Is there a replacement that features the full trader language? We expose that in ktraderclient: https://invent.kde.org/plasma/kde-cli-tools/-/blob/master/ktraderclient/ktraderclient.cpp

No, we determined that the trader language wasn't the best way to write C++ code.
ktraderclient was mostly written as a debugging helper. I suggest to strip out the constraint functionality from ktraderclient.

I've hit a minor problem while porting:

In https://invent.kde.org/frameworks/kio/-/blob/master/src/core/kprotocolmanager.cpp#L729 KIO uses KMimeTypeTrader to find the preferred browser part and extracts the default user agent from it.

The idiomatic solution would be to port to KParts::PartLoader, but since KParts depends on KIO that would create a cyclic dependency between KIO and KParts

My thoughts on that code are the following: T14307. But actually I just realized something.... I'll add a comment there.

This was done with https://invent.kde.org/frameworks/kservice/-/commit/041005e3c38959bee61c7ff3d6830ff477ce178c