Experimenting with the proposed Cross-Origin Storage API in Transformers.js

Back to Articles Experimenting with the proposed Cross-Origin Storage API in Transformers.js Published June 23, 2026 Update on GitHub Upvote 6 Thomas Steiner tomayac Follow google (This is a guest post by Developer Relations Engineer Thomas Steiner from the Chrome team at Google.) Transformers.js provides Web developers with a simple way to use the power of transformers in their Web apps through task-specific pipelines. To run inference in the browser, developers create an instance of pipeline() and specify a task they want to use the pipeline for. As a concrete example, the following snippet shows how to set up an automatic speech recognition (ASR) pipeline. import { pipeline } from 'https://cdn.jsdelivr.net/npm/@huggingface/[email protected]'; const asr = await pipeline( 'automatic-speech-recognition', 'Xenova/whisper-tiny.en', { device: 'webgpu' }, ); const result = await asr('jfk.wav'); console.log(result); The cache challenge You will notice in the source code that I specified Xenova/whisper-tiny.en as the model, which is a very decent choice for common English automatic speech recognition tasks. In fact, it's even the default model according to the Transformers.js default model resolution, as per the linked excerpt. Model resources When you run this example in the browser, Transformers.js automatically takes care of downloading and caching the relevant model resources and Wasm files. The following screenshot shows the Chrome DevTools Cache storage section after visiting the app. When you reload the page, the resources are served from the Cache API, and the model returns results almost instantly. However, Xenova/whisper-tiny.en being a popular model (and, as mentioned before, even being the ASR default model in Transformers.js), you can well imagine that more than just one app that you visit would use it. To simulate this situation, here's the same example app from before, but served from a different origin. When you visit this different origin app, rather than being usable almost instantly, the browser instead has to download and cache all the model resources again, even if they're byte-by-byte the same as before. Even in this toy example, this adds up to 177 MB of duplicate download and storage, as you can examine in the Storage section of the Chrome DevTools Application panel. You can imagine that this quickly adds up. Wasm runtime resources But it gets worse. Let's add a second pipeline to the toy example: sentiment analysis. Sentiment analysis by default uses the Xenova/distilbert-base-uncased-finetuned-sst-2-english model. By not specifying the model, Transformers.js' default model resolution automatically picks it for you. const classifier = await pipeline('sentiment-analysis'); const sentiment = await classifier(result.text); console.log(sentiment); Two entirely different AI models, but they depend on the same 4,733 kB ort-wasm-simd-threaded.asyncify.wasm WebAssembly (Wasm) runtime file from the underlying ONNX Runtime library that Transformers.js is built on top of. Open the extended demo on a different origin, and you will notice in the Network tab how also the Wasm runtime gets downloaded and cached again. So even if you run apps that don't share the same AI models, your browser still makes redundant requests for shared Wasm resources you already have, and on top of that also caches them again, which consumes space on your hard disk. Cache isolation AI model resources serving By default, AI model resources come from the Hugging Face Hub, and ultimately the Hugging Face CDN. The browser makes a request for a resource like https://huggingface.co/Xenova/distilbert-base-uncased-finetuned-sst-2-english/resolve/main/config.json which then gets redirected to the final CDN URL like https://huggingface.co/api/resolve-cache/models/Xenova/distilbert-base-uncased-finetuned-sst-2-english/0b6928efcb76139cae2c6881d49cda67fe119f42/config.json?%2FXenova%2Fdistilbert-base-uncased-finetuned-sst-2-english%2Fresolve%2Fmain%2Fconfig.json=&etag=%223c36342ef1f74de2797d667c68c6b7b988d0b87c%22 in this case. Wasm runtime resources serving The Wasm runtime resources are served from the jsDelivr CDN by default. For example, ort-wasm-simd-threaded.asyncify.wasm comes from https://cdn.jsdelivr.net/npm/[email protected]/dist/ort-wasm-simd-threaded.asyncify.wasm at the time of this writing. Now you may say that if different apps, even though running on different origins, in the end serve their resources from the same CDN URLs, caching shouldn't be a problem, as long as the final URLs are the same. Unfortunately, this is not how caching works in browsers for a long time. The article Gaining security and privacy by partitioning the cache goes into all the details, but essentially, caches are isolated by origin to prevent timing attacks: the time a website takes to respond to HTTP requests can reveal that the browser has accessed the same resource in the past, which makes the browser vulnerable to security and privacy leaks. Chrome's implementation The concrete implementation may vary by browser, but in Chrome, cached resources are keyed using a Network Isolation Key in addition to the resource URL. The Network Isolation Key is composed of the top-level site and the current-frame site. Take the previous toy examples hosted on the origins https://googlechrome.github.io and https://rawcdn.rawgit.net. If they both use the Wasm runtime from https://cdn.jsdelivr.net/npm/[email protected]/dist/ort-wasm-simd-threaded.asyncify.wasm, their cache keys will look like in the following table. Network Isolation Key Resource URL Top-level site Current-frame site https://googlechrome.github.io https://googlechrome.github.io https://cdn.jsdelivr.net/npm/[email protected]/dist/ort-wasm-simd-threaded.asyncify.wasm https://rawcdn.rawgit.net https://rawcdn.rawgit.net https://cdn.jsdelivr.net/npm/[email protected]/dist/ort-wasm-simd-threaded.asyncify.wasm So even if the resource URLs are exactly the same, since the Network Isolation Keys don't match, there's no cache hit, which means duplicate download and duplicate storage. This is the challenge that the Cross-Origin Storage proposal aims to tackle. Enter the Cross-Origin Storage API 💡 Note: The Cross-Origin Storage API is an early-stage proposal that isn't final. While the proposed API is not yet natively implemented in any browser, you don't have to wait to experiment with it. Install the Cross-Origin Storage extension to inject the navigator.crossOriginStorage polyfill on all pages and test the complete flow. The proposed Cross-Origin Storage (COS) API introduces a dedicated navigator.crossOriginStorage interface through which web apps can store and retrieve large files across origin boundaries, identified not by a URL, but by a cryptographic hash. That last point about cryptographic hashes is key. Because COS identifies files by their hash rather than by their URL or origin, the same ort-wasm-simd-threaded.asyncify.wasm Wasm runtime you downloaded while visiting https://googlechrome.github.io is recognized as identical to the one https://rawcdn.rawgit.net is about to request, no matter where either of the two origins fetched it from. See the following code snippet that illustrates the basic flow. const hash = { algorithm: 'SHA-256', value: '8f434346648f6b96df89dda901c5176b10a6d83961dd3c1ac88b59b2dc327aa4', }; try { const handle = await navigator.crossOriginStorage.requestFileHandle(hash); // Cache hit! Get the file as a Blob and use it directly. const fileBlob = await handle.getFile(); } catch { // Cache miss. Download from network, then store for next time. const fileBlob = await fetch('https://cdn.jsdelivr.net/.../ort-wasm-simd-threaded.asyncify.wasm') .then(r => r.blob()); const handle = await navigator.crossOriginStorage.requestFileHandle( hash, { create: true, origins: '*' }, ); const writableStream = await handle