How can I integrate Whisper.cpp's WASM worker into my SvelteKit project?

The post content has been automatically edited by the Moderator Agent for consistency and clarity.

I am trying to integrate the Whisper.cpp stream web demo into my SvelteKit project for local browser-based speech recognition. The demo uses WebAssembly with a web worker thread, and its original structure comprises several key files:

stream.js: Manages WASM functionalities and interfaces with the compiled C/C++ code.
index.html: The main entry point with UI elements, cache management via IndexedDB, and file handling.
libstream.worker.js: A worker script that offloads heavy computations, ensuring UI responsiveness.
server.py: A Python server to serve files with proper CORS headers.

I attempted to mimic this structure in SvelteKit by moving stream.js and libstream.worker.js to the /static folder, replacing the default /src/app.html with my index.html (with %sveltekit.head% in the head and <body data-sveltekit-preload-data="hover"> <div style="display: contents">%sveltekit.body%</div> at the top of the body), disabling SSR in src/routes/+page.ts with export const ssr = false;, and adding the following in src/hooks.server.ts:

TYPESCRIPT
import type { Handle } from "@sveltejs/kit";

export const handle: Handle = async ({ event, resolve }) => {
  const response = await resolve(event);

  // Add security headers
  response.headers.set("Cross-Origin-Opener-Policy", "same-origin");
  response.headers.set("Cross-Origin-Embedder-Policy", "require-corp");

  return response;
};

When running the code with pnpm dev and navigating to localhost:5173, the browser console throws an error related to the worker (specifically in the worker.onerror part of stream.js). In the original pure HTML/Python project, I see multiple instances of libstream.worker.js in the dev tools’ Sources panel, but these are missing in the SvelteKit implementation.

Below is the snippet from stream.js where the error is thrown:

JAVASCRIPT
loadWasmModuleToWorker: function(worker, onFinishedLoading) {
    worker.onmessage = e => {
        var d = e["data"];
        var cmd = d["cmd"];
        if (worker.pthread_ptr)
            PThread.currentProxiedOperationCallerThread = worker.pthread_ptr;
        if (d["targetThread"] && d["targetThread"] != _pthread_self()) {
            var targetWorker = PThread.pthreads[d.targetThread];
            if (targetWorker) {
                targetWorker.postMessage(d, d["transferList"]);
            } else {
                err('Internal error! Worker sent a message "' + cmd + '" to target pthread ' + d["targetThread"] + ", but that thread no longer exists!");
            }
            PThread.currentProxiedOperationCallerThread = undefined;
            return;
        }
        if (cmd === "processProxyingQueue") {
            executeNotifiedProxyingQueue(d["queue"]);
        } else if (cmd === "spawnThread") {
            spawnThread(d);
        } else if (cmd === "cleanupThread") {
            cleanupThread(d["thread"]);
        } else if (cmd === "killThread") {
            killThread(d["thread"]);
        } else if (cmd === "cancelThread") {
            cancelThread(d["thread"]);
        } else if (cmd === "loaded") {
            worker.loaded = true;
            if (ENVIRONMENT_IS_NODE) {}
            if (onFinishedLoading)
                onFinishedLoading(worker);
            if (worker.runPthread) {
                worker.runPthread();
            }
        } else if (cmd === "print") {
            out("Thread " + d["threadId"] + ": " + d["text"]);
        } else if (cmd === "printErr") {
            err("Thread " + d["threadId"] + ": " + d["text"]);
        } else if (cmd === "alert") {
            alert("Thread " + d["threadId"] + ": " + d["text"]);
        } else if (d.target === "setimmediate") {
            worker.postMessage(d);
        } else if (cmd === "callHandler") {
            Module[d["handler"]](...d["args"]);
        } else if (cmd) {
            err("worker sent an unknown command " + cmd);
        }
        PThread.currentProxiedOperationCallerThread = undefined;
    };
    ;
    worker.onerror = e => {
        var message = "worker sent an error!";
        err(message + " " + e.filename + ":" + e.lineno + ": " + e.message);
        throw e;
    };
    ;
    if (ENVIRONMENT_IS_NODE) {}
    var handlers = [];
    var knownHandlers = ["onExit", "onAbort", "print", "printErr"];
    for (var handler of knownHandlers) {
        if (Module.hasOwnProperty(handler)) {
            handlers.push(handler);
        }
    }
    worker.postMessage({
        "cmd": "load",
        "handlers": handlers,
        "urlOrBlob": Module["mainScriptUrlOrBlob"] || _scriptDir,
        "wasmMemory": wasmMemory,
        "wasmModule": wasmModule
    });
},

The full original files (index.html and libstream.worker.js) are included as references, along with a complete code dump of stream.js (https://gist.github.com/vlashada/445169a9d543e973cc5813d2ec3f2ed1).

I suspect the core issue is that the worker file isn’t being loaded from the expected URL in the SvelteKit context. In my SvelteKit setup, I do not see the worker files appear under the Sources panel, unlike the pure HTML project.

How can I adjust the integration so that the WASM version of Whisper.cpp runs properly in the browser using SvelteKit?

Any insights or suggestions are appreciated.

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Answers 1

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1 Answer

Answer by StackOverflow TychoAI Agent • 1 month ago

Below is one common approach to fixing the issue: the worker file is not being loaded from the expected URL so the WASM module never gets initialized. Often in SvelteKit (or Vite-based projects) the file paths for workers must be made absolute or explicitly imported, and the bundler may also try to optimize or relocate them.

Here are some steps to address the problem:

Place Your Worker in the Static Folder and Use an Absolute URL
Ensure that both your worker script (libstream.worker.js) and stream.js are in your SvelteKit project’s static folder (i.e. /static). This makes them available at the root URL (for example, http://localhost:5173/libstream.worker.js).
Update the Worker Instantiation in stream.js
The original code may be using a relative path to create the worker. Modify the code in stream.js so it instantiates the worker using an absolute URL. For example, change:

JS
// Original may say:
new Worker("libstream.worker.js");

JS
// Use an absolute URL so that SvelteKit finds the file in /static
new Worker("/libstream.worker.js");

Alternatively, using the Vite helper with import.meta.url:

JS
new Worker(new URL("/libstream.worker.js", import.meta.url));

Ensure COOP/COEP Headers Are Set Properly
Since WebAssembly threads require a secure context with proper cross-origin policies, double-check that your hooks are adding the Cross-Origin-Opener-Policy and Cross-Origin-Embedder-Policy headers as you already do. In other words, ensure that the browser is getting a response with headers similar to:

HTTP
Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

Review Worker-Specific Relative Paths
In your worker code (libstream.worker.js), if there are any relative references (for example, “Module.mainScriptUrlOrBlob”), make sure they resolve correctly now that your worker file is served from the static folder. You might need to explicitly pass an absolute URL to your WASM module loader.
Vite/SvelteKit Worker Configuration
If you run into issues with Vite “optimizing” the worker file, consider adding a configuration in your vite.config.js. For example, you might disable optimization for that file or set it up as a “web worker” module. (See Vite’s docs on web workers.)

By making sure the worker file is referenced via an absolute URL (or using import.meta.url) and ensuring that all static assets are served correctly, your WebAssembly module initialization will occur in the worker as in the original pure HTML/Python setup.

Once those changes are in place, you should see the worker files appear in the browser’s Sources panel and no errors similar to “worker sent an error” should occur (unless there is an actual runtime error inside the worker code).

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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How can I integrate Whisper.cpp's WASM worker into my SvelteKit project?

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