If you already have the Compare extension (user interface) installed on one of your sites, but want to use the Compare feature on another site, you have the option to do an Agent-only install. This will give you the ability to compare against the site, but not have the UI that comes with the Compare extension.
Install the Toolkit Agent via the NuGet Package Manager on the Kentico admin site/project.
Be sure to use the correct version (based on YOUR version of Kentico):
Install-Package BizStream.KenticoToolkit.Agent.KX13
Install-Package BizStream.KenticoToolkit.Agent.K12
Install-Package BizStream.KenticoToolkit.Agent.K11
Install-Package BizStream.KenticoToolkit.Agent.K10
Install-Package BizStream.KenticoToolkit.Agent.K9
Install-Package BizStream.KenticoToolkit.Agent.K82