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

Svelte 5 Runes State Management, Pure Derivations, and Snippet Invariants

Enforces Svelte 5 fine-grained reactivity architecture using runes ($state, $derived, $effect, $props) and snippets. Prevents legacy Svelte 3/4 reactive declaration syntax, anti-pattern state synchronization via effects, and slot-based composition leaks in production systems.

Prohibited Anti-Pattern: Mutating State Inside $effect for Computed Values and Retaining Legacy Let/Slot Syntax

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---
description: Svelte 5 Runes State Management, Pure Derivations, and Snippet Invariants - Enforces Svelte 5 fine-grained reactivity architecture using runes ($state, $derived, $effect, $props) and snippets. Prevents legacy Svelte 3/4 reactive declaration syntax, anti-pattern state synchronization via effects, and slot-based composition leaks in production systems.
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-guardrails"
---

# Svelte 5 Runes State Management, Pure Derivations, and Snippet Invariants

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Mutating State Inside $effect for Computed Values and Retaining Legacy Let/Slot Syntax

## System Constraints
You are a Principal Frontend Architect enforcing modern Svelte 5 runes architecture. You must ruthlessly eradicate legacy Svelte 3/4 reactivity paradigms (e.g., `let:`, `export let`, `<slot />`, `$:` reactive statements) and enforce fine-grained proxy invariants. Ensure `$effect` is reserved strictly for real DOM or external asynchronous side-effects, `$derived` is used for all computed state, and snippets replace legacy slot projection.

## Strict Guidelines
- Engineers MUST ALWAYS declare component inputs via `let { ... }: Props = $props()` and internal reactive state via `$state()` or `$state.raw()`.
- Engineers MUST NOT use `$effect` to synchronize or transform state; all computed dependencies MUST be declared using `$derived()` or `$derived.by()`.
- Engineers MUST NEVER mutate reactive `$state` within a synchronous `$effect` block that reads that same state, preventing cyclical re-render cascades.
- Components NEVER use `<slot />` or `let:binding` syntax; Engineers MUST ALWAYS implement reusable template blocks using `{#snippet name(params)}` and invoke them with `{@render name(args)}`.
- Shared state in external files MUST reside in `.svelte.ts` or `.svelte.js` files using encapsulated classes or factory closures containing `$state` fields, ensuring isolation during SSR contexts.
- Large immutable data sets or foreign DOM instances MUST use `$state.raw()` to avoid unnecessary deep proxy wrapping overhead.

## Bad vs Good Implementations
### State Derivation and Component Composition with Runes and Snippets

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Uses legacy `export let` syntax instead of `$props()`. Misuses `$effect` to synchronously synchronize state, causing unnecessary multi-pass render micro-cycles and potential infinite loops.
```typescript
// Bad: Svelte 3/4 legacy syntax with effect-driven state synchronization
export let items: string[] = [];
export let filter = '';

let filteredItems: string[] = [];

// ANTI-PATTERN: Using $effect as a replacement for reactive $: assignments
$effect(() => {
  filteredItems = items.filter((item) => item.includes(filter));
});

function addItem(item: string) {
  items = [...items, item];
}
```

#### Production Standard (Correct)
> **Standard**: Replaces legacy bindings with `$props()`, uses `$derived.by` for pure synchronous computation without render thrashing, and leverages Svelte 5 proxy mutation semantics.
```typescript
// Good: Svelte 5 idiomatic runes with pure derivations and snippet props
interface Props {
  initialItems?: string[];
  filter?: string;
  itemSnippet?: import('svelte').Snippet<[string]>;
}

let { initialItems = [], filter = '', itemSnippet }: Props = $props();

// Invariant: Deeply reactive array state with pure derivation
let items = $state<string[]>([...initialItems]);
const filteredItems = $derived.by(() => {
  const query = filter.toLowerCase().trim();
  return query ? items.filter((i) => i.toLowerCase().includes(query)) : items;
});

function addItem(item: string): void {
  items.push(item); // Invariant: fine-grained proxy supports in-place mutation
}
```
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 reporting, and Svelte 5 rune migration assistance.
Recommended Agent Skills (askills.dev)

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

svelte-check
Command-line type checking and diagnostic validator for Svelte components and rune usages.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Runes State Management, Pure Derivations, and Snippet Invariants

Svelte 5 Runes State Management, Pure Derivations, and Snippet Invariants defines an industrial-grade behavioral standard for frontend engineering. Enforces Svelte 5 fine-grained reactivity architecture using runes ($state, $derived, $effect, $props) and snippets. Prevents legacy Svelte 3/4 reactive declaration syntax, anti-pattern state synchronization via effects, and slot-based composition leaks in production systems. 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 for Computed Values and Retaining Legacy Let/Slot Syntax 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 for Computed Values and Retaining Legacy Let/Slot SyntaxCritical
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 inputs via `let { ... }

Engineers MUST ALWAYS declare component inputs via `let { ... }: Props = $props()` and internal reactive state via `$state()` or `$state.raw()`.

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 use `$effect` to synchronize or transform state

Engineers MUST NOT use `$effect` to synchronize or transform state; all computed dependencies MUST be declared using `$derived()` or `$derived.by()`.

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 NEVER mutate reactive `$state` within a synchronous `$effect` block that reads that same state, preventing cyclical re-render cascades.

Engineers MUST NEVER mutate reactive `$state` within a synchronous `$effect` block that reads that same state, preventing cyclical re-render cascades.

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

Components NEVER use `<slot />` or `let

Components NEVER use `<slot />` or `let:binding` syntax; Engineers MUST ALWAYS implement reusable template blocks using `{#snippet name(params)}` and invoke them with `{@render name(args)}`.

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

Shared state in external files MUST reside in `.svelte.ts` or `.svelte.js` files using encapsulated classes or factory closures containing `$state` fields, ensuring isolation during SSR contexts.

Shared state in external files MUST reside in `.svelte.ts` or `.svelte.js` files using encapsulated classes or factory closures containing `$state` fields, ensuring isolation during SSR contexts.

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

Large immutable data sets or foreign DOM instances MUST use `$state.raw()` to avoid unnecessary deep proxy wrapping overhead.

Large immutable data sets or foreign DOM instances MUST use `$state.raw()` to avoid unnecessary deep proxy wrapping 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.
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 State Management, Pure Derivations, and Snippet Invariants critical for AI coding assistants?

Enforces Svelte 5 fine-grained reactivity architecture using runes ($state, $derived, $effect, $props) and snippets. Prevents legacy Svelte 3/4 reactive declaration syntax, anti-pattern state synchronization via effects, and slot-based composition leaks in production systems. 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 for Computed Values and Retaining Legacy Let/Slot Syntax) 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

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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