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frontendcritical PriorityID: svelte-5-runes-reactivity-boundary-ownership-invariant

Svelte 5 Runes Reactivity Boundary & Ownership Invariant

Enforces strict state ownership and unidirectional data flow within Svelte 5 Runes architectures. Prohibits state mutation inside $effect runes and forbids leaking mutable reactive proxies across decoupled micro-frontend boundaries to eliminate infinite update loops and non-deterministic state synchronization.

Prohibited Anti-Pattern: Mutating $state inside $effect Runes and Leaking Mutable Reactive Proxies Across Micro-Frontend Boundaries

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---
description: Svelte 5 Runes Reactivity Boundary & Ownership Invariant - Enforces strict state ownership and unidirectional data flow within Svelte 5 Runes architectures. Prohibits state mutation inside $effect runes and forbids leaking mutable reactive proxies across decoupled micro-frontend boundaries to eliminate infinite update loops and non-deterministic state synchronization.
globs: src/**/*.{svelte,svelte.ts,ts}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/svelte-5-runes-reactivity-boundary-ownership-invariant"
---

# Svelte 5 Runes Reactivity Boundary & Ownership Invariant

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Mutating $state inside $effect Runes and Leaking Mutable Reactive Proxies Across Micro-Frontend Boundaries

## System Constraints
You are a Principal Software Architect enforcing Svelte 5 Runes Reactivity Boundaries. You must mandate strict state ownership semantics, disallow state mutations within $effect or $effect.pre blocks, enforce pure derivation via $derived/$derived.by, and mandate immutable state serialization across micro-frontend boundaries using $state.snapshot().

## Strict Guidelines
- Engineers MUST ALWAYS use $derived or $derived.by for values computed from other reactive state instead of synchronizing state via $effect.
- Engineers MUST NOT mutate any $state inside an $effect or $effect.pre block; side-effects MUST be strictly limited to external DOM manipulations, analytics, or logging.
- Engineers MUST serialize all state crosses micro-frontend boundaries or CustomEvent dispatches using $state.snapshot() to avoid leaking reactive runtime proxies.
- Code SHOULD encapsulate complex rune state within dedicated TypeScript domain model classes using private fields and explicit mutation methods rather than raw public $state.
- Components NEVER accept writable runes directly as props across micro-frontend boundaries; communication MUST be unidirectional via explicit callback functions or typed EventTarget messages.
- Services MUST cleanly dispose of cross-boundary event listeners and external subscriptions using the cleanup return function inside $effect.

## Bad vs Good Implementations
### State Derivation and Micro-Frontend Event Synchronization

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Mutating reactive state ('doubled') inside '$effect' forces unnecessary microtask scheduling, risks 'ERR_SVELTE_TOO_MANY_UPDATES' infinite loops, and leaking mutable proxy references to external micro-frontends violates boundary encapsulation.
```typescript
import { untrack } from 'svelte';

// ANTI-PATTERN: Using $effect to synchronize state and leaking raw proxy across MFE boundary
let count = $state(0);
let doubled = $state(0);

$effect(() => {
  // Mutating state within $effect causes cascading updates and circular render loops
  doubled = count * 2;
  
  // Leaking reactive proxy to external micro-frontend host
  window.dispatchEvent(new CustomEvent('mfe:count-updated', {
    detail: { count, doubled }
  }));
});

export function increment() {
  count++;
}
```

#### Production Standard (Correct)
> **Standard**: Uses '$derived' for zero-cost reactive computation with no cascading mutations. State is safely encapsulated inside a domain class and serialized across the MFE boundary with '$state.snapshot()' to prevent proxy leakage.
```typescript
import { untrack } from 'svelte';

// PRODUCTION PATTERN: Idiomatic $derived projection and snapshot-isolated boundary publishing
class CounterDomainService {
  #count = $state<number>(0);
  #doubled = $derived(this.#count * 2);

  public get count(): number { return this.#count; }
  public get doubled(): number { return this.#doubled; }

  public increment(): void {
    this.#count += 1;
  }
}

export const counterService = new CounterDomainService();

$effect(() => {
  // Accessing reactive property tracks it for this side-effect
  const payload = $state.snapshot({
    count: counterService.count,
    doubled: counterService.doubled
  });

  // Unidirectional dispatch of a plain, immutable snapshot across MFE boundary
  window.dispatchEvent(new CustomEvent('mfe:count-updated', { detail: payload }));

  return () => {
    // Ensure teardown if necessary
  };
});
```
Recommended MCP Servers (mcp-serverlist.com)

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

Svelte AST & Diagnostics Inspector
Validates Svelte 5 AST structures and rune semantics against runtime reactivity invariants.
Recommended Agent Skills (askills.dev)

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

Svelte Check CLI
Executes static type and reactivity validation on Svelte 5 rune components and modules.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Runes Reactivity Boundary & Ownership Invariant

Svelte 5 Runes Reactivity Boundary & Ownership Invariant defines an industrial-grade behavioral standard for frontend engineering. Enforces strict state ownership and unidirectional data flow within Svelte 5 Runes architectures. Prohibits state mutation inside $effect runes and forbids leaking mutable reactive proxies across decoupled micro-frontend boundaries to eliminate infinite update loops and non-deterministic state synchronization. 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: Mutating $state inside $effect Runes and Leaking Mutable Reactive Proxies Across Micro-Frontend 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 ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftMutating $state inside $effect Runes and Leaking Mutable Reactive Proxies Across Micro-Frontend BoundariesCritical
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 ALWAYS use $derived or $derived.by for values computed from other reactive state instead of synchronizing state via $effect.

Engineers MUST ALWAYS use $derived or $derived.by for values computed from other reactive state instead of synchronizing state via $effect.

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 NOT mutate any $state inside an $effect or $effect.pre block

Engineers MUST NOT mutate any $state inside an $effect or $effect.pre block; side-effects MUST be strictly limited to external DOM manipulations, analytics, or logging.

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

Engineers MUST serialize all state crosses micro-frontend boundaries or CustomEvent dispatches using $state.snapshot() to avoid leaking reactive runtime proxies.

Engineers MUST serialize all state crosses micro-frontend boundaries or CustomEvent dispatches using $state.snapshot() to avoid leaking reactive runtime proxies.

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

Code SHOULD encapsulate complex rune state within dedicated TypeScript domain model classes using private fields and explicit mutation methods rather than raw public $state.

Code SHOULD encapsulate complex rune state within dedicated TypeScript domain model classes using private fields and explicit mutation methods rather than raw public $state.

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

Components NEVER accept writable runes directly as props across micro-frontend boundaries

Components NEVER accept writable runes directly as props across micro-frontend boundaries; communication MUST be unidirectional via explicit callback functions or typed EventTarget messages.

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

Services MUST cleanly dispose of cross-boundary event listeners and external subscriptions using the cleanup return function inside $effect.

Services MUST cleanly dispose of cross-boundary event listeners and external subscriptions using the cleanup return function inside $effect.

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 Reactivity Boundary & Ownership Invariant critical for AI coding assistants?

Enforces strict state ownership and unidirectional data flow within Svelte 5 Runes architectures. Prohibits state mutation inside $effect runes and forbids leaking mutable reactive proxies across decoupled micro-frontend boundaries to eliminate infinite update loops and non-deterministic state synchronization. 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 (Mutating $state inside $effect Runes and Leaking Mutable Reactive Proxies Across Micro-Frontend Boundaries) 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.

Open Specification · CC BY 4.0·Free for Personal, Team & Commercial In-Repo Use

Citation & Open Source Attribution Guidelines

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