TFS Aggregator is a extension for Team Foundation Server (TFS) and Visual Studio Team Services (VSTS)
that enables running custom script when Work Items change, allowing dynamic calculation of field values in TFS and more.
You can use the Server Plugin, for TFS 2013 update 2 up to TFS 2017 update 2,
or the Web Service version, for VSTS or TFS 2015 and later.
TFS Server API changes frequently: the Server Plugin contains specific code for each TFS version, so the binaries will work only with a specific TFS version.
The Web Service has not such a problem.