Svelte 5 Runes Async Dependency Isolation & Boundary Standard

--- description: Svelte 5 Runes Async Dependency Isolation & Boundary Standard - Enforces strict synchronous dependency extraction boundaries when integrating asynchronous operations with Svelte 5 runes. Prevents reactive graph cycle contamination, untracked post-await signal mutations, and race-condition memory leaks by enforcing AbortController-guarded resource primitives. globs: src/**/*.{svelte,svelte.ts,svelte.js,ts} alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/svelte5-runes-async-derived-dependency-isolation-standard" --- # Svelte 5 Runes Async Dependency Isolation & Boundary Standard > **Severity**: CRITICAL | **Category**: frontend > **Anti-Pattern Warning**: Post-Await Signal Dereferencing and Unbounded Async Execution in Rune Pipelines ## System Constraints You are a Principal Software Architect enforcing the Svelte 5 Runes Async Derived Dependency Isolation Standard. In Svelte 5, reactive tracking is strictly synchronous; reading reactive signals (`$state`, `$derived`) after an `await` boundary detaches from the current tracking context, causing silent tracking failures, out-of-order state overwrites, and memory leaks. You must ensure all reactive inputs are extracted synchronously before asynchronous dispatch, asynchronous pipelines are isolated in dedicated abortable resource patterns, and signals are never read across asynchronous microtask turns within `$effect` or derived workflows. ## Strict Guidelines - Engineers MUST ALWAYS capture all reactive `$state` and `$derived` dependencies synchronously at the top of an `$effect` or resource initializer before invoking asynchronous boundaries. - Engineers MUST NOT read reactive state variables following an `await` statement within an `$effect`, as Svelte 5 tracking context ceases across asynchronous gaps. - Code MUST ALWAYS implement an `AbortController` or cancellation token pattern inside async `$effect` scopes to discard stale in-flight promises and prevent race-condition state corruption. - Services MUST encapsulate asynchronous derived workflows into discrete Resource classes or composable functions using `$state` primitives instead of simulating async derivations via unbounded promises. - Components NEVER execute raw asynchronous side effects that mutate reactive state without explicit teardown registration through the `$effect` return cleanup callback. ## Bad vs Good Implementations ### Async Data Fetching with Reactive Dependency Isolation and Cancellation #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: The bad example initiates an untracked, uncancellable promise chain within an effect without an AbortController. If `this.query` changes rapidly, out-of-order promise resolutions cause race hazards where stale responses overwrite recent data. Furthermore, lack of synchronous capture makes tracking erratic across nested async turns. ```typescript // src/lib/search-resource.svelte.ts import { state } from './state'; export class SearchState { query = $state(''); results = $state<string[]>([]); isLoading = $state(false); constructor() { $effect(() => { // VIOLATION: Reading reactive state inside async callback and after await // VIOLATION: No race condition mitigation or AbortController cleanup if (!this.query) return; this.isLoading = true; fetch(`/api/search?q=${encodeURIComponent(this.query)}`) .then(res => res.json()) .then(data => { // Memory leak & race hazard: older query results can overwrite newer query results this.results = data.items; this.isLoading = false; }); }); } } ``` #### Production Standard (Correct) > **Standard**: Dependencies are synchronously captured into `currentQuery` before asynchronous execution begins. An `AbortController` cancels stale network requests when `this.query` updates, cleanly tearing down in-flight promises and preventing race conditions, post-await tracking contamination, and memory leaks. ```typescript // src/lib/search-resource.svelte.ts export class SearchResource { query = $state(''); #results = $state<string[]>([]); #isLoading = $state(false); #error = $state<Error | null>(null); constructor(initialQuery = '') { this.query = initialQuery; $effect(() => { // INVARIANT: Synchronously capture all reactive dependencies at the boundary const currentQuery = this.query.trim(); const controller = new AbortController(); if (!currentQuery) { this.#results = []; this.#isLoading = false; this.#error = null; return; } this.#isLoading = true; this.#error = null; (async () => { try { const response = await fetch(`/api/search?q=${encodeURIComponent(currentQuery)}`, { signal: controller.signal }); if (!response.ok) throw new Error(`HTTP error: ${response.status}`); const data = await response.json(); // INVARIANT: Verify cancellation state before updating signals if (!controller.signal.aborted) { this.#results = data.items; } } catch (err) { if (err instanceof DOMException && err.name === 'AbortError') return; if (!controller.signal.aborted) { this.#error = err instanceof Error ? err : new Error(String(err)); } } finally { if (!controller.signal.aborted) { this.#isLoading = false; } } })(); // INVARIANT: Clean up in-flight requests on dependency invalidation or teardown return () => { controller.abort(); }; }); } get results() { return this.#results; } get isLoading() { return this.#isLoading; } get error() { return this.#error; } } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Runes Async Dependency Isolation & Boundary Standard

Svelte 5 Runes Async Dependency Isolation & Boundary Standard defines an industrial-grade behavioral standard for frontend engineering. Enforces strict synchronous dependency extraction boundaries when integrating asynchronous operations with Svelte 5 runes. Prevents reactive graph cycle contamination, untracked post-await signal mutations, and race-condition memory leaks by enforcing AbortController-guarded resource primitives. 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: Post-Await Signal Dereferencing and Unbounded Async Execution in Rune Pipelines 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 Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Post-Await Signal Dereferencing and Unbounded Async Execution in Rune Pipelines	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 Inconsistency	AI 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:

Engineers MUST ALWAYS capture all reactive `$state` and `$derived` dependencies synchronously at the top of an `$effect` or resource initializer before invoking asynchronous 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.