In TeamCity an agent tool (i.e. a set of files/a binary distribution) is a type of plugin without any classes loaded into the runtime; agent tools are used to only distribute binary files to agents.

TeamCity allows you to install / remove additional tools on all the agents, which is especially useful in the environments with a large number of build agents as you can distribute tools to or remove them from all build agents at once, centralize configuration files distribution (e.g. you want to distribute a custom configuration file/library to all agents), etc.

The Administration | Tools page provides a unified interface to set up tools to be used by appropriate plugins. You can install different versions of a tool and/or change the default one. The tools will be automatically distributed to all build agents to be used in the related runners. 

The following types of tools can be managed up via the Administration | Tools page:

You can see that the tool appears on the agent in the TeamCity Web UI by checking configuration parameters reported by the agent in the form teamcity.tool.<the installed tool id>. You can use this parameter in your build: reference this parameter in the TeamCity Web UI (anywhere where the %parameter% format is supported) or refer to this parameters in your build as an environment or a system parameter.

 

To distribute a tool to all agents, TeamCity places them into the  <TeamCity Data Directory>/plugins/.tools and monitors the content of this folder. Agents will restart in the process of obtaining the tool.