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frontendcritical PriorityID: svelte-5-runes-ssr-cross-request-leak-prevention

Svelte 5 Runes Cross-Request SSR Scope Isolation Standard

Enforces strict request-level boundary isolation for Svelte 5 runes ($state, $derived) during Server-Side Rendering (SSR) and asynchronous streaming. Prohibits module-level reactive rune singletons that persist in Node/worker process memory across concurrent HTTP requests, preventing cross-user data contamination and race conditions.

Prohibited Anti-Pattern: Module-Level Rune State Instantiation in SSR Contexts Leaking Cross-Request Data

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---
description: Svelte 5 Runes Cross-Request SSR Scope Isolation Standard - Enforces strict request-level boundary isolation for Svelte 5 runes ($state, $derived) during Server-Side Rendering (SSR) and asynchronous streaming. Prohibits module-level reactive rune singletons that persist in Node/worker process memory across concurrent HTTP requests, preventing cross-user data contamination and race conditions.
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-cross-request-leak-prevention"
---

# Svelte 5 Runes Cross-Request SSR Scope Isolation Standard

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Module-Level Rune State Instantiation in SSR Contexts Leaking Cross-Request Data

## System Constraints
You are a Principal Software Architect enforcing zero-leakage concurrency rules for Svelte 5 streaming SSR. Svelte 5 runes ($state, $derived) instantiated at the module level in `.svelte.ts` files reside in process memory shared across all concurrent server requests. You must mandate that all mutable reactive state is strictly encapsulated within per-request component context (setContext/getContext) or initialized within request-scoped lifecycle factories. Never allow module-level rune exports holding tenant, session, or user state.

## Strict Guidelines
- Engineers MUST NEVER declare top-level, module-scoped `$state` or `$derived` variables in `.svelte.ts` or `.ts` files intended for SSR execution.
- Engineers MUST encapsulate shared reactive state within classes or factory functions that are instantiated exclusively inside Svelte component initialization lifecycles or SvelteKit load boundaries.
- Components MUST propagate request-scoped rune instances down the component tree using Svelte's `setContext` and `getContext` APIs with strictly-typed symbols.
- Engineers MUST NOT mutate module-level singletons within asynchronous streaming resolvers or unawaited promises during SSR rendering.
- Services handling per-request dynamic data MUST receive state explicitly via component props or SvelteKit `event.locals`, never via imported mutable global runes.
- Code SHOULD enforce immutability or factory patterns for any utility runes shared across client and server boundaries.

## Bad vs Good Implementations
### Module-Level Rune Singleton vs Context-Scoped State Factory

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: In Node.js or edge runtimes, the module is evaluated once. `currentSession` is shared across all concurrent HTTP requests. If Request A sets the session and awaits an asynchronous streaming promise, Request B can mutate `currentSession`, causing Request A to stream sensitive tenant data belonging to Request B.
```typescript
// src/lib/stores/user-session.svelte.ts
// VULNERABILITY: Module-level rune is a process-wide singleton during SSR.
// Concurrent requests will overwrite and read each other's session state.
export interface SessionData {
  userId: string | null;
  tenantId: string | null;
}

export const currentSession = $state<SessionData>({
  userId: null,
  tenantId: null
});

export function setSession(userId: string, tenantId: string) {
  currentSession.userId = userId;
  currentSession.tenantId = tenantId;
}
```

#### Production Standard (Correct)
> **Standard**: By encapsulating the runes in a class and injecting it via Svelte's `setContext` during component tree initialization (e.g., in `+layout.svelte`), each SSR request obtains an isolated instance tied uniquely to that request's component tree. Process-level memory leakage across concurrent requests is impossible.
```typescript
// src/lib/stores/user-session.svelte.ts
import { getContext, setContext } from 'svelte';

export interface SessionData {
  userId: string | null;
  tenantId: string | null;
}

export class UserSession {
  userId = $state<string | null>(null);
  tenantId = $state<string | null>(null);

  constructor(initialData?: SessionData) {
    if (initialData) {
      this.userId = initialData.userId;
      this.tenantId = initialData.tenantId;
    }
  }
}

const SESSION_KEY = Symbol('USER_SESSION');

export function setSessionContext(initialData?: SessionData): UserSession {
  const session = new UserSession(initialData);
  setContext(SESSION_KEY, session);
  return session;
}

export function getSessionContext(): UserSession {
  const session = getContext<UserSession>(SESSION_KEY);
  if (!session) {
    throw new Error('UserSession context not initialized in component tree.');
  }
  return session;
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Svelte AST Context Linter
Static AST analyzer verifying rune placement and preventing module-scope reactive variables in Svelte 5 projects.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

svelte-check
Diagnostic CLI tool for verifying Svelte 5 typing, runes usage, and compilation safety across SSR builds.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Runes Cross-Request SSR Scope Isolation Standard

Svelte 5 Runes Cross-Request SSR Scope Isolation Standard defines an industrial-grade behavioral standard for frontend engineering. Enforces strict request-level boundary isolation for Svelte 5 runes ($state, $derived) during Server-Side Rendering (SSR) and asynchronous streaming. Prohibits module-level reactive rune singletons that persist in Node/worker process memory across concurrent HTTP requests, preventing cross-user data contamination and race conditions. 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: Module-Level Rune State Instantiation in SSR Contexts Leaking Cross-Request Data 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 ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftModule-Level Rune State Instantiation in SSR Contexts Leaking Cross-Request DataCritical
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 InconsistencyAI 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:

1

Engineers MUST NEVER declare top-level, module-scoped `$state` or `$derived` variables in `.svelte.ts` or `.ts` files intended for SSR execution.

Engineers MUST NEVER declare top-level, module-scoped `$state` or `$derived` variables in `.svelte.ts` or `.ts` files intended for SSR execution.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Engineers MUST encapsulate shared reactive state within classes or factory functions that are instantiated exclusively inside Svelte component initialization lifecycles or SvelteKit load boundaries.

Engineers MUST encapsulate shared reactive state within classes or factory functions that are instantiated exclusively inside Svelte component initialization lifecycles or SvelteKit load boundaries.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Components MUST propagate request-scoped rune instances down the component tree using Svelte's `setContext` and `getContext` APIs with strictly-typed symbols.

Components MUST propagate request-scoped rune instances down the component tree using Svelte's `setContext` and `getContext` APIs with strictly-typed symbols.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

Engineers MUST NOT mutate module-level singletons within asynchronous streaming resolvers or unawaited promises during SSR rendering.

Engineers MUST NOT mutate module-level singletons within asynchronous streaming resolvers or unawaited promises during SSR rendering.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
5

Services handling per-request dynamic data MUST receive state explicitly via component props or SvelteKit `event.locals`, never via imported mutable global runes.

Services handling per-request dynamic data MUST receive state explicitly via component props or SvelteKit `event.locals`, never via imported mutable global runes.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
6

Code SHOULD enforce immutability or factory patterns for any utility runes shared across client and server boundaries.

Code SHOULD enforce immutability or factory patterns for any utility runes shared across client and server boundaries.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Svelte 5 Runes Cross-Request SSR Scope Isolation Standard critical for AI coding assistants?

Enforces strict request-level boundary isolation for Svelte 5 runes ($state, $derived) during Server-Side Rendering (SSR) and asynchronous streaming. Prohibits module-level reactive rune singletons that persist in Node/worker process memory across concurrent HTTP requests, preventing cross-user data contamination and race conditions. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Module-Level Rune State Instantiation in SSR Contexts Leaking Cross-Request Data) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

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Citation & Open Source Attribution Guidelines

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