TeamCity agent cannot reconnect to server: plugin download fails with “Connection reset”

Symptoms

• Agents enter an infinite loop downloading plugins and never reconnect.

• The teamcity-agent.log file on the agent side shows repeated failures to download a plugin file from the server, e.g.:

  [2026-01-29 13:23:48,088]   INFO -    jetbrains.buildServer.AGENT - Downloading http://<server_name>/update/plugins/<plugin_name>.zip ==> C:\BuildAgent\update\plugins\<plugin_name>.zip
  [2026-01-29 13:25:07,587]   WARN -    jetbrains.buildServer.AGENT - Error downloading file "C:\BuildAgent\update\plugins\<plugin_name>.zip". Will try to re-download the file. Error: Failed to download file http://<server name>/update/plugins/<plugin_name>.zip to C:\BuildAgent\update\plugins\<plugin_name>.zip. Connection reset
  [2026-01-29 13:25:08,089]   INFO -    jetbrains.buildServer.AGENT - Downloading http://<server name>/update/plugins/<plugin_name>.zip ==> C:\BuildAgent\update\plugins\<plugin_name>.zip
  

Cause

Possible causes include:

• Antivirus or security software is terminating the TCP connection during file transfer.
• Reverse proxy/firewall (e.g., Nginx/IIS) blocking or timing out the download.

Resolution

To narrow down the cause of this issue:

• Temporarily disable antivirus or security software on the agent host.
• Bypass any proxy between the agent and the TeamCity server.

If the issue persists, contact TeamCity support and provide:

• All logs from the affected agent: <Agent home directory>/logs/*
• All teamcity-server.log* files (from Administration → Diagnostics → Server Logs)
• Confirmation whether the file mentioned in the error message (e.g., http://<server_name>/update/plugins/<plugin_name>.zip) is downloadable manually from the affected agent (e.g., curl -O http://<server_name>/update/plugins/<plugin_name>.zip)