How to enable debug output for the Minimal Download Strategy (MDS)?

Question

I need to know why MDS failover for a SharePoint page is triggered.

In the browser address bar I see start.aspx when loading a page (as expected), then the browser is redirected to the actual page URL (without start.aspx). That means the MDS engine detected a page that cannot be served via MDS.

Now I need to know why this happens. I remember some time in the past seeing lots of MDS-related debug messages in the browser console. But if I look today (SP 2016) there is nothing.

How can I see/enable debug output for MDS to have more insight into what is happening?

Answer 1

The ULS log contains entries pointing to the cause of the MDS failover.

In my case:

Message: MDSFailover: script in markup

This is written as Verbose message with area SharePoint Foundation and category Unified Logging Service.

This can also be seen in the developer dashboard, no need to go to the server.