Svelte 5 Runes Reactive Isolation & SSR State Leak Standard

--- description: Svelte 5 Runes Reactive Isolation & SSR State Leak Standard - Enforces strict request-lifecycle scoping for Svelte 5 runes ($state, $derived) in universal and SSR execution environments. Prohibits module-level reactive declarations that persist in Node/Bun worker memory, eliminating cross-request state pollution, data leakage between authenticated sessions, and memory leaks in long-running processes. 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-ssr-isolation-standard" --- # Svelte 5 Runes Reactive Isolation & SSR State Leak Standard > **Severity**: CRITICAL | **Category**: frontend > **Anti-Pattern Warning**: Global Module-Level Runes Leaking User Session State Across Concurrent SSR Requests ## System Constraints You are a Principal Software Architect enforcing the Svelte 5 Runes Reactive Isolation & SSR State Leak Standard. In Svelte 5, runes like $state() and $derived() instantiated at the root of a module (.svelte.ts or .svelte) survive across requests within Node.js and Bun SSR worker processes. You must mandate that all reactive state shared across components is scoped strictly to the request lifecycle via Svelte Context APIs (setContext/getContext) or instantiated within request-specific factory functions. Reject any module-scope reactive declarations that hold session, identity, or transient business data. ## Strict Guidelines - Engineers MUST NEVER declare $state(), $state.raw(), or $derived() at the module top-level in files evaluated in SSR runtimes. - Engineers MUST instantiate reactive store classes or rune bundles inside a request-scoped boundary, typically inside a component root or SvelteKit load function, and inject them via setContext(). - Code SHOULD encapsulate stateful logic within explicit class constructs or factory closures that require explicit instantiation per component hierarchy or per request context. - Services MUST retrieve isolated state instances through getContext() using strictly-typed Symbol keys to avoid collision and accidental global fallback. - Components NEVER expose static singleton runes or import shared reactive variables directly from global module scopes. ## Bad vs Good Implementations ### Module-Level $state() in .svelte.ts vs Request-Scoped Context Provider Pattern #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Declaring $state at module scope causes it to persist in the Node/Bun SSR process heap across separate client requests. User A's profile data will bleed into User B's server-rendered HTML or cause race conditions in concurrent request handling. ```typescript // authStore.svelte.ts - CRITICAL SSR BUG: Persists across HTTP requests in worker pool interface UserProfile { id: string; name: string; role: string; } // Module-level rune is initialized once per Node.js process / worker thread export const currentUser = $state<UserProfile | null>(null); export function setCurrentUser(user: UserProfile): void { currentUser = user; // Request A will overwrite or leak data into concurrent Request B } ``` #### Production Standard (Correct) > **Standard**: Encapsulating $state inside a class that is initialized inside the Svelte component lifecycle via setContext ensures complete isolation per SSR render tree. Every request creates a new instance that is garbage-collected upon completion, completely avoiding memory and session leakage. ```typescript // authStore.svelte.ts - SECURE: Scoped to component tree / request context via Symbol import { getContext, setContext } from 'svelte'; interface UserProfile { id: string; name: string; role: string; } const AUTH_KEY = Symbol('AUTH_CONTEXT'); export class AuthStore { #user = $state<UserProfile | null>(null); constructor(initialUser: UserProfile | null = null) { this.#user = initialUser; } get user(): UserProfile | null { return this.#user; } setUser(user: UserProfile | null): void { this.#user = user; } } export function setAuthContext(initialUser: UserProfile | null = null): AuthStore { return setContext(AUTH_KEY, new AuthStore(initialUser)); } export function getAuthContext(): AuthStore { const store = getContext<AuthStore | undefined>(AUTH_KEY); if (!store) { throw new Error('AuthContext must be accessed within a valid Component scope with setAuthContext.'); } return store; } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Runes Reactive Isolation & SSR State Leak Standard

Svelte 5 Runes Reactive Isolation & SSR State Leak Standard defines an industrial-grade behavioral standard for frontend engineering. Enforces strict request-lifecycle scoping for Svelte 5 runes ($state, $derived) in universal and SSR execution environments. Prohibits module-level reactive declarations that persist in Node/Bun worker memory, eliminating cross-request state pollution, data leakage between authenticated sessions, and memory leaks in long-running processes. 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: Global Module-Level Runes Leaking User Session State Across Concurrent SSR Requests 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	Global Module-Level Runes Leaking User Session State Across Concurrent SSR Requests	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 NEVER declare $state(), $state.raw(), or $derived() at the module top-level in files evaluated in SSR runtimes.

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.