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On starting TeamCity server (and updating agent distribution or plugins on the server), TeamCity agents connected to the server and correctly installed are automatically updated to the version corresponding to the server. This occurs for both server upgrades and downgrades. If there is a running build on the agent, the build finishes. No new builds are started on the agent unless the agent is up to date with the server.
Since TeamCity 2018.2, before starting the agent upgrade, the agent is checked for free disk space, 3 gb by default. To modify the value required for the upgrade, configure the teamcity.agent.upgrade.ensure.free.space agent property.
The agent update procedure is as follows: The agent (agent.bat, agent.sh, or agent service) will download the current agent package from the TeamCity server. When the download is complete and the agent is idle, it will start the upgrade process (the agent is stopped, the agent files are updated, and the agent is restarted). This process may take several minutes depending on the agent hardware and network bandwidth. Do not interrupt the upgrade process, as doing so may cause the upgrade to fail and you will need to manually reinstall the agent.
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