reactive runes inside standalone TypeScript modules (`.svelte.ts`) outside component component tree lifecycles.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.