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Svelte 5 Runes SSR Reactive Context Isolation Standard

Enforces strict isolation of Svelte 5 runes ($state, $derived) and signal graphs to component subtrees and request boundaries during Server-Side Rendering (SSR). Prohibits module-level reactive singletons in shared modules to prevent cross-tenant state leaks and uncollected signal nodes across concurrent HTTP requests.

Prohibited Anti-Pattern: Module-Scope $state Singletons Causing Cross-Request Data Leaks in SSR

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---
description: Svelte 5 Runes SSR Reactive Context Isolation Standard - Enforces strict isolation of Svelte 5 runes ($state, $derived) and signal graphs to component subtrees and request boundaries during Server-Side Rendering (SSR). Prohibits module-level reactive singletons in shared modules to prevent cross-tenant state leaks and uncollected signal nodes across concurrent HTTP requests.
globs: src/**/*.svelte, src/**/*.svelte.ts, src/**/*.svelte.js, src/**/*.ts
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/svelte-5-runes-ssr-context-isolation"
---

# Svelte 5 Runes SSR Reactive Context Isolation Standard

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Module-Scope $state Singletons Causing Cross-Request Data Leaks in SSR

## System Constraints
You are a Principal Software Architect enforcing Svelte 5 Runes SSR Reactive Context Isolation. In SSR environments, Node/Bun runtimes persist module scopes across incoming HTTP requests. You must strictly forbid top-level module-scoped `$state` declarations in shared code and mandate request-isolated, context-bound reactive stores.

## Strict Guidelines
- Engineers MUST NEVER declare `$state` or `$derived` at the root module scope of `.svelte.ts` or `.svelte.js` files that are imported across SSR pipelines.
- Engineers MUST encapsulate shared reactive state within factory functions or classes that are instantiated per-request and injected via Svelte's `setContext` and `getContext` APIs.
- Components MUST consume context-driven state via typed getters or explicit context accessors initialized during component initialization (in `<script>` execution scope).
- Services MUST ensure that any manual reactive boundaries created with `$effect.root` outside components are deterministically disposed within the same request-response lifecycle.
- Code MUST use SvelteKit's `event.locals` or dedicated page data contracts for request-scoped identity and state passing, never cross-module reactive variables.
- Components NEVER store sensitive user data, auth credentials, or tenant metadata in static module closures.

## Bad vs Good Implementations
### Module-level state leak vs. Context-scoped state encapsulation

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Declaring `$state` at the top level of a module creates a single in-memory reactive object shared across all server requests. User A's items will be visible to and modified by User B on subsequent or concurrent SSR renders.
```typescript
// src/lib/stores/cart.svelte.ts
// FATAL: Module-scoped $state is evaluated once per Node.js process.
// All concurrent SSR requests share this exact state instance, leaking user cart data.
export const cart = $state({
  items: [] as string[],
  userId: null as string | null
});

export function addItem(item: string) {
  cart.items.push(item);
}
```

#### Production Standard (Correct)
> **Standard**: By encapsulating `$state` inside a class instantiated per-request inside `setContext` (typically in `+layout.svelte`), each SSR render cycle gets a completely isolated reactive graph that is automatically collected by the garbage collector after the request completes.
```typescript
// src/lib/stores/cart.svelte.ts
import { getContext, setContext } from 'svelte';

const CART_KEY = Symbol('CART_CONTEXT');

export class CartState {
  items = $state<string[]>([]);
  userId = $state<string | null>(null);

  constructor(initialUserId: string | null = null) {
    this.userId = initialUserId;
  }

  addItem(item: string): void {
    this.items.push(item);
  }
}

export function initCartContext(userId: string | null = null): CartState {
  const cart = new CartState(userId);
  setContext(CART_KEY, cart);
  return cart;
}

export function getCartContext(): CartState {
  const context = getContext<CartState>(CART_KEY);
  if (!context) {
    throw new Error('Cart context must be accessed within an initialized subtree.');
  }
  return context;
}
```

Recommended MCP Servers (mcp-serverlist.com)

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

Svelte AST Linter Server

Monitors AST nodes in `.svelte.ts` files to flag top-level rune declarations in server build targets.

Recommended Agent Skills (askills.dev)

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

Svelte SSR Check Runner

Executes svelte-check with strict SSR boundary analysis to catch shared reactive states.

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Runes SSR Reactive Context Isolation Standard

Svelte 5 Runes SSR Reactive Context Isolation Standard defines an industrial-grade behavioral standard for frontend engineering. Enforces strict isolation of Svelte 5 runes ($state, $derived) and signal graphs to component subtrees and request boundaries during Server-Side Rendering (SSR). Prohibits module-level reactive singletons in shared modules to prevent cross-tenant state leaks and uncollected signal nodes across concurrent HTTP requests. 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-Scope $state Singletons Causing Cross-Request Data Leaks in SSR 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	Module-Scope $state Singletons Causing Cross-Request Data Leaks in SSR	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:

1

Engineers MUST NEVER declare `$state` or `$derived` at the root module scope of `.svelte.ts` or `.svelte.js` files that are imported across SSR pipelines.

Engineers MUST NEVER declare `$state` or `$derived` at the root module scope of `.svelte.ts` or `.svelte.js` files that are imported across SSR pipelines.

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 factory functions or classes that are instantiated per-request and injected via Svelte's `setContext` and `getContext` APIs.

Engineers MUST encapsulate shared reactive state within factory functions or classes that are instantiated per-request and injected via Svelte's `setContext` and `getContext` APIs.

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 consume context-driven state via typed getters or explicit context accessors initialized during component initialization (in `<script>` execution scope).

Components MUST consume context-driven state via typed getters or explicit context accessors initialized during component initialization (in `<script>` execution scope).

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

Services MUST ensure that any manual reactive boundaries created with `$effect.root` outside components are deterministically disposed within the same request-response lifecycle.

Services MUST ensure that any manual reactive boundaries created with `$effect.root` outside components are deterministically disposed within the same request-response lifecycle.

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

Code MUST use SvelteKit's `event.locals` or dedicated page data contracts for request-scoped identity and state passing, never cross-module reactive variables.

Code MUST use SvelteKit's `event.locals` or dedicated page data contracts for request-scoped identity and state passing, never cross-module reactive variables.

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

Components NEVER store sensitive user data, auth credentials, or tenant metadata in static module closures.

Components NEVER store sensitive user data, auth credentials, or tenant metadata in static module closures.

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 SSR Reactive Context Isolation Standard critical for AI coding assistants?

Enforces strict isolation of Svelte 5 runes ($state, $derived) and signal graphs to component subtrees and request boundaries during Server-Side Rendering (SSR). Prohibits module-level reactive singletons in shared modules to prevent cross-tenant state leaks and uncollected signal nodes across concurrent HTTP requests. 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-Scope $state Singletons Causing Cross-Request Data Leaks in SSR) 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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