If you've ever seen this in the console: Uncaught ReferenceError: SharedArrayBuffer is not...
COOP + COEP headers enable WebAssembly multithreading (post-Spectre isolation); CORP must also be configured on subresources. Misconfiguration silently degrades to single-thread or breaks third-party assets—a critical infrastructure oversight in production.
If you've ever seen this in the console:
Uncaught ReferenceError: SharedArrayBuffer is not defined
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or your multithreaded WebAssembly quietly fell back to a single thread, the cause is almost always the same thing: your page is not cross-origin isolated. Here's what that means, why the browser does it, and exactly how to fix it.
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