Svelte 5 Fine-Grained Signal Topological Diamond Dependency Settling Standard

--- description: Svelte 5 Fine-Grained Signal Topological Diamond Dependency Settling Standard - Enforces synchronous topological graph evaluation across Svelte 5 runes ($state, $derived, $effect) to eliminate diamond dependency glitches and torn reads. Prohibits breaking the reactive directed acyclic graph (DAG) via side-effect-driven state mutations and asynchronous signal propagation. 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-signal-diamond-settling-standard" --- # Svelte 5 Fine-Grained Signal Topological Diamond Dependency Settling Standard > **Severity**: CRITICAL | **Category**: frontend > **Anti-Pattern Warning**: Mutating $state inside $effect to bridge derived branches, introducing diamond torn reads and cascading microtask churn ## System Constraints You are a Principal Software Architect enforcing Svelte 5 fine-grained reactive signal invariants. You must prevent diamond dependency glitches (torn reads) by ensuring the reactive graph evaluates topologically and synchronously through pure $derived and $derived.by runes. Disallow using $effect to synchronize intermediate state values. ## Strict Guidelines - Engineers MUST ALWAYS model converging intermediate calculations strictly as pure derivations via $derived or $derived.by to preserve Svelte 5 push-pull topological settling. - Engineers MUST NOT mutate any $state inside an $effect or $derived to propagate values across parallel graph branches. - Code SHOULD avoid mixing asynchronous microtasks (Promises, setTimeout) within signal computation boundaries that split atomic state settling. - Services MUST expose pure reactive signals using .svelte.ts modules rather than imperative event-emitter patterns that bypass signal graph synchronization. - Components NEVER capture transient intermediate signal states by reading state inside untracked boundaries or uncoordinated lifecycle hooks during active diamond propagation. ## Bad vs Good Implementations ### Resolving Signal Diamond Dependency Convergence #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Mutating $state inside $effect to compute dependent values breaks synchronous topological ordering. Svelte schedules effects post-render or in distinct passes, creating intermediate frames where effectivePrice evaluates with a mix of stale and updated signals. ```typescript // Broken: Uses $effect to synchronize downstream branches of a diamond graph. // Causes torn reads where 'effectivePrice' briefly evaluates with stale 'tax' and fresh 'discount'. let basePrice = $state(100); let discount = $state(0); let tax = $state(0); let effectivePrice = $state(100); $effect(() => { // Branch A discount = basePrice > 50 ? basePrice * 0.1 : 0; }); $effect(() => { // Branch B tax = (basePrice - discount) * 0.2; }); $effect(() => { // Convergence point: evaluated before Branch B completes, reading torn intermediate values effectivePrice = basePrice - discount + tax; }); ``` #### Production Standard (Correct) > **Standard**: Declaring intermediate branches as $derived runes forms a unified reactive DAG. When basePrice changes, Svelte 5 synchronously flags the subgraph as dirty and pulls updated values in strict topological order, preventing glitches and duplicate layout passes. ```typescript // Correct: Idiomatic Svelte 5 pure topological signal graph. // Svelte's push-pull signal runtime guarantees atomic synchronous settling without torn reads. let basePrice = $state(100); // Branch A: Pure derived signal let discount = $derived(basePrice > 50 ? basePrice * 0.1 : 0); // Branch B: Pure derived signal depending on root and Branch A let tax = $derived((basePrice - discount) * 0.2); // Convergence: Evaluates topologically once all upstream dependencies settle synchronously let effectivePrice = $derived(basePrice - discount + tax); ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Fine-Grained Signal Topological Diamond Dependency Settling Standard

Svelte 5 Fine-Grained Signal Topological Diamond Dependency Settling Standard defines an industrial-grade behavioral standard for frontend engineering. Enforces synchronous topological graph evaluation across Svelte 5 runes ($state, $derived, $effect) to eliminate diamond dependency glitches and torn reads. Prohibits breaking the reactive directed acyclic graph (DAG) via side-effect-driven state mutations and asynchronous signal propagation. 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: Mutating $state inside $effect to bridge derived branches, introducing diamond torn reads and cascading microtask churn 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	Mutating $state inside $effect to bridge derived branches, introducing diamond torn reads and cascading microtask churn	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 model converging intermediate calculations strictly as pure derivations via $derived or $derived.by to preserve Svelte 5 push-pull topological settling.

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.