Svelte 5 Runes Cross-Island Reactive Boundary Isolation Standard

--- description: Svelte 5 Runes Cross-Island Reactive Boundary Isolation Standard - Enforces strict boundary encapsulation and lifecycle disposal for Svelte 5 reactive graphs operating across micro-frontend islands. Prevents cross-context memory leaks and uncontrolled cascade loops by prohibiting shared mutable $state proxies and uncontained reactive subscriptions across isolated runtime contexts. globs: src/**/*.{svelte,svelte.ts,svelte.js,ts,js} alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/svelte-5-runes-boundary-isolation-standard" --- # Svelte 5 Runes Cross-Island Reactive Boundary Isolation Standard > **Severity**: CRITICAL | **Category**: frontend > **Anti-Pattern Warning**: Leaking Raw Mutable $state Proxies and Detached $effect Subscriptions Across Micro-Frontend Island Boundaries ## System Constraints You are a Principal Engineer enforcing Svelte 5 Runes Reactive Graph Boundary Isolation Standard. Mandate explicit ownership semantics, boundary immutability, and deterministic disposal for reactive signals crossing micro-frontend or island boundaries. ## Strict Guidelines - Engineers MUST ALWAYS isolate Svelte 5 reactive signal graphs to the owning island's lifecycle using explicit boundary adaptors. - Engineers MUST NOT export or assign raw mutable `$state` proxy references to global scopes (e.g., `window`), shared registries, or foreign island props. - Engineers MUST ALWAYS serialize state crossings using `$state.snapshot()` or explicit immutable Data Transfer Objects (DTOs) before sending across boundary channels. - Cross-island reactive bridges MUST wrap external listeners inside an explicit `$effect.root()` and return an idempotent teardown function to prevent zombie reactive nodes. - Components NEVER bind `$bindable()` directly to foreign island models or non-owned micro-frontend references. ## Bad vs Good Implementations ### Cross-Island State Communication via Shared Mutable Proxy vs Isolated Event-Bridge with Snapshot #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Exporting raw $state instances across island boundaries creates cross-context reactive coupling. Island A's subscriber graph is kept alive by globalIslandState even after Island A unmounts, producing detached zombie effects and memory leaks. Direct cross-island mutation bypasses change governance, risking infinite feedback loops. ```typescript // shared-store.svelte.ts - Anti-pattern: Shared reactive proxy across island boundaries export const globalIslandState = $state({ selectedCustomerId: null as string | null, payload: {} as Record<string, unknown> }); // Island A (Consumer) // Registers an untracked effect against a foreign reactive object export function mountIslandA() { $effect(() => { // Leaks subscriber reference in globalIslandState when Island A is unmounted console.log('Customer changed:', globalIslandState.selectedCustomerId); }); } // Island B (Producer) export function updateCustomer(id: string) { // Triggers silent reactive cascade loops if Island A triggers back globalIslandState.selectedCustomerId = id; } ``` #### Production Standard (Correct) > **Standard**: State crossing the boundary is serialized as an immutable, non-reactive DTO via CustomEvent. The consuming island manages its own isolated reactive graph via an explicit $effect.root(), ensuring complete teardown on unmount without retaining foreign references or causing reactive graph cascades. ```typescript // island-bridge.ts - Production invariant: Event-driven isolation with deep snapshotting export interface CustomerPayload { readonly id: string; readonly timestamp: number; } // Strict boundary transport: Typed, serializable CustomEvents const DISPATCH_CHANNEL = 'mfe:customer:selection'; export function dispatchCustomerSelection(id: string): void { const payload: CustomerPayload = { id, timestamp: Date.now() }; window.dispatchEvent(new CustomEvent<CustomerPayload>(DISPATCH_CHANNEL, { detail: payload })); } // Island A Local Boundary Controller (Svelte 5 Context) export class IslandACustomerBridge { // Owned local island state private state = $state<{ activeId: string | null }>({ activeId: null }); private cleanupRoot: (() => void) | null = null; constructor() { // Enforce isolated reactive root with deterministic disposal this.cleanupRoot = $effect.root(() => { const handler = (event: Event) => { const customEvent = event as CustomEvent<CustomerPayload>; // Mutate ONLY island-owned state; isolate reactive dependency graph this.state.activeId = customEvent.detail.id; }; window.addEventListener(DISPATCH_CHANNEL, handler); return () => { window.removeEventListener(DISPATCH_CHANNEL, handler); }; }); } // Expose only immutable snapshots or read-only primitives get activeId(): string | null { return this.state.activeId; } public destroy(): void { if (this.cleanupRoot) { this.cleanupRoot(); this.cleanupRoot = null; } } } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Runes Cross-Island Reactive Boundary Isolation Standard

Svelte 5 Runes Cross-Island Reactive Boundary Isolation Standard defines an industrial-grade behavioral standard for frontend engineering. Enforces strict boundary encapsulation and lifecycle disposal for Svelte 5 reactive graphs operating across micro-frontend islands. Prevents cross-context memory leaks and uncontrolled cascade loops by prohibiting shared mutable $state proxies and uncontained reactive subscriptions across isolated runtime contexts. 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 Raw Mutable $state Proxies and Detached $effect Subscriptions Across Micro-Frontend Island 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 Raw Mutable $state Proxies and Detached $effect Subscriptions Across Micro-Frontend Island 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 isolate Svelte 5 reactive signal graphs to the owning island's lifecycle using explicit boundary adaptors.

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.