yarn install
yarn serve --project YOUR_PROJECT_ID

1. Access to http://localhost:5001/YOUR_PROJECT_ID/us-central1/checkUA
2. You'll see your user agent.

Example

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36

1. Access to http://localhost:5000/foobar
2. You'll see firebasecli/9.1.0 regardless of your user agent.