Svelte 5 Universal Runes Reactive State and Micro-Frontend Boundary Guardrail

--- description: Svelte 5 Universal Runes Reactive State and Micro-Frontend Boundary Guardrail - Enforces fine-grained signal reactivity via Svelte 5 universal runes ($state, $derived, $effect) across module boundaries and micro-frontend envelopes. Guarantees that internal proxy-wrapped state mutations remain encapsulated and are bridged to external foreign frameworks (React, Vue, Web Components) through explicit snapshots or readonly getter protocols rather than leaking raw reactive proxies across boundaries. globs: src/**/*.{svelte,svelte.ts,svelte.js,ts} alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/svelte-5-universal-runes-microfrontend-contract" --- # Svelte 5 Universal Runes Reactive State and Micro-Frontend Boundary Guardrail > **Severity**: CRITICAL | **Category**: frontend > **Anti-Pattern Warning**: Leaking Svelte 5 Deep Reactive Proxies Across Framework Boundaries and Bypassing Universal Module Contracts ## System Constraints You are a Principal Software Architect enforcing Svelte 5 Universal Runes Reactive State Architecture. Ensure all shared reactive state leverages universal `.svelte.ts` files, signal proxies are properly encapsulated, and boundary crossing into micro-frontends or non-Svelte targets uses `$state.snapshot` or explicit pub/sub event emitters to avoid cross-realm proxy desynchronization and memory leaks. ## Strict Guidelines - Engineers MUST ALWAYS house shared reactive state in files ending with `.svelte.ts` or `.svelte.js` to enable the Svelte compiler's rune transformation pipeline outside of single-file components. - Engineers MUST NOT expose raw `$state` proxy references directly across micro-frontend boundaries or external event buses; use `$state.snapshot()` to decouple consumer environments. - Code SHOULD encapsulate `$state` mutations within explicit model/store class methods or closures rather than allowing external callers to directly mutate deep proxy properties across domains. - Services MUST prefer `$derived` and `$derived.by` over side-effectful `$effect` synchronizations for pure transactional state derivations. - Components NEVER instantiate untracked global `$state` directly within module bodies without scoping to a root-managed lifecycle or context when running in multi-tenant micro-frontend sandboxes. ## Bad vs Good Implementations ### Universal Reactive State Encapsulation with Cross-MFE Bridge #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Exporting raw objects transformed via $state inside ordinary .ts files bypasses universal rune transforms or creates leaky reactive proxies that can be mutated outside Svelte's reactive graph, crashing foreign micro-frontends without a VDOM reconciler. ```typescript // state.ts (Missing .svelte.ts extension, raw proxy exported, cross-boundary mutation leak) export const sharedState = { user: { name: 'Alice', authenticated: false } }; // Inside a Svelte 5 component let state = $state(sharedState); // Leaking internal proxy to external micro-frontend window.dispatchEvent(new CustomEvent('mfe:user-sync', { detail: state // CRITICAL: Leaks internal reactive proxy; consumers mutating this will cause unhandled re-entrancy })); ``` #### Production Standard (Correct) > **Standard**: Uses the universal `.svelte.ts` extension, encapsulates internal proxy state using private class fields, and relies on `$state.snapshot()` to emit non-reactive, clean POJOs to cross-framework micro-frontend consumers. ```typescript // user-session.svelte.ts (Universal Rune Module) export class UserSessionStore { #user = $state({ name: 'Alice', authenticated: false }); // Readonly accessor for local Svelte consumers get user() { return this.#user; } // Immutable snapshot bridge for foreign micro-frontends get snapshot() { return $state.snapshot(this.#user); } updateName(newName: string): void { this.#user.name = newName; // Explicit boundary broadcast with pure POJO window.dispatchEvent(new CustomEvent('mfe:user-sync', { detail: this.snapshot })); } } export const sessionStore = new UserSessionStore(); ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Universal Runes Reactive State and Micro-Frontend Boundary Guardrail

Svelte 5 Universal Runes Reactive State and Micro-Frontend Boundary Guardrail defines an industrial-grade behavioral standard for frontend engineering. Enforces fine-grained signal reactivity via Svelte 5 universal runes ($state, $derived, $effect) across module boundaries and micro-frontend envelopes. Guarantees that internal proxy-wrapped state mutations remain encapsulated and are bridged to external foreign frameworks (React, Vue, Web Components) through explicit snapshots or readonly getter protocols rather than leaking raw reactive proxies across boundaries. 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: Leaking Svelte 5 Deep Reactive Proxies Across Framework Boundaries and Bypassing Universal Module Contracts 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	Leaking Svelte 5 Deep Reactive Proxies Across Framework Boundaries and Bypassing Universal Module Contracts	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 house shared reactive state in files ending with `.svelte.ts` or `.svelte.js` to enable the Svelte compiler's rune transformation pipeline outside of single-file components.

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.