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This page offers troubleshooting tips, solutions to known problems, and tricks - often contributed by our Community members. Don't see a fix to your problem here? Let us know by opening an issue!


WebContainers won't run properly (or at all) if cookie blockers, third-party or browser built-in ones, are enabled. Check the on('error') event and StackBlitz docs to learn more.

Slow boot time

To improve your project's boot time, pass both package-lock.json and package.json to the file system. Without package-lock.json Turbo, our npm client, first generates a fresh lockfile and only then downloads dependencies, resulting in a slower boot time.

Tip from the community

SvelteKit bypasses Turbo install by bundling the files so they can be cached and written straight to the virtual file system. Including package-lock.json was not an ideal solution given that different sections of their tutorial have different dependencies.

Alternatively, you can create and populate the node_modules folder yourself if you want your app to be runnable via script commands like npm run.

Turbo fails to load

In case lockfile resolution times out, you will see the following error:

Access to fetch at [URL] from origin [URL] has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled

To prevent the timeout, add package-lock.json to the file system which will speed up the dependency installation. For more information, see the "Slow boot time" tip.

Proxy error

If the boot() is called more than once, you may see the following error:

Uncaught (in promise) Error: Proxy has been released and is not usable

Make sure that boot() is only called once, even in the presence of HMR (hot module replacement).

WebContainers not loading and postMessage error

If all repeat requests are served without any headers and with the 304 status, access to SharedArrayBuffers will be denied. For example, if you're getting the following error message:

Uncaught (in promise) Error: Failed to execute ‘postMessage’ on ‘Worker’: SharedArrayBuffer transfer requires self.crossOriginIsolated.

You might have not set the COOP/COEP headers. If you have, make sure to restart the dev server and then refresh the preview page with a hard refresh (cmd+shift+r on MacOS and ctrl+shift+r on Windows).

Tip from the community

Solid.js developers added the following snippet to the Vite plugins array to include a plugin that always adds CORS in their Vite-based app:

      name: 'add-cors',

      configureServer(server) {
        server.middlewares.use((_req, res, next) => {
          res.setHeader('Cross-Origin-Opener-Policy', 'same-origin');
          res.setHeader('Cross-Origin-Embedder-Policy', 'require-corp');
          res.setHeader('Cross-Origin-Resource-Policy', 'cross-origin');

For deployment, they used netlify _headers file.

Embedding cross-origin-isolated site

In order to embed a cross-origin-isolated site, make sure that both the embed and embedder have the same COOP/COEP settings. The WebContainer API requires require-corp so both need to be set up as require-corp. Additionally, add allow="cross-origin-isolated" as an attribute on the embedded iframe.

To learn more, check out this article about COOP/COEP.

Running commands in a particular directory

Tip from the community

To enable running commands in a particular directory, Astro team followed these steps in the setup:

  1. Install Astro globally
  2. Mount each project in a separate directory
  3. Call each project via a package manager

which is reflected in the following code:


webcontainer.spawn('turbo', ['--cwd', `~/projects/${id}`, 'exec', 'astro', 'dev']);

Out of memory error

It may happen that having a few StackBlitz projects open at the same time may cause your browser to run out of memory. In your browser's console, you will see the following error:

RangeError: WebAssembly.instantiate(): Out of memory: wasm memory

When too much memory is allocated to browser processes, browsers will run out of memory. This can happen if you open too many StackBlitz projects at the same time. This includes our editor, Codeflow IDE, and WebContainer API app - or if a page features too many StackBlitz embeds running at the same time.

To free up memory, close other StackBlitz projects in other tabs or windows, and refresh the page.

Cannot load native addon error

Currently, WebContainers can only execute languages that are natively supported on the Web, including JavaScript and WebAssembly. It is not possible to run native addons which are usually implemented using native languages such as C++, unless they can be compiled to WebAssembly. Therefore, loading native addons is disabled by default via --no-addons in WebContainers. As a result, you may encounter an error that says:

Cannot load native addon because loading addons is disabled.

The solution to this is to use an alternative to the native addon which is either fully implemented in JavaScript or can be compiled to WebAssembly.

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