Svelte 5 Rune-Derived Streaming Hydration Boundary Standard

--- description: Svelte 5 Rune-Derived Streaming Hydration Boundary Standard - Enforces deterministic client reconciliation boundaries for asynchronous and derived rune primitives during progressive HTML streaming. Prevents client-side hydration mismatches, DOM tearing, and microtask race conditions caused by evaluating unstable $derived expressions against streamed promise payloads. globs: src/**/*.svelte, src/**/*.svelte.ts, src/**/*.svelte.js alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/svelte-5-rune-streaming-hydration-boundary-standard" --- # Svelte 5 Rune-Derived Streaming Hydration Boundary Standard > **Severity**: CRITICAL | **Category**: frontend > **Anti-Pattern Warning**: Directly evaluating raw streamed promises inside synchronous $derived runes without hydration boundary guards ## System Constraints You are a Principal Frontend Architect enforcing deterministic hydration and streaming invariants in Svelte 5. You must eliminate DOM tearing and client-server reconciliation mismatches caused by asynchronous streamed data interacting with synchronous Svelte 5 runes ($derived, $derived.by). Mandate strict boundary isolation between unresolved promises and synchronous derived reactive state. ## Strict Guidelines - Engineers MUST ALWAYS isolate streamed, deferred promises behind an {#await} template boundary or an explicit reactive stream container before binding to $derived runes. - Engineers MUST NOT execute synchronous property access or transformations via $derived or $derived.by on raw Promise instances received from load functions. - Components NEVER render intermediate derived state on the client that contradicts the initial SSR chunk flushed over the wire. - Code MUST initialize $state wrappers for streamed data with deterministic fallback structures matching the exact SSR payload schema. - Services MUST leverage Svelte 5's untrack() when reading non-reactive streaming markers during component mounting to prevent hydration thrashing. - Engineers MUST ensure that any client-side reconciliation fallback explicitly mirrors the deferred skeleton structure emitted by server-side streaming. ## Bad vs Good Implementations ### Streaming Hydration Invariant: Raw Deferred Promise vs. Hydration Boundary Resource #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Accessing properties directly on a streamed promise inside a synchronous $derived.by rune causes the client to evaluate undefined properties during hydration, resulting in an immediate hydration mismatch exception, DOM tearing, and flash of empty content. ```typescript import type { PageData } from './$types'; // ANTI-PATTERN: Direct synchronous $derived calculation over a streamed promise. // When SvelteKit progressively streams `streamedUserData`, the client reconciles // an unresolved Promise object, triggering hydration mismatch and DOM tearing. let { data }: { data: PageData } = $props(); const formattedMetrics = $derived.by(() => { // Bad: synchronous assumption on potentially unresolved stream payload const raw = data.streamedUserData as any; return raw.scores ? raw.scores.map((s: number) => s * 100) : []; }); ``` #### Production Standard (Correct) > **Standard**: Wrapping the streamed promise within a dedicated reactive resource container guarantees that the hydration phase observes a deterministic initial state matching the server skeleton, postponing derived mutations until the promise resolves. ```typescript import type { PageData } from './$types'; // PRODUCTION PATTERN: Deterministic boundary isolating asynchronous streamed state interface UserData { scores: number[]; } let { data }: { data: PageData & { streamedUserData: Promise<UserData> } } = $props(); class StreamHydrationResource<T> { current = $state<T | null>(null); error = $state<Error | null>(null); loading = $state(true); constructor(promise: Promise<T>, initialFallback: T | null = null) { this.current = initialFallback; promise .then((resolved) => { this.current = resolved; this.loading = false; }) .catch((err) => { this.error = err instanceof Error ? err : new Error(String(err)); this.loading = false; }); } } const userResource = $derived(new StreamHydrationResource(data.streamedUserData)); const formattedScores = $derived( userResource.current ? userResource.current.scores.map((s) => s * 100) : [] ); ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Rune-Derived Streaming Hydration Boundary Standard

Svelte 5 Rune-Derived Streaming Hydration Boundary Standard defines an industrial-grade behavioral standard for frontend engineering. Enforces deterministic client reconciliation boundaries for asynchronous and derived rune primitives during progressive HTML streaming. Prevents client-side hydration mismatches, DOM tearing, and microtask race conditions caused by evaluating unstable $derived expressions against streamed promise payloads. 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: Directly evaluating raw streamed promises inside synchronous $derived runes without hydration boundary guards 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	Directly evaluating raw streamed promises inside synchronous $derived runes without hydration boundary guards	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 isolate streamed, deferred promises behind an {#await} template boundary or an explicit reactive stream container before binding to $derived runes.

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.