web-preview.mdx

title	Web Preview
sidebarTitle	Web Preview
icon	browser

Codegen's Web Preview feature lets you start a development server in your sandbox and view your running application directly in the Codegen interface.

How it Works

Define your web server startup commands just like Setup Commands. Instead of taking a snapshot, Codegen keeps the server running as a long-lived process.

A "View Web Preview" button appears on the agent trace page once the server starts. Click it to open your running application in a new tab through Codegen's secure proxy.

Configuration

You configure Web Preview commands in a manner similar to Setup Commands, likely within the same repository settings area (e.g., https://codegen.com/{your_org}/{repo_name}/settings/web-preview).

You'll provide the command(s) necessary to start your development server. Ensure that your server is configured to listen on an appropriate host (often 127.0.0.1) and a predictable port that Codegen can then expose.

The web server started for Web Preview **MUST** listen on port 3000. Codegen is specifically configured to look for and expose applications running on this port within the sandbox.

Common Examples

The primary use case is starting a development web server:

# For a Node.js/npm project
npm run dev

# For a Python/Django project
python manage.py runserver 127.0.0.1:3000

# For a Ruby on Rails project
bundle exec rails server -b 127.0.0.1 -p 3000

The Web Preview server runs within the same sandbox environment as your other agent tasks, meaning it has access to the same file system (including any changes made by the agent) and the [Environment Variables](./environment-variables). The `CG_PREVIEW_URL` environment variable is automatically set and contains the URL where your web preview will be accessible. Use this in your application code when you need to reference the preview URL programmatically (e.g., for CORS configuration, webhooks, or generating absolute URLs). The web preview is intended for development and debugging purposes. The server is typically only accessible while the agent run is active or for a short period afterward, and it's not designed for public hosting.