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Configuring your browser to run WebContainers

While all supported browsers can run WebContainers, some browser options can stop WebContainers from running correctly.

You can work around those issues by adding exceptions for StackBlitz domains in your browser’s settings. Here’s how.

Why do I need to add exceptions for StackBlitz?

WebContainers use a combination of browser technologies, such as Service Worker and WebAssembly, across several domains. Each running project has its own domain and needs to install a Service Worker for that domain to work properly.

In some browsers, this feature is blocked by “third-party cookie” or “third-party storage” restrictions. These are legitimate restrictions when the third-party domain is an ad server or a tracking server, but in the case of WebContainers the third-party domain is where your project code runs.


With Chrome's defaults, starting with version 118 or later, you should get a pretty good experience out of the box.

Read on if you run into issues or want an even smoother experience.

Previews in a separate tab

Previews opened in a separate tab are not connected by default to the editor and require an extra step. To complete this step, you are usually prompted to click a button.

This can happen automatically if your popups settings, chrome://settings/content/popups, are adjusted with the following exception:

Chrome popups settings showing the * origins as exceptions

Once done, whenever the preview needs to reconnect to the editor, it will open a popup that gets immediately closed.

Lastly, if Chrome's memory saver is turned on, this step might be necessary more often than required. To avoid it, you can add the following exception in chrome://settings/performance:

Your performance settings should be similar to this:

Chrome performance settings showing the memory saver section with an exception for

Enabling Service Workers

If you use the “Block Third Party Cookies” option in Chrome and you have “Third-party Storage Partitioning” disabled, you will need to either:

  • Enable Storage partitioning
  • Or add exceptions for StackBlitz projects.

The first option should be preferred because this is a new feature of Chrome which improves your privacy when you visit websites that embed third-party sites.

Enable Storage partitioning

Visit chrome://flags/#third-party-storage-partitioning and choose Enabled in the drop down. In recent version of Chrome, choosing the Default value should work too.

Chrome flags showing the third-party Storage Partitioning option enabled.

Or add exceptions for Stackblitz

To allow all StackBlitz projects to use Service Workers, go to your browser’s cookie preferences, and add exceptions for the following URL patterns:


For instance, in Chrome, go to chrome://settings/cookies and add those exceptions in the “Sites that can always use cookies” section.

Chrome cookie settings with an empty list of “Sites that can always use cookies”.Chrome cookie settings showing a modal dialog for adding a site to list of third-party cookie exceptions.Chrome cookie settings with exceptions for and domains.
Advanced: enabling Service Workers for a single project

If you don’t want to allow Service Workers and third-party cookies for all StackBlitz projects, you can add exceptions for individual projects instead.

Note that this may not be very practical, because the exact domain for a project can change between sessions, especially in Codeflow IDE.

In Chrome, click the lock icon in the navigation bar:

Clicking the lock icon in the navigation bar shows a menu with a “Cookies” entry.A modal dialog titled “Cookies in use” shows that cookies for a couple subdomains of and were blocked. Select a domain and click “Allow” to add an exception.

If that is the case, choose to “Allow” cookies for those third-party domains. After that, the browser will prompt you to reload the page and you will be able to run the current project.

Note that the list of blocked domains might look different for you. In particular, a domain might not be present. WebContainers-based projects use different domains dynamically, depending on what the runtime is doing (for instance, whether it is serving HTTP content or not), so you might need to check the list of blocked domains later if you see something off again.


Enabling Service Workers

When Firefox’s Enhanced Tracking Protection is configured in “Custom” mode with the cookie blocking option set to “All cross-site cookies”, Firefox will block the Service Workers used by WebContainers.

You can work around this issue in a couple ways:

  1. Use the “Strict” mode instead of “Custom” mode. It’s important to note that it's slightly more permissive than “Custom” mode with very strict options. If this is not something you'd like to allow, you can...
  2. Add exceptions to cookie blocking for the domains used to run StackBlitz projects.

To do so, visit about:preferences#privacy to check what your current settings look like.

The Firefox “Privacy & Security” settings page showing that Enhanced Tracking Protection is set to “Custom”, and to “Cookies: All cross-site cookies (may cause websites to break)”.

To add exceptions for StackBlitz, scroll down to the “Cookies and Site Data” section, click on “Manage Exceptions…”, and add exceptions for the following sites:

The Cookies and site data exceptions management window in Firefox settings. It shows a couple entries for and, both with the value “Allow”.

Don’t forget to click “Save Changes”. Then you should be able to reload the tab with your StackBlitz project, and hopefully everything should work!

Previews in a separate tab

Previews opened in a separate tab are not connected by default to the editor and require an extra step. To complete this step, you are usually prompted to click a button.

This can happen automatically if your popups settings, about:preferences#privacy, are adjusted with the following exception:
Firefox popups settings showing the site allowed as an exception

Brave: enabling Service Workers

By default, Brave’s “Shields” feature blocks Service Workers and cookies from third-party domains.

To allow WebContainers to run in Brave, you will need to add an exception for StackBlitz:

  1. Visit a WebContainers-based project, for instance The project’s boot sequence might stay stuck on the “Running start command” step:
Screenshot of Brave on a WebContainers project with the Brave Shields feature on. Loading the project’s web server is stuck on the last step
  1. Click on the “Shields” icon at the right of the address bar, then click on “Advanced View”.
Screenshot showing the Shields configuration popup for
  1. In the advanced view, change the “Cross-site cookies blocked” option to “All cookies allowed”.
Screenshot showing the advanced view of the Shields configuration popup, with a drop-down selector for cross-site cookie permissions.

Brave will reload the page, and you should get a working project:

Screenshot of Brave on a WebContainers project with the Brave Shields feature tweaked to allow third-party cookies and Service Workers. Loading the web server works, and shows the default page for Next.js’s starter project.

Edge: enabling WebAssembly

Microsoft Edge disables usage of WebAssembly when you set its “Enhance your security on the web” option to “Strict”.

To allow StackBlitz projects to run, either set this option to “Balanced” or add exceptions for a couple domains used by StackBlitz projects.

  1. Go to edge://settings/privacy → “Enhance your security on the web”.
Edge privacy settings showing the “Enhance your security on the web” section.
  1. Then click “Exceptions”, and add exceptions for the following domains:
Edge privacy settings showing two entries added under the label “Enhanced security is turned off for these sites”.