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frontendcritical PriorityID: svelte-5-runes-reactive-state-guardrails

Svelte 5 Fine-Grained Reactivity and Snippet Architecture Standard

Mandates strict adherence to Svelte 5 runes architecture ($state, $derived, $props, $effect, and snippets), strictly prohibiting legacy Svelte 3/4 patterns ($:, export let, <slot>). Enforces precise reactive dependency tracking, prevents infinite render loops from effect-driven derivations, and ensures snippet-based component composition.

Prohibited Anti-Pattern: Mutating State Inside Effects and Retaining Legacy Svelte 3/4 Reactive Declarations ($:, export let, slots)

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---
description: Svelte 5 Fine-Grained Reactivity and Snippet Architecture Standard - Mandates strict adherence to Svelte 5 runes architecture ($state, $derived, $props, $effect, and snippets), strictly prohibiting legacy Svelte 3/4 patterns ($:, export let, <slot>). Enforces precise reactive dependency tracking, prevents infinite render loops from effect-driven derivations, and ensures snippet-based component composition.
globs: src/**/*.{svelte,svelte.ts,svelte.js,ts}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/svelte-5-runes-reactive-state-guardrails"
---

# Svelte 5 Fine-Grained Reactivity and Snippet Architecture Standard

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Mutating State Inside Effects and Retaining Legacy Svelte 3/4 Reactive Declarations ($:, export let, slots)

## System Constraints
You are a Principal Software Architect enforcing Svelte 5 Runes architecture. You must mandate fine-grained reactivity invariants using $state, $derived, $props, and $effect. Reject all legacy Svelte 3/4 syntax including '$:', 'export let', and '<slot />'. Enforce zero-effect derivations, strict signal hygiene, and type-safe snippet renders.

## Strict Guidelines
- Engineers MUST ALWAYS use $state() or $state.raw() for mutable reactive state instead of bare 'let' declarations.
- Engineers MUST ALWAYS use $derived() or $derived.by() for values computed from other state; NEVER synchronize state via $effect().
- Engineers MUST ALWAYS declare component parameters via the $props() rune, utilizing TypeScript interfaces with Snippet types for layout slots.
- Engineers MUST NOT use legacy Svelte 3/4 syntax, specifically '$:', 'export let', '$$props', '$$slots', or '<slot />' in modern components.
- Engineers MUST NOT mutate state synchronously inside $effect() unless handling explicit bidirectional sync with non-reactive third-party DOM APIs.
- Components NEVER expose bare slot fallbacks; Snippets MUST be invoked via '{@render snippetName(args)}' with explicit type guards or optional chaining if optional.
- Code SHOULD leverage $state.raw() for large immutable structures, arrays, or external class instances where deep proxying induces performance overhead.
- Services and Shared State modules MUST use '.svelte.ts' file extensions to host module-level runes such as $state or $derived outside .svelte component boundaries.

## Bad vs Good Implementations
### Migrating Derived Logic from $effect to $derived.by

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Using $effect to update intermediate state variables forces multiple microtask render passes, risks infinite cascading re-renders, and breaks fine-grained dependency graph resolution.
```typescript
// Bad: Using $effect to synchronize dependent calculations causing multi-pass render loops
let count = $state(0);
let double = $state(0);
let status = $state('even');

$effect(() => {
  // Anti-pattern: Synchronizing derived state within an effect
  double = count * 2;
  status = double % 2 === 0 ? 'even' : 'odd';
});
```

#### Production Standard (Correct)
> **Standard**: $derived and $derived.by establish pure reactive nodes that resolve synchronously without unnecessary DOM render ticks or state desynchronization.
```typescript
// Good: Pure, synchronous fine-grained derivation using $derived and $derived.by
let count = $state(0);
const double = $derived(count * 2);
const status = $derived.by(() => {
  return double % 2 === 0 ? 'even' : 'odd';
});
```

### Migrating Legacy Props and Slots to $props() and Snippets

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Legacy 'export let' and '<slot>' mechanisms cannot participate in Svelte 5 fine-grained signal propagation and break strict compile-time type validation.
```typescript
// Bad: Svelte 3/4 legacy export let and slot architecture in Svelte 5
// <script lang="ts">
//   export let title: string;
//   export let disabled: boolean = false;
// </script>
// <div class="card">
//   <h2>{title}</h2>
//   <slot name="icon" />
//   <slot />
// </div>
export {};
```

#### Production Standard (Correct)
> **Standard**: $props() cleanly destructures properties with default fallbacks, and typed Snippet instances provide type-checked arguments to child layout blocks via @render.
```typescript
// Good: Svelte 5 type-safe $props() contract with typed Snippets
import type { Snippet } from 'svelte';

interface CardProps {
  title: string;
  disabled?: boolean;
  icon?: Snippet;
  children: Snippet<[{ active: boolean }]>;
}

const { title, disabled = false, icon, children }: CardProps = $props();

// Template usage:
// <div class="card" class:disabled>
//   <h2>{title}</h2>
//   {#if icon}{@render icon()}{/if}
//   {@render children({ active: !disabled })}
// </div>
```
Recommended MCP Servers (mcp-serverlist.com)

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

Svelte Language Server MCP
Provides live diagnostics, rune validation, and compiler diagnostics for Svelte 5 codebases.
Recommended Agent Skills (askills.dev)

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

Svelte Check CLI
Performs static type-checking and compiler verification on Svelte components to catch rune misuse.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Fine-Grained Reactivity and Snippet Architecture Standard

Svelte 5 Fine-Grained Reactivity and Snippet Architecture Standard defines an industrial-grade behavioral standard for frontend engineering. Mandates strict adherence to Svelte 5 runes architecture ($state, $derived, $props, $effect, and snippets), strictly prohibiting legacy Svelte 3/4 patterns ($:, export let, <slot>). Enforces precise reactive dependency tracking, prevents infinite render loops from effect-driven derivations, and ensures snippet-based component composition. 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 Effects and Retaining Legacy Svelte 3/4 Reactive Declarations ($:, export let, slots) 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 Effects and Retaining Legacy Svelte 3/4 Reactive Declarations ($:, export let, slots)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 $state() or $state.raw() for mutable reactive state instead of bare 'let' declarations.

Engineers MUST ALWAYS use $state() or $state.raw() for mutable reactive state instead of bare 'let' declarations.

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 $derived() or $derived.by() for values computed from other state

Engineers MUST ALWAYS use $derived() or $derived.by() for values computed from other state; NEVER synchronize 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.
3

Engineers MUST ALWAYS declare component parameters via the $props() rune, utilizing TypeScript interfaces with Snippet types for layout slots.

Engineers MUST ALWAYS declare component parameters via the $props() rune, utilizing TypeScript interfaces with Snippet types for layout slots.

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 use legacy Svelte 3/4 syntax, specifically '$

Engineers MUST NOT use legacy Svelte 3/4 syntax, specifically '$:', 'export let', '$$props', '$$slots', or '<slot />' in modern components.

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 mutate state synchronously inside $effect() unless handling explicit bidirectional sync with non-reactive third-party DOM APIs.

Engineers MUST NOT mutate state synchronously inside $effect() unless handling explicit bidirectional sync with non-reactive third-party DOM 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.
6

Components NEVER expose bare slot fallbacks

Components NEVER expose bare slot fallbacks; Snippets MUST be invoked via '{@render snippetName(args)}' with explicit type guards or optional chaining if optional.

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

Code SHOULD leverage $state.raw() for large immutable structures, arrays, or external class instances where deep proxying induces performance overhead.

Code SHOULD leverage $state.raw() for large immutable structures, arrays, or external class instances where deep proxying induces performance overhead.

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

Services and Shared State modules MUST use '.svelte.ts' file extensions to host module-level runes such as $state or $derived outside .svelte component boundaries.

Services and Shared State modules MUST use '.svelte.ts' file extensions to host module-level runes such as $state or $derived outside .svelte component 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 Fine-Grained Reactivity and Snippet Architecture Standard critical for AI coding assistants?

Mandates strict adherence to Svelte 5 runes architecture ($state, $derived, $props, $effect, and snippets), strictly prohibiting legacy Svelte 3/4 patterns ($:, export let, <slot>). Enforces precise reactive dependency tracking, prevents infinite render loops from effect-driven derivations, and ensures snippet-based component composition. 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 Effects and Retaining Legacy Svelte 3/4 Reactive Declarations ($:, export let, slots)) 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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