A TeamCity Build Agent is a piece of software which listens for the commands from the TeamCity server and starts the actual build processes. It is installed and configured separately from the TeamCity server. An agent can be installed on the same computer as the server or on a different machine (the latter is a preferred setup for server performance reasons); an agent can run the same operating system (OS) as the TeamCity server or a different OS.
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An Agent typically checks out the source code, downloads artifacts of other builds and runs the build process. An agent can run a single build at a time. The number of agents basically limits the number of parallel builds and environments in which your build processes are run.
An Agent can run builds of any compatible build configuration.
The TeamCity server monitors all the connected agents and assigns queued builds to the agents based on compatibility requirements, Agent Pools, Build Configuration restrictions configured for an agent and the selection algorithm described here.
Build Agent Status
In TeamCity, a build agent can have following statuses:
An agent is connected if it is registered on the TeamCity server and responds to server commands, otherwise it is disconnected. This status is determined automatically.
Agents are manually authorized via the web UI on the Agents page. Only authorized build agents can run builds. The number of simultaneously authorized agents cannot exceed the number of agent licenses in your license pool. When an agent is unauthorized, a license is freed and a different build agent can be authorized. Purchase additional licenses to expand the number of agents that can concurrently run builds. When a new agent is registered on the server for the first time, it is unauthorized by default and requires manual authorization to run the builds.
Agents are manually enabled/disabled via the web UI. The TeamCity server only distributes builds to agents that are enabled.
Disabled agents can still run builds, when the build is assigned to a special agent (e.g. by Triggering a Custom Build).
All agents connected to the server must have unique agent names.
Only users with certain roles can manage agents. See Role and Permission for more information.
For a build agent configuration, refer to the Build Agent Configuration section.
A TeamCity agent is upgraded automatically when necessary. The process involves downloading new agent files from the TeamCity server and restarting the agent on the new files. In order to successfully accomplish this, the user under whose account the agent runs should have enough permissions.
Typically, an agent upgrade happens when: