Mozilla's attachment CDN can answer a Chrome user agent with HTTP 406, and that 406 carries cache-control: public, max-age=31536000, immutable. Chromium stored it in the browsing session's disk cache, and the user agent is not part of the cache key, so every later net.fetch of the engine wasm got the cached 406 whatever User-Agent header it sent. net.fetch does send the header; the cache answered before the network was asked. Translation downloads now go through an in-memory session, peek-translations, whose user agent is Peek/<app version> (on-device translation), and every request asks for cache: 'no-store'. Nothing the session receives outlives the run, it keeps away from browsing cookies, and it keeps Chromium's proxy and certificate handling. model-download.ts stays transport-agnostic. Checked in Electron main against a copy of the affected profile's cache: net.fetch with the Peek header got 406 for the wasm; the translation session got 200 for the wasm, the models-v2 records and the server root.