AgentRules.org
RulesStacksClientsAuditorPrunerSelf-HealRFCsBenchmarkCLICI/CDAuthorsSubmit
This specification is maintained by the AgentRules Working Group. Have enhancements or want to claim maintainership?Submit Specification →
frontendcritical PriorityID: svelte-5-runes-reactivity

Svelte 5 Runes ($state, $derived, $effect) & Legacy Reactivity Elimination

Production standard for Svelte 5. Mandates using modern Runes ($state, $derived, $effect) and strictly eliminates deprecated Svelte 3/4 reactive declarations ($: variable = ...) and mutable let bindings.

Prohibited Anti-Pattern: AI writes deprecated $: reactive declarations or let count = 0 inside Svelte 5 components, failing to trigger the new signals-based reactivity engine.

Export Ready-to-Drop Configuration

Select format below to copy code or download file directly.

---
description: Svelte 5 Runes ($state, $derived, $effect) & Legacy Reactivity Elimination - Production standard for Svelte 5. Mandates using modern Runes ($state, $derived, $effect) and strictly eliminates deprecated Svelte 3/4 reactive declarations ($: variable = ...) and mutable let bindings.
globs: **/*.svelte, src/**/*.svelte.ts
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/svelte-5-runes-reactivity"
---

# Svelte 5 Runes ($state, $derived, $effect) & Legacy Reactivity Elimination

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: AI writes deprecated $: reactive declarations or let count = 0 inside Svelte 5 components, failing to trigger the new signals-based reactivity engine.

## System Constraints
You are a Svelte 5 Principal Frontend Architect. In Svelte 5, never use Svelte 3/4 legacy reactivity ($: label = expr) or plain let declarations for reactive state. Always use modern Runes: $state() for reactive values, $derived() for computed values, and $effect() for side-effects.

## Strict Guidelines
- MUST declare reactive state using let state = $state(initialValue) instead of plain let.
- MUST derive computed properties using let double = $derived(count * 2) instead of $: double = count * 2.
- MUST isolate asynchronous or DOM-sync side-effects inside $effect(() => { ... }) with explicit cleanup returns.
- MUST NOT mutate props directly; use $props() destructuring with bindable declarations where two-way binding is intended.

## Bad vs Good Implementations
### Migrating Reactive Counters from Legacy $: to Svelte 5 Runes

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: In Svelte 5, plain let bindings are non-reactive, and the $: syntax is obsolete and unoptimized for the new signals reactivity engine.
```svelte
<!-- FATAL: Deprecated Svelte 4 reactivity in Svelte 5 project -->
<script>
  let count = 0; // Not reactive in Svelte 5!
  $: doubled = count * 2; // DEPRECATED: Legacy label syntax
  
  function increment() {
    count += 1;
  }
</script>

<button on:click={increment}>{doubled}</button>
```

#### Production Standard (Correct)
> **Standard**: Uses $state and $derived runes to hook into the fine-grained signals graph. Renders at sub-microsecond DOM reconciliation speeds.
```svelte
<!-- Production Standard: Svelte 5 Signals-Driven Runes -->
<script lang="ts">
  let count = $state(0);
  let doubled = $derived(count * 2);

  function increment() {
    count += 1;
  }

  $effect(() => {
    // Clean side-effect logging with auto-tracked dependencies
    console.log(`Current count: ${count}`);
  });
</script>

<button onclick={increment}>
  Count: {count} (Doubled: {doubled})
</button>
```
Recommended MCP Servers (mcp-serverlist.com)

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

Recommended Agent Skills (askills.dev)

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

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Runes ($state, $derived, $effect) & Legacy Reactivity Elimination

Svelte 5 Runes ($state, $derived, $effect) & Legacy Reactivity Elimination defines an industrial-grade behavioral standard for frontend engineering. Production standard for Svelte 5. Mandates using modern Runes ($state, $derived, $effect) and strictly eliminates deprecated Svelte 3/4 reactive declarations ($: variable = ...) and mutable let bindings. 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: AI writes deprecated $: reactive declarations or let count = 0 inside Svelte 5 components, failing to trigger the new signals-based reactivity engine. 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 DriftAI writes deprecated $: reactive declarations or let count = 0 inside Svelte 5 components, failing to trigger the new signals-based reactivity engine.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

MUST declare reactive state using let state = $state(initialValue) instead of plain let.

MUST declare reactive state using let state = $state(initialValue) instead of plain 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

MUST derive computed properties using let double = $derived(count * 2) instead of $

MUST derive computed properties using let double = $derived(count * 2) instead of $: double = count * 2.

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

MUST isolate asynchronous or DOM-sync side-effects inside $effect(() => { ... }) with explicit cleanup returns.

MUST isolate asynchronous or DOM-sync side-effects inside $effect(() => { ... }) with explicit cleanup returns.

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

MUST NOT mutate props directly

MUST NOT mutate props directly; use $props() destructuring with bindable declarations where two-way binding is intended.

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, $derived, $effect) & Legacy Reactivity Elimination critical for AI coding assistants?

Production standard for Svelte 5. Mandates using modern Runes ($state, $derived, $effect) and strictly eliminates deprecated Svelte 3/4 reactive declarations ($: variable = ...) and mutable let bindings. 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 (AI writes deprecated $: reactive declarations or let count = 0 inside Svelte 5 components, failing to trigger the new signals-based reactivity engine.) 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.

Related FRONTEND Rules

View all rules →
frontend
119

SolidStart 1.0 Server Functions & Async Boundaries Production Standard

SolidStart 1.0 relies on the Vinxi bundler to split `server$` closures into independent HTTP POST RPC endpoints. Direct lexical closure over client signals within `server$` scopes or failure to demarcate async boundaries with `<Suspense>` causes silent serialization failures, client-bundle secret leaks, and SSR hydration mismatches. Enforcing strict parameter passing, runtime input validation, and Solid async primitives maintains security and fine-grained reactivity across network boundaries.

frontend
119

Astro 5 Actions & Content Layer Production Standard

Enforces strict type safety and architectural correctness for Astro 5 Server Actions and the zero-waterfall Content Layer API. Prevents runtime contract mismatches, hydration mismatches, and unhandled mutation exceptions across dynamic and prerendered boundaries. Failure to follow these patterns leads to silent data fetching waterfalls, build-time cache corruptions, and unhandled server action exceptions.

frontend
51

Nuxt 4 Nitro Server Engine & Shared Directory Production Standard

Nuxt 4 enforces strict boundary segregation between client UI in the app/ directory, backend endpoints in server/, and universal contracts in shared/. Violating cross-boundary isolation or maintaining module-scoped state in Nitro server handlers causes catastrophic cross-request data corruption and hydration mismatches under concurrent production traffic. Architects must enforce request-isolated H3 event contexts, universal serialization schemas, and strict path-alias boundaries.

1 Rule in Stack

Svelte