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frontendcritical PriorityID: svelte5-runes-reactivity-snippets-invariant

Svelte 5 Runes Reactivity, State Encapsulation, and Snippets Invariant

Enforces deterministic state synchronization and composability in Svelte 5 using the runes reactive engine ($state, $derived, $props, $effect) and snippets. Prohibits mixing legacy Svelte 3/4 reactivity paradigms (e.g., export let, $:, <slot />) and prevents reactivity breakage caused by primitive destructuring or mutating derived values.

Prohibited Anti-Pattern: Mixing legacy Svelte 3/4 reactivity with Svelte 5 runes and breaking reactive proxies via primitive destructuring or derived mutations.

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---
description: Svelte 5 Runes Reactivity, State Encapsulation, and Snippets Invariant - Enforces deterministic state synchronization and composability in Svelte 5 using the runes reactive engine ($state, $derived, $props, $effect) and snippets. Prohibits mixing legacy Svelte 3/4 reactivity paradigms (e.g., export let, $:, <slot />) and prevents reactivity breakage caused by primitive destructuring or mutating derived values.
globs: src/**/*.{svelte,svelte.ts,svelte.js}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/svelte5-runes-reactivity-snippets-invariant"
---

# Svelte 5 Runes Reactivity, State Encapsulation, and Snippets Invariant

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Mixing legacy Svelte 3/4 reactivity with Svelte 5 runes and breaking reactive proxies via primitive destructuring or derived mutations.

## System Constraints
You are a Principal Software Architect enforcing Svelte 5 Runes and modern template paradigms. Svelte 5 introduces deep reactive proxies via $state, functional derivations via $derived, unidirectional contract interfaces via $props, and template lambdas via snippets. You must reject legacy Svelte 3/4 syntax including 'export let', '$:', and '<slot />'. Ensure agents adhere to reactivity boundaries, prevent infinite dependency cycles in $effect, avoid premature destructuring of primitive proxy properties, and cleanly separate pure derivations from external side effects.

## Strict Guidelines
- Engineers MUST ALWAYS use $props() to declare component inputs and NEVER use legacy 'export let'.
- Engineers MUST ALWAYS use {#snippet name(args)} and {@render name(args)} instead of legacy <slot /> or let:directives.
- Engineers MUST ALWAYS compute derived states using pure functions via $derived(expression) or $derived.by(() => block) with zero side effects.
- Engineers MUST NOT mutate values produced by $derived or assign to read-only derived proxies.
- Engineers MUST NOT destructure primitive properties directly out of $state objects if downstream consumers require two-way binding or continuous reactive updates; access them by property path or pass accessor functions.
- Engineers MUST NOT trigger state mutations inside $effect blocks that read the same state synchronously, preventing cyclical execution loops.
- Engineers MUST use .svelte.ts or .svelte.js file extensions when creating shared non-component reactive stores powered by runes.
- Engineers MUST clean up subscriptions, DOM listeners, and asynchronous tasks using return teardown functions inside $effect.

## Bad vs Good Implementations
### Legacy Props & Reactive Declarations vs. Svelte 5 $props & $derived

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Uses deprecated 'export let', legacy '$:' reactive declarations with hazardous self-mutating side effects, and legacy <slot /> syntax which is incompatible with modern Svelte 5 runes architecture.
```svelte
<script lang="ts">
  // ANTI-PATTERN: Svelte 3/4 legacy syntax mixed into Svelte 5 project
  export let count: number = 0;
  export let multiplier: number = 2;

  // Legacy reactive statement causing implicit dependency tracking and mutation risks
  $: doubled = count * multiplier;
  $: if (doubled > 100) {
    count = 0; // Reactive cycle hazard
  }
</script>

<button on:click={() => count++}>
  <slot name="label">Default Count:</slot> {doubled}
</button>
```

#### Production Standard (Correct)
> **Standard**: Implements modern Svelte 5 idioms: $props for inputs, $state for local mutation, $derived for pure synchronous calculations, $effect for bounded side effects, and snippet rendering instead of slots.
```svelte
<script lang="ts">
  import type { Snippet } from 'svelte';

  interface Props {
    count?: number;
    multiplier?: number;
    label?: Snippet<[{ value: number }]>;
  }

  let { count = 0, multiplier = 2, label }: Props = $props();
  let internalCount = $state(count);

  // Pure reactive calculation without side-effect cascades
  const doubled = $derived(internalCount * multiplier);

  $effect(() => {
    if (doubled > 100) {
      internalCount = 0;
    }
  });
</script>

<button onclick={() => internalCount++}>
  {#if label}
    {@render label({ value: doubled })}
  {:else}
    Default Count: {doubled}
  {/if}
</button>
```

### Destructuring Reactive State in External Modules

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Destructuring state.count extracts the primitive number at initial execution time. Consumers reading cart.count will never observe updates when state.count increments.
```typescript
// ANTI-PATTERN: cart.svelte.ts breaks reactivity via destructuring
export function createCart() {
  const state = $state({ items: [] as string[], count: 0 });

  // Destructuring primitive 'count' strips its reactive proxy connection
  const { count, items } = state;

  function addItem(item: string) {
    items.push(item);
    state.count++;
  }

  return { count, items, addItem };
}
```

#### Production Standard (Correct)
> **Standard**: Exporting explicit getters maintains fine-grained reactive read access to the $state proxy across boundaries without disconnecting reactive tracking.
```typescript
// PRODUCTION FIX: cart.svelte.ts preserving reactive getters and proxy references
export function createCart() {
  const state = $state({ items: [] as string[], count: 0 });

  function addItem(item: string) {
    state.items.push(item);
    state.count++;
  }

  return {
    get count() { return state.count; },
    get items() { return state.items; },
    addItem
  };
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

Svelte Language Server MCP
Provides AST inspection, diagnostic validation, and automated runes verification for Svelte 5 codebases.
Recommended Agent Skills (askills.dev)

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

Svelte 5 Migration CLI
CLI utility to automatically upgrade legacy Svelte components to modern runes and snippet templates.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Runes Reactivity, State Encapsulation, and Snippets Invariant

Svelte 5 Runes Reactivity, State Encapsulation, and Snippets Invariant defines an industrial-grade behavioral standard for frontend engineering. Enforces deterministic state synchronization and composability in Svelte 5 using the runes reactive engine ($state, $derived, $props, $effect) and snippets. Prohibits mixing legacy Svelte 3/4 reactivity paradigms (e.g., export let, $:, <slot />) and prevents reactivity breakage caused by primitive destructuring or mutating derived values. 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: Mixing legacy Svelte 3/4 reactivity with Svelte 5 runes and breaking reactive proxies via primitive destructuring or derived mutations. 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 DriftMixing legacy Svelte 3/4 reactivity with Svelte 5 runes and breaking reactive proxies via primitive destructuring or derived mutations.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 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 $props() to declare component inputs and NEVER use legacy 'export let'.

Engineers MUST ALWAYS use $props() to declare component inputs and NEVER use legacy 'export let'.

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 ALWAYS use {#snippet name(args)} and {@render name(args)} instead of legacy <slot /> or let

Engineers MUST ALWAYS use {#snippet name(args)} and {@render name(args)} instead of legacy <slot /> or let:directives.

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 ALWAYS compute derived states using pure functions via $derived(expression) or $derived.by(() => block) with zero side effects.

Engineers MUST ALWAYS compute derived states using pure functions via $derived(expression) or $derived.by(() => block) with zero side effects.

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 values produced by $derived or assign to read-only derived proxies.

Engineers MUST NOT mutate values produced by $derived or assign to read-only derived 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.
5

Engineers MUST NOT destructure primitive properties directly out of $state objects if downstream consumers require two-way binding or continuous reactive updates

Engineers MUST NOT destructure primitive properties directly out of $state objects if downstream consumers require two-way binding or continuous reactive updates; access them by property path or pass accessor functions.

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

Engineers MUST NOT trigger state mutations inside $effect blocks that read the same state synchronously, preventing cyclical execution loops.

Engineers MUST NOT trigger state mutations inside $effect blocks that read the same state synchronously, preventing cyclical execution loops.

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.
7

Engineers MUST use .svelte.ts or .svelte.js file extensions when creating shared non-component reactive stores powered by runes.

Engineers MUST use .svelte.ts or .svelte.js file extensions when creating shared non-component reactive stores powered by 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.
8

Engineers MUST clean up subscriptions, DOM listeners, and asynchronous tasks using return teardown functions inside $effect.

Engineers MUST clean up subscriptions, DOM listeners, and asynchronous tasks using return teardown functions 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, State Encapsulation, and Snippets Invariant critical for AI coding assistants?

Enforces deterministic state synchronization and composability in Svelte 5 using the runes reactive engine ($state, $derived, $props, $effect) and snippets. Prohibits mixing legacy Svelte 3/4 reactivity paradigms (e.g., export let, $:, <slot />) and prevents reactivity breakage caused by primitive destructuring or mutating derived values. 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 (Mixing legacy Svelte 3/4 reactivity with Svelte 5 runes and breaking reactive proxies via primitive destructuring or derived mutations.) 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

This specification is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are completely free to adapt and share this rule in your workspace. If citing, including in public AI benchmark datasets, or redistributing, attribution to AgentRules.org is required.

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