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frontendcritical PriorityID: svelte-5-runes-async-effect-isolation

Svelte 5 Runes Async Boundary & Effect Teardown Isolation

Enforces synchronous dependency tracking and explicit cancellation isolation for asynchronous operations inside Svelte 5 $effect runes. In Svelte 5, reactive signal reads after an await boundary lose tracking, and async callbacks return Promises that drop cleanup teardowns. This standard guarantees lifecycle-safe asynchronous task execution and prevents orphaned listeners during client-side route transitions.

Prohibited Anti-Pattern: Asynchronous Effect Cleanup Loss and Post-Await Reactivity Tracking Leaks

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---
description: Svelte 5 Runes Async Boundary & Effect Teardown Isolation - Enforces synchronous dependency tracking and explicit cancellation isolation for asynchronous operations inside Svelte 5 $effect runes. In Svelte 5, reactive signal reads after an await boundary lose tracking, and async callbacks return Promises that drop cleanup teardowns. This standard guarantees lifecycle-safe asynchronous task execution and prevents orphaned listeners during client-side route transitions.
globs: src/**/*.svelte, src/**/*.svelte.ts, src/**/*.svelte.js, src/**/*.ts
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/svelte-5-runes-async-effect-isolation"
---

# Svelte 5 Runes Async Boundary & Effect Teardown Isolation

> **Severity**: CRITICAL | **Category**: frontend
> **Anti-Pattern Warning**: Asynchronous Effect Cleanup Loss and Post-Await Reactivity Tracking Leaks

## System Constraints
You are a Principal Software Architect enforcing Svelte 5 Runes Async State Disconnect and Effect Lifecycle Isolation. In Svelte 5, $effect tracks dependencies synchronously; any state accessed across an 'await' boundary is disconnected from the reactivity graph. Furthermore, passing an async arrow function to $effect returns a Promise, completely bypassing Svelte's teardown mechanism. Mandate rigorous separation of synchronous dependency reads, synchronous teardown registration, and cancellation-aware async execution.

## Strict Guidelines
- Engineers MUST NEVER pass an 'async' callback directly to '$effect' or '$effect.pre'; asynchronous functions return a Promise, causing Svelte to drop effect cleanup functions.
- Engineers MUST register teardown logic synchronously by returning a cleanup function directly from the outer synchronous '$effect' block.
- Engineers MUST read all reactive dependencies ($state, $derived, props) synchronously at the top of the '$effect' prior to triggering any asynchronous invocation or Promise resolution.
- Engineers MUST pass an 'AbortController.signal' or explicit cancellation sentinel to asynchronous operations to prevent microtask state updates after component unmount or route transitions.
- Engineers MUST guard asynchronous writes to '$state' variables against aborted or superseded task runs.
- Code SHOULD isolate long-running async side-effects into reusable '.svelte.ts' state controllers with explicit disposers rather than burying nested Promises in component script roots.
- Components NEVER assume reactivity triggers after an 'await' keyword inside helper closures executed within an effect context.

## Bad vs Good Implementations
### Async Effect Cleanup Trap vs. Isolated Abortable Task

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: In Svelte 5, $effect expects either void or a synchronous teardown function. An async callback returns a Promise; Svelte ignores this return value, causing memory leaks, dangling listeners, and race conditions where late-resolving promises overwrite updated state.
```typescript
import { fetchUserData } from './api';

let { userId } = $props<{ userId: string }>();
let userData = $state<UserData | null>(null);
let error = $state<string | null>(null);

// ANTI-PATTERN: Passing an async function directly to $effect.
// 1. Returns a Promise, discarding the cleanup return function.
// 2. userId accessed post-await would not be tracked (if moved below).
// 3. Stale network responses can resolve after route navigation and overwrite state.
$effect(async () => {
  try {
    const data = await fetchUserData(userId);
    userData = data;
  } catch (err: any) {
    error = err.message;
  }

  return () => {
    // THIS CLEANUP NEVER RUNS in Svelte 5 because the effect returns a Promise!
    console.log('Cleanup dropped');
  };
});
```

#### Production Standard (Correct)
> **Standard**: Dependencies are read synchronously before any micro-task boundary. The cleanup function is returned synchronously to the runtime, ensuring that if the route changes or dependencies update, ongoing requests are aborted immediately and unmounted components do not execute state mutations.
```typescript
import { fetchUserData } from './api';
import type { UserData } from './types';

let { userId } = $props<{ userId: string }>();
let userData = $state<UserData | null>(null);
let error = $state<string | null>(null);

// CORRECT: Synchronous effect with immediate dependency registration,
// cancellation token passing, and synchronous cleanup return.
$effect(() => {
  // 1. Synchronously capture dependencies into local scope
  const currentUserId = userId;
  const controller = new AbortController();

  async function executeFetch() {
    try {
      const data = await fetchUserData(currentUserId, { signal: controller.signal });
      if (controller.signal.aborted) return;
      userData = data;
      error = null;
    } catch (err: unknown) {
      if (controller.signal.aborted) return;
      error = err instanceof Error ? err.message : 'Unknown error';
    }
  }

  void executeFetch();

  // 2. Synchronous teardown returned directly to Svelte
  return () => {
    controller.abort();
  };
});
```
Recommended MCP Servers (mcp-serverlist.com)

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

Svelte Language Tools MCP
Validates Svelte 5 runes syntax, compiler warnings, and effect lifecycle constraints.
Recommended Agent Skills (askills.dev)

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

svelte-check
CLI diagnostic tool for detecting invalid rune usage, bad effect signatures, and reactivity traps in Svelte projects.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Runes Async Boundary & Effect Teardown Isolation

Svelte 5 Runes Async Boundary & Effect Teardown Isolation defines an industrial-grade behavioral standard for frontend engineering. Enforces synchronous dependency tracking and explicit cancellation isolation for asynchronous operations inside Svelte 5 $effect runes. In Svelte 5, reactive signal reads after an await boundary lose tracking, and async callbacks return Promises that drop cleanup teardowns. This standard guarantees lifecycle-safe asynchronous task execution and prevents orphaned listeners during client-side route transitions. 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: Asynchronous Effect Cleanup Loss and Post-Await Reactivity Tracking Leaks 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 DriftAsynchronous Effect Cleanup Loss and Post-Await Reactivity Tracking LeaksCritical
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 NEVER pass an 'async' callback directly to '$effect' or '$effect.pre'

Engineers MUST NEVER pass an 'async' callback directly to '$effect' or '$effect.pre'; asynchronous functions return a Promise, causing Svelte to drop effect cleanup 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.
2

Engineers MUST register teardown logic synchronously by returning a cleanup function directly from the outer synchronous '$effect' block.

Engineers MUST register teardown logic synchronously by returning a cleanup function directly from the outer synchronous '$effect' block.

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 read all reactive dependencies ($state, $derived, props) synchronously at the top of the '$effect' prior to triggering any asynchronous invocation or Promise resolution.

Engineers MUST read all reactive dependencies ($state, $derived, props) synchronously at the top of the '$effect' prior to triggering any asynchronous invocation or Promise resolution.

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 pass an 'AbortController.signal' or explicit cancellation sentinel to asynchronous operations to prevent microtask state updates after component unmount or route transitions.

Engineers MUST pass an 'AbortController.signal' or explicit cancellation sentinel to asynchronous operations to prevent microtask state updates after component unmount or route transitions.

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 guard asynchronous writes to '$state' variables against aborted or superseded task runs.

Engineers MUST guard asynchronous writes to '$state' variables against aborted or superseded task runs.

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

Code SHOULD isolate long-running async side-effects into reusable '.svelte.ts' state controllers with explicit disposers rather than burying nested Promises in component script roots.

Code SHOULD isolate long-running async side-effects into reusable '.svelte.ts' state controllers with explicit disposers rather than burying nested Promises in component script roots.

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 assume reactivity triggers after an 'await' keyword inside helper closures executed within an effect context.

Components NEVER assume reactivity triggers after an 'await' keyword inside helper closures executed within an effect context.

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 Async Boundary & Effect Teardown Isolation critical for AI coding assistants?

Enforces synchronous dependency tracking and explicit cancellation isolation for asynchronous operations inside Svelte 5 $effect runes. In Svelte 5, reactive signal reads after an await boundary lose tracking, and async callbacks return Promises that drop cleanup teardowns. This standard guarantees lifecycle-safe asynchronous task execution and prevents orphaned listeners during client-side route transitions. 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 (Asynchronous Effect Cleanup Loss and Post-Await Reactivity Tracking Leaks) 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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