Svelte 5 Runes Reactive State, Snippets, and Effects Invariant Standard

--- description: Svelte 5 Runes Reactive State, Snippets, and Effects Invariant Standard - Enforces deterministic, fine-grained reactivity in Svelte 5 by strictly mandating universal runes syntax ($state, $derived, $effect, $props) and snippets over legacy Svelte 3/4 runtime paradigms. Prevents state desynchronization caused by proxy destructuring, impure derived calculations, and uncontrolled effect cycles. globs: src/**/*.{svelte,svelte.ts,svelte.js,ts,js} alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/svelte-5-runes-reactive-state-and-snippets-standard" --- # Svelte 5 Runes Reactive State, Snippets, and Effects Invariant Standard > **Severity**: CRITICAL | **Category**: frontend > **Anti-Pattern Warning**: Destructuring fine-grained reactive state proxies, abusing $effect for derived computations, and relying on legacy slot syntax. ## System Constraints You are a Principal Engineer enforcing Svelte 5 Runes Reactive State and Snippet architectures. Mandate fine-grained proxy invariants: reject legacy export let, $: reactive declarations, and <slot /> elements. Eliminate effect-driven state synchronization in favor of pure $derived runes, and guarantee proxy references remain intact by banning reactive object destructuring. ## Strict Guidelines - Engineers MUST ALWAYS declare reactive state using $state() or $state.raw() for non-mutated large collections, and declare derived values using $derived() or $derived.by() for multi-statement computations. - Engineers MUST NOT destructure properties directly from $state() proxy objects or $props(), as destructuring permanently strips reactivity; access properties directly or use getter functions. - Components NEVER utilize legacy Svelte 3/4 syntax including 'export let', '$: reactivity', or '<slot />'; engineers MUST migrate to $props() and {#snippet} blocks. - Code MUST NEVER synchronize state within $effect() or $effect.pre(); $effect runes are reserved exclusively for untracked DOM operations, external subscriptions, or third-party imperative APIs. - Services and shared stores MUST utilize universal .svelte.ts modules exposing fine-grained $state instances or explicit classes rather than legacy writable/readable store contracts. ## Bad vs Good Implementations ### State Derivation and Destructuring Preservation #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Relies on legacy compiler semantics ('export let' and '$:'), loses fine-grained reactivity by destructuring object properties from dynamic arrays, and utilizes deprecated slot projection. ```svelte <script lang="ts"> // ANTI-PATTERN: Legacy export let and $: reactivity export let items: { id: string; price: number; quantity: number }[] = []; let total = 0; // Anti-pattern: using reactive statement for derived state $: total = items.reduce((acc, item) => acc + item.price * item.quantity, 0); // Anti-pattern: destructuring kills fine-grained reactivity const { id, price } = items[0] ?? { id: '', price: 0 }; </script> <slot name="header" /> <div>Total: {total} - Primary: {id} ({price})</div> ``` #### Production Standard (Correct) > **Standard**: Adheres to Svelte 5 runes: declares typed $props(), isolates derived logic using $derived.by() without re-triggering mutation loops, uses {@render} snippets instead of slots, and maintains reactivity via direct property reads on derived accessors. ```svelte <script lang="ts"> import type { Snippet } from 'svelte'; interface Item { id: string; price: number; quantity: number; } interface Props { items: Item[]; header?: Snippet; } // INVARIANT: Pure runes with typed $props const { items, header }: Props = $props(); // INVARIANT: $derived.by() for functional computations without $effect side-effects const total = $derived.by(() => { return items.reduce((acc, item) => acc + item.price * item.quantity, 0); }); // INVARIANT: Preserving proxy reference access through derived accessors const firstItem = $derived(items[0]); </script> {#if header} {@render header()} {/if} <div> Total: {total} - Primary: {firstItem?.id ?? 'N/A'} ({firstItem?.price ?? 0}) </div> ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Runes Reactive State, Snippets, and Effects Invariant Standard

Svelte 5 Runes Reactive State, Snippets, and Effects Invariant Standard defines an industrial-grade behavioral standard for frontend engineering. Enforces deterministic, fine-grained reactivity in Svelte 5 by strictly mandating universal runes syntax ($state, $derived, $effect, $props) and snippets over legacy Svelte 3/4 runtime paradigms. Prevents state desynchronization caused by proxy destructuring, impure derived calculations, and uncontrolled effect cycles. 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: Destructuring fine-grained reactive state proxies, abusing $effect for derived computations, and relying on legacy 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 Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Destructuring fine-grained reactive state proxies, abusing $effect for derived computations, and relying on legacy slot syntax.	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 declare reactive state using $state() or $state.raw() for non-mutated large collections, and declare derived values using $derived() or $derived.by() for multi-statement computations.

Engineers MUST ALWAYS declare reactive state using $state() or $state.raw() for non-mutated large collections, and declare derived values using $derived() or $derived.by() for multi-statement computations.

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.