# How enabling cross-origin isolation silently broke our multi-threaded WASM image compressor

A production postmortem. We shipped browser-side image compression (Rust → WASM + WebGPU), turned on cross-origin isolation for speed, and watched every format crash with compression worker crashed . Here's the root cause and the fix. The setup We built an image compressor that runs 100% in the browser — Rust compiled to WASM for the codec work, WebGPU for the heavy ML passes (background removal, denoise, watermark). No upload, so users' pixels never leave the device. Privacy is the whole selling point. For the multi-threaded code paths we rely on shared memory + atomics , which in the browser requires crossOriginIsolated . So we served the document with: Cross-Origin-Embedder-Policy: require-corp Cross-Origin-Opener-Policy: same-origin That gives us crossOriginIsolated === true , unlocks SharedArrayBuffer , and lets the *‑threaded WASM builds actually spawn workers. The build uses a nightly toolchain ( nightly-2025-06-01 + -Z build-std ) with: RUSTFLAGS = "--cfg=... +atomics,+bulk-memory --shared-memory --import-memory" and a custom rayon handle pool ( with_turbo_pool ) instead of build_global , so we control worker lifecycle and can abort/self-heal. The incident After flipping COEP to require-corp in production, every format started crashing with the same message: compression worker crashed Not one codec — JPG, PNG, WebP, AVIF, all of them. It was a P0: the core feature was dead for every user. What made it nasty: it only reproduced under real cross-origin isolation . Local dev without COEP was fine. Staging without the header was fine. So the bug hid until it hit production traffic. Root cause The *‑threaded WASM packages spin up nested rayon workers to parallelize the codec. Under COI + COEP require-corp , those nested workers get blocked by Cross-Origin-Resource-Policy / COEP — the spawned worker script is treated as a cross-origin response without the right CORP header, so the browser refuses it. No worker → the rayon pool never initializes → the compression c