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6/18/2026
Fix 'SharedArrayBuffer is not defined': a practical guide to cross-origin isolation

Fix 'SharedArrayBuffer is not defined': a practical guide to cross-origin isolation

Short summary

Cross-origin isolation, enforced via COOP + COEP headers, gates SharedArrayBuffer and multithreaded WebAssembly—a post-Spectre browser security measure. Configure both headers on your document, verify with self.crossOriginIsolated, and audit third-party assets for CORS/CORP compliance. Common gotcha: CORP isolates subresources, not your page; COOP+COEP isolate the document.

  • •SharedArrayBuffer is blocked unless your page is cross-origin isolated
  • •Set COOP: same-origin + COEP: require-corp on your document response
  • •CORP (for subresources) and COOP/COEP (for pages) are distinct; confusion is a common mistake

Generated with AI, which can make mistakes.

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