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frontendcritical PriorityID: svelte-5-runes-reactivity-snippets-invariants

Svelte 5 Runes: Reactive State Proxies, Derived Tracking, and Snippet Composition

Svelte 5 introduces fine-grained reactive proxies via runes ($state, $derived, $props, $effect) and replaces slots with explicit snippets. Legacy Svelte 3/4 reactivity paradigms ($:, export let, <slot />) and reactive destructuring violate signal graph propagation, creating broken reactive boundaries and hydration mismatches.

Prohibited Anti-Pattern: Destructuring Reactive Proxies, Using $effect for Synchronous Computations, and Mixing Legacy Svelte 4 Reactivity

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---
description: Svelte 5 Runes: Reactive State Proxies, Derived Tracking, and Snippet Composition - Svelte 5 introduces fine-grained reactive proxies via runes ($state, $derived, $props, $effect) and replaces slots with explicit snippets. Legacy Svelte 3/4 reactivity paradigms ($:, export let, <slot />) and reactive destructuring violate signal graph propagation, creating broken reactive boundaries and hydration mismatches.
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-snippets-invariants"
---

# Svelte 5 Runes: Reactive State Proxies, Derived Tracking, and Snippet Composition

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Destructuring Reactive Proxies, Using $effect for Synchronous Computations, and Mixing Legacy Svelte 4 Reactivity

## System Constraints
You are a Principal Software Architect enforcing Svelte 5 Runes and Snippets production standards. Mandate the strict separation of fine-grained reactive proxies from legacy syntax, ensure clean reactivity preservation when passing props or state across modules, enforce $derived over side-effect-driven mutations, and mandate type-safe snippets in place of legacy slots.

## Strict Guidelines
- Engineers MUST ALWAYS declare component parameters via $props() and type them with strict TypeScript interfaces or types.
- Engineers MUST NOT destructure reactive properties from $props() or $state() objects if two-way binding ($bindable()) or dynamic getter tracking is required across closures.
- Engineers MUST use $derived or $derived.by for all synchronous computations dependent on reactive state instead of synchronizing state inside $effect.
- Engineers MUST restrict $effect exclusively to external side effects, DOM integrations, or cleanup routines; $effect MUST NEVER mutate local state synchronously to prevent cascading renders.
- Engineers MUST replace all legacy slot elements (<slot />, <slot name="...">) with typed Snippet parameters and explicit {@render snippetName(args)} syntax.
- Engineers MUST use $state.raw() for large arrays, third-party class instances, or objects that do not require deep fine-grained proxy mutation tracking.
- Components NEVER use legacy Svelte 4 syntax including '$: ', 'export let', '<svelte:component>', or '<slot>' within Svelte 5 projects.

## Bad vs Good Implementations
### State Computation and Prop Passing via Runes and Snippets

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Mixing legacy 'export let' with runes triggers compiler deprecations and runtime ambiguity. Destructuring props or declaring non-rune variables breaks proxy tracking. Using $effect to synchronize 'doubled' creates unnecessary microtask scheduling, render cascades, and risks infinite loops.
```typescript
// Anti-pattern: Legacy syntax, destructuring props, mutating state in $effect
import type { Snippet } from 'svelte';

// Bad: Legacy export let and reactive declarations mixed with runes
export let initialCount: number = 0;
let count = initialCount;
let doubled: number;

// Bad: Cascading updates and infinite loop hazard using $effect for derivation
$effect(() => {
  doubled = count * 2;
});

function increment() {
  count += 1;
}

// Bad: Using legacy slot tags in markup
// <button onclick={increment}>{count} - {doubled}</button>
// <slot name="footer"></slot>
```

#### Production Standard (Correct)
> **Standard**: Props are cleanly declared with $props(), reactive state is encapsulated in a $state proxy, derivations are handled deterministically via $derived without $effect overhead, and component extension uses type-safe Snippets with {@render}.
```typescript
// Production Standard: Strict Svelte 5 Runes with $props, $state, $derived, and Snippets
import type { Snippet } from 'svelte';

interface CounterProps {
  initialCount?: number;
  multiplier?: number;
  header?: Snippet<[{ current: number }]>;
  footer?: Snippet;
}

// Good: Strict $props typing with defaults, maintaining property access
const { initialCount = 0, multiplier = 2, header, footer }: CounterProps = $props();

// Good: Fine-grained reactive proxy
let count = $state(initialCount);

// Good: Pure, synchronous, fine-grained derivation
const doubled = $derived(count * multiplier);

function increment(): void {
  count += 1;
}

// In markup:
// {#if header}
//   {@render header({ current: count })}
// {/if}
// <button type="button" onclick={increment}>Count: {count} | Doubled: {doubled}</button>
// {#if footer}
//   {@render footer()}
// {/if}
```
Recommended MCP Servers (mcp-serverlist.com)

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

Svelte Language Tools MCP
Provides AST inspection, rune type-checking diagnostics, and migration validation for Svelte 5 components.
Recommended Agent Skills (askills.dev)

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

Svelte Check CLI
Runs diagnostic type-checking across Svelte components to detect invalid rune usage and legacy syntax mixing.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Runes: Reactive State Proxies, Derived Tracking, and Snippet Composition

Svelte 5 Runes: Reactive State Proxies, Derived Tracking, and Snippet Composition defines an industrial-grade behavioral standard for frontend engineering. Svelte 5 introduces fine-grained reactive proxies via runes ($state, $derived, $props, $effect) and replaces slots with explicit snippets. Legacy Svelte 3/4 reactivity paradigms ($:, export let, <slot />) and reactive destructuring violate signal graph propagation, creating broken reactive boundaries and hydration mismatches. 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: Destructuring Reactive Proxies, Using $effect for Synchronous Computations, and Mixing Legacy Svelte 4 Reactivity 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 DriftDestructuring Reactive Proxies, Using $effect for Synchronous Computations, and Mixing Legacy Svelte 4 ReactivityCritical
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 declare component parameters via $props() and type them with strict TypeScript interfaces or types.

Engineers MUST ALWAYS declare component parameters via $props() and type them with strict TypeScript interfaces or types.

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 destructure reactive properties from $props() or $state() objects if two-way binding ($bindable()) or dynamic getter tracking is required across closures.

Engineers MUST NOT destructure reactive properties from $props() or $state() objects if two-way binding ($bindable()) or dynamic getter tracking is required across 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.
3

Engineers MUST use $derived or $derived.by for all synchronous computations dependent on reactive state instead of synchronizing state inside $effect.

Engineers MUST use $derived or $derived.by for all synchronous computations dependent on reactive state instead of synchronizing state 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.
4

Engineers MUST restrict $effect exclusively to external side effects, DOM integrations, or cleanup routines

Engineers MUST restrict $effect exclusively to external side effects, DOM integrations, or cleanup routines; $effect MUST NEVER mutate local state synchronously to prevent cascading renders.

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 replace all legacy slot elements (<slot />, <slot name="...">) with typed Snippet parameters and explicit {@render snippetName(args)} syntax.

Engineers MUST replace all legacy slot elements (<slot />, <slot name="...">) with typed Snippet parameters and explicit {@render snippetName(args)} syntax.

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 use $state.raw() for large arrays, third-party class instances, or objects that do not require deep fine-grained proxy mutation tracking.

Engineers MUST use $state.raw() for large arrays, third-party class instances, or objects that do not require deep fine-grained proxy mutation tracking.

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

Components NEVER use legacy Svelte 4 syntax including '$

Components NEVER use legacy Svelte 4 syntax including '$: ', 'export let', '<svelte:component>', or '<slot>' within Svelte 5 projects.

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: Reactive State Proxies, Derived Tracking, and Snippet Composition critical for AI coding assistants?

Svelte 5 introduces fine-grained reactive proxies via runes ($state, $derived, $props, $effect) and replaces slots with explicit snippets. Legacy Svelte 3/4 reactivity paradigms ($:, export let, <slot />) and reactive destructuring violate signal graph propagation, creating broken reactive boundaries and hydration mismatches. 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 (Destructuring Reactive Proxies, Using $effect for Synchronous Computations, and Mixing Legacy Svelte 4 Reactivity) 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

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