Unsupported browser or Safari version: WhatsApp, Slack, etc.

[kofigumbs]

Some sites, like WhatsApp and Slack, disable functionality based on the user-agent your browser provides.
The easiest way to workaround issues like this is to pretend to be Safari:

{
  "tabs": [
    {
      "title": "WhatsApp",
      "url": "https://web.whatsapp.com",
      "userAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/15.6 Safari/605.1.15"
    }
  ]
}

Historical note

Before v2.2.0, Multi included a hard-coded user-agent that made it appear like Safari.
This behavior caused subtle issues and confusion when the hard-coded user-agent didn't reflect the system WebKit version.
Recent Multi versions remove the hard-coded user-agent, but now sites like Slack and WhatsApp complain that your browser is out of date. (Ideally these sites would use feature detection instead of user-agent sniffing to gracefully degrade behavior; alas, the world does not work ideally.)