Skip to main content
Sometimes you may need to access a running sandbox application and preview the content in real time in a front-end client. This is useful for example to instantly preview React code generated by a codegen AI agent. You can do this via a preview URL that routes to a specific port on your sandbox (e.g. port 3000 for npm run dev -- --host 0.0.0.0 --port 3000 &). This preview URL can be either public (does not require you to be authenticated to access it) or private (see below). They will look something like this: 
https://tkmu0oj2bf6iuoag6mmlt8.us-pdx-1.preview.bl.run
If you see a 502 error when accessing the preview URL, the most common cause is that your application server is not reachable externally. To resolve this, configure your server to bind to IP address 0.0.0.0, so that it listens on all available network interfaces. Sample server startup commands are shown below:
npm run dev -- --host 0.0.0.0
# or
npm serve -- --host 0.0.0.0
Read more about this in our troubleshooting page.
You can also set a custom domain on a preview URL (see down below).

​
Current limitations of real-time previews

JavaScript module bundlers handle real-time previewing. Here are the key compatibility requirements and limitations:
  • Module bundler must implement ping-pong.
  • Both Webpack and Turbopack (v16.1.1) have been tested and confirmed to work.
  • Blaxel has a 15-minute connection timeout. To maintain previews beyond this limit, ensure your bundler implements automatic reconnection.
  • You cannot create a preview on port 80 which is reserved for system.
Using a Webpack server but unable to hot reload your previews? Check our troubleshooting page for possible solutions.

​
Private preview URLs

When you create a private preview URL a token is required to access the URL. You must include the token as:

​
Manage preview URLs

To whitelist sandbox traffic in your network, you can retrieve the public IP addresses used by Blaxel.

​
Blaxel console

You can create a preview URL for a sandbox from the Blaxel Console, on the overview of a sandbox: Screenshot 2025-05-06 at 10.50.49 PM.png

​
Blaxel SDK

The Blaxel SDK authenticates with your workspace using credentials from these sources, in priority order:
  1. when running on Blaxel, authentication is handled automatically
  2. variables in your .env file (BL_WORKSPACE and BL_API_KEY, or see this page for other authentication options).
  3. environment variables from your machine
  4. configuration file created locally when you log in through Blaxel CLI (or deploy on Blaxel)
When developing locally, the recommended method is to just log in to your workspace with Blaxel CLI. This allows you to run Blaxel SDK functions that will automatically connect to your workspace without additional setup. When you deploy on Blaxel, this connection persists automatically.When running Blaxel SDK from a remote server that is not Blaxel-hosted, we recommend using environment variables as described in the third option above.
Create and manage a sandbox’s public preview URL: 
import { SandboxInstance } from "@blaxel/core";

const sandbox = await SandboxInstance.get("my-sandbox");
// Create public preview
const preview = await sandbox.previews.create({
    metadata: { name: "app-preview" },
    spec: {
        port: 3000,
        public: true,
        responseHeaders: {
            "Access-Control-Allow-Origin": "https://YOUR-DOMAIN",
            "Access-Control-Allow-Methods": "GET, POST, PUT, DELETE, OPTIONS, PATCH",
            "Access-Control-Allow-Headers": "Content-Type, Authorization, X-Requested-With, X-Blaxel-Workspace, X-Blaxel-Preview-Token, X-Blaxel-Authorization",
            "Access-Control-Allow-Credentials": "true",
            "Access-Control-Expose-Headers": "Content-Length, X-Request-Id",
            "Access-Control-Max-Age": "86400",
            "Vary": "Origin"
        }
    }
});

// Get preview URL
const url = preview.spec?.url;
Or create a private preview: 
import { SandboxInstance } from "@blaxel/core";

const sandbox = await SandboxInstance.get("my-sandbox");

// Create private preview
const preview = await sandbox.previews.create({
    metadata: { name: "private-preview" },
    spec: {
      port: 3000,
      public: false
    }
});

// Create access token (10 minutes expiry)
const expiresAt = new Date(Date.now() + 10 * 60 * 1000);
const token = await preview.tokens.create(expiresAt);

// How to access the preview with the token
const url = preview.spec?.url;
const response = await fetch(`${url}/health?bl_preview_token=${token.value}`);

​
URL prefix

You can customize the preview URL with a custom string prefix using the prefixUrl argument: 
const sandbox = await SandboxInstance.get("my-sandbox");

const preview = await sandbox.previews.create({
    metadata: { name: "app-preview" },
    spec: {
        port: 3000,
        public: true,
        prefixUrl: "my-prefix"
    }
});
When using a prefix URL, the workspace name is automatically added to the prefix and included in the final structure for the preview URL - for example, https://myprefix-workspace-xxx.preview.bl.run.

​
Create if not exists

Just like for sandboxes, this helper function either retrieves an existing preview or creates a new one if it doesn’t exist. Blaxel first checks for an existing preview with the provided name and either retrieves it or creates a new one using your specified configuration. 
const preview = await sandbox.previews.createIfNotExists({
    metadata: {
        name: "preview-name"
    },
    spec: {
        port: 443,
        public: false
    }
})

​
Custom domains

To set up a custom domain for your sandbox preview:
  • Register a custom domain to your Blaxel workspace and complete the verification process
  • Use this verified custom domain when creating a new preview:
const preview = await sandbox.previews.create({
    metadata: {
        name: "preview-custom-domain"
    },
    spec: {
        port: 443,
        public: false,
        customDomain: ‘your.custom.domain.dev’
    }
})
When you register a custom domain, you also enable the use of wildcard subdomains for that domain. For example, if you register mycompany.com, you can configure preview URLs to use any *.mycompany.com subdomain.