This is awesome!!

Clarifying question - re: "special tables that only support row-based sampling", would this be tables with custom/non-default storage definitions?

I'd recommend looking at what (other than valid JSON) is in result.HttpResponse.Data. Also worth looking at result.HttpResponse.StatusCode - is it 400 or 404?

I'm not aware of any JSON schema capability.

%Dictionary has two sets of classes: *Definition and Compiled*. If you want to see inherited members you need to look at Compiled* (e.g., CompiledClass / CompiledProperty). If you want to know from which class, there's an "Origin" property.

##class(%SYS.ProcessQuery).GetOSUsername()

These would be set via the Configuration Registry, though it seems they aren't discussed in the docs for it. See: https://docs.intersystems.com/irisforhealthlatest/csp/docbook/DocBook.UI.Page.cls?KEY=HXREG_ch_configuration_registry

Here's a sample that loads interoperability system defaults from an XML file:

Class Sample.CCREventHandler Extends %Studio.SourceControl.CCREventHandler
{

/// This method is called during the loading of an ItemSet, after the contents of the ItemSet have been loaded into the namespace, 
/// and after the ImplementCCR routine has been run
Method ItemSetAfterLoadToNS() As %Status
{
    set key = ""
    for {
        set item = ..ItemSetItemList.GetNext(.key)
        quit:(key="")
        if (key [ "backup/ens/defaults.xml") {
            $$$ThrowOnError(##class(Ens.Config.DefaultSettings).%Import(^Sources_"backup/ens/defaults.xml"))
            quit
        }
    }
    Quit ##super()
}

/// This method is called by the CCR Refresh logic, after the items have been refreshed into the namespace.  It is intended for any additional configuration work which 
/// may be necessary (e.g. initialization of reference tables, building of 3rd party sources, etc)
Method RefreshFinalize() As %Status
{
    $$$ThrowOnError(##class(Ens.Config.DefaultSettings).%Import(^Sources_"backup/ens/defaults.xml"))
    Quit ##super()
}

}

Things I mentioned in the kickoff webinar:

%SQL.AbstractFind/%Library.FunctionalIndex implementations:
Semantic version index:
https://github.com/intersystems-community/zpm/blob/master/src/%25ZPM/Ge…
https://github.com/intersystems-community/zpm/blob/master/src/%25ZPM/Ge…
https://github.com/intersystems-community/zpm/blob/master/src/%25ZPM/Re…
Spatial index: https://github.com/intersystems-ru/spatialindex
Image color index: https://openexchange.intersystems.com/package/iris-image-index-demo
Inspiration for a project:
Implement a hierarchical index that operates on a table with a self-referential field (e.g., an employee table with a manager field) to return (via a %SQL.AbstractFind implementation) all records above (directly/indirectly), below (directly/indirectly), or in the same tree (or graph - need to consider circular references) as a given record.

Some things that would help diagnose the issue:
write $zv
Modify TestCoverage.Manager to output the following as soon as possible after the error occurs:
write $zu(56,2)

It would also help to know what arguments are being passed to %Monitor.System.LineByLine:Start.

I've published TestCoverage 2.1.3 on the Open Exchange / zpm - in the situations leading to this, you'll get a more descriptive error message now and the system won't be left in a bad state.

@Alex Kogan is this just a matter of = vs. ==? (The worst thing when switching between ObjectScript and JS...)