WasmGC SharedArrayBuffer Cross-Origin Isolation and COOP/COEP Enforcement

Enforces strict Cross-Origin-Opener-Policy (COOP) and Cross-Origin-Embedder-Policy (COEP) invariants across all root documents, service workers, and micro-frontend entrypoints to guarantee window.crossOriginIsolated is true. This prevents catastrophic runtime crashes when allocating shared WebAssembly.Memory instances for hardware-accelerated multithreaded WasmGC workloads. It also ensures all cross-origin subresources and workers declare appropriate Cross-Origin Resource Policy (CORP) headers.

--- description: WasmGC SharedArrayBuffer Cross-Origin Isolation and COOP/COEP Enforcement - Enforces strict Cross-Origin-Opener-Policy (COOP) and Cross-Origin-Embedder-Policy (COEP) invariants across all root documents, service workers, and micro-frontend entrypoints to guarantee window.crossOriginIsolated is true. This prevents catastrophic runtime crashes when allocating shared WebAssembly.Memory instances for hardware-accelerated multithreaded WasmGC workloads. It also ensures all cross-origin subresources and workers declare appropriate Cross-Origin Resource Policy (CORP) headers. globs: src/**/*.{ts,tsx,js,jsx}, public/**/*.{ts,js,html}, vite.config.{ts,js}, next.config.{js,mjs,ts}, server/**/*.{ts,js} alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/wasmgc-sharedarraybuffer-cross-origin-isolation-invariant" --- # WasmGC SharedArrayBuffer Cross-Origin Isolation and COOP/COEP Enforcement > **Severity**: CRITICAL | **Category**: frontend > **Anti-Pattern Warning**: Instantiating Multithreaded WasmGC or SharedArrayBuffer Without Complete Cross-Origin Isolation and CORP Verification ## System Constraints You are a Principal Software Architect enforcing the WasmGC SharedArrayBuffer Cross-Origin Isolation Invariant. Multithreaded WebAssembly Garbage Collection (WasmGC) requires window.crossOriginIsolated to be strictly true; otherwise, SharedArrayBuffer constructors and shared WebAssembly.Memory instantiation fail with runtime ReferenceErrors or TypeErrors. You must mandate strict COOP/COEP header propagation, cross-origin resource isolation (CORP), fallback feature-gating, and secure Web Worker instantiation across all micro-frontends and asset pipelines. ## Strict Guidelines - Engineers MUST ALWAYS verify window.crossOriginIsolated before constructing WebAssembly.Memory with shared: true or accessing SharedArrayBuffer. - Engineers MUST ALWAYS configure host edge proxies, CDNs, and micro-frontend orchestrators to return 'Cross-Origin-Opener-Policy: same-origin' and 'Cross-Origin-Embedder-Policy: require-corp' (or 'credentialless'). - Engineers MUST NOT instantiate multi-threaded WasmGC instances directly on the main thread without a robust single-threaded fallback when cross-origin isolation cannot be verified. - Code MUST explicitly set crossorigin="anonymous" and ensure upstream servers provide 'Cross-Origin-Resource-Policy: cross-origin' for all federated micro-frontend chunks, Wasm binaries, and worker scripts. - Workers MUST be instantiated with module type { type: 'module' } and must not rely on cross-origin scripts that omit CORP headers, as COEP will block execution. - Components NEVER assume SharedArrayBuffer availability based on browser user-agent sniffing alone; runtime feature detection of crossOriginIsolated and WebAssembly.validate() is required. ## Bad vs Good Implementations ### Cross-Origin Isolated WasmGC Memory and Worker Instantiation #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Attempting to allocate a shared WebAssembly.Memory without verifying self.crossOriginIsolated leads to immediate runtime exceptions if the host document or micro-frontend shell lacks COOP/COEP headers. Spawning a worker without validating cross-origin script policies and CORP headers causes silent worker network failures. ```typescript // Anti-Pattern: Unchecked SharedArrayBuffer allocation and worker spawn in unisolated micro-frontend export async function initWasmRuntime(wasmUrl: string, workerUrl: string) { // Crashes with ReferenceError: SharedArrayBuffer is not defined if COOP/COEP headers are missing const memory = new WebAssembly.Memory({ initial: 256, maximum: 1024, shared: true, }); // Unchecked worker spawn will fail COEP if script lacks CORP headers or cross-origin origin mismatch occurs const worker = new Worker(workerUrl); const response = await fetch(wasmUrl); const bytes = await response.arrayBuffer(); const module = await WebAssembly.compile(bytes); worker.postMessage({ module, memory }); return { worker, memory }; } ``` #### Production Standard (Correct) > **Standard**: The good example rigorously validates globalThis.crossOriginIsolated and SharedArrayBuffer availability before attempting shared memory creation. It gracefully falls back to single-threaded execution if the runtime lacks isolation, stream-compiles the Wasm module, and guarantees typed module worker instantiation. ```typescript // Production-Grade: Runtime isolation verification, header-safe fallback, and validated worker initialization interface WasmInstanceResult { isShared: boolean; memory: WebAssembly.Memory; workerPool?: Worker[]; } export async function initWasmRuntime( wasmUrl: string, workerUrl: string, concurrency: number = navigator.hardwareConcurrency || 4 ): Promise<WasmInstanceResult> { const isIsolated = typeof globalThis.crossOriginIsolated === 'boolean' && globalThis.crossOriginIsolated; // Check if SharedArrayBuffer and shared memory are supported under current isolation state const supportsSharedMemory = isIsolated && typeof WebAssembly.validate === 'function' && typeof SharedArrayBuffer !== 'undefined'; if (!supportsSharedMemory) { console.warn( '[WasmGC] Environment is not cross-origin isolated. Falling back to single-threaded WasmGC runtime.' ); const unsharedMemory = new WebAssembly.Memory({ initial: 256, maximum: 1024, shared: false, }); return { isShared: false, memory: unsharedMemory }; } // Allocate hardware-accelerated shared memory backing for multithreaded WasmGC const sharedMemory = new WebAssembly.Memory({ initial: 256, maximum: 1024, shared: true, }); const response = await fetch(wasmUrl, { credentials: 'same-origin' }); if (!response.ok) { throw new Error(`Failed to fetch Wasm binary: ${response.statusText}`); } const module = await WebAssembly.compileStreaming(response); // Spawn workers using module syntax, inheriting cross-origin isolation context const workers = Array.from({ length: concurrency }, () => { const worker = new Worker(workerUrl, { type: 'module' }); worker.postMessage({ type: 'INIT_SHARED_WASMIC', module, memory: sharedMemory }); return worker; }); return { isShared: true, memory: sharedMemory, workerPool: workers, }; } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: WasmGC SharedArrayBuffer Cross-Origin Isolation and COOP/COEP Enforcement

WasmGC SharedArrayBuffer Cross-Origin Isolation and COOP/COEP Enforcement defines an industrial-grade behavioral standard for frontend engineering. Enforces strict Cross-Origin-Opener-Policy (COOP) and Cross-Origin-Embedder-Policy (COEP) invariants across all root documents, service workers, and micro-frontend entrypoints to guarantee window.crossOriginIsolated is true. This prevents catastrophic runtime crashes when allocating shared WebAssembly.Memory instances for hardware-accelerated multithreaded WasmGC workloads. It also ensures all cross-origin subresources and workers declare appropriate Cross-Origin Resource Policy (CORP) headers. 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: Instantiating Multithreaded WasmGC or SharedArrayBuffer Without Complete Cross-Origin Isolation and CORP Verification 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	Instantiating Multithreaded WasmGC or SharedArrayBuffer Without Complete Cross-Origin Isolation and CORP Verification	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 verify window.crossOriginIsolated before constructing WebAssembly.Memory with shared

Engineers MUST ALWAYS verify window.crossOriginIsolated before constructing WebAssembly.Memory with shared: true or accessing SharedArrayBuffer.

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.