Jenkins Agent Offline - Node Disconnected and Builds Won’t Start

An agent is marked offline, so jobs targeting it never start. Unlike a mid-build drop, this is a launch/connection problem: the agent failed to connect or was taken offline, often due to launch config, Java, disk, or a secret/clock mismatch.

What this error means

On Manage Jenkins → Nodes the agent shows offline (or "Disconnected"), and jobs with that label sit queued. The agent log shows the launch attempt failing rather than a build error.

Jenkins agent log

agent-windows-2 is offline
Launch failed - cleaning up connection
java.io.IOException: Unexpected termination of the channel
SEVERE: Failed to connect: connection refused

Common causes

Launch method misconfigured

A wrong SSH host/key/credential, a blocked JNLP inbound port, or an incorrect agent JAR/secret prevents the agent from completing its connection handshake.

Wrong or missing Java on the agent

The agent host has no compatible JRE (or the wrong major version), so the remoting agent cannot start.

Agent taken offline by a monitor

Built-in monitors mark a node offline when free disk, temp space, or response time crosses a threshold - the node is healthy network-wise but disqualified.

How to fix it

Read the agent log and fix the launch

Open Manage Jenkins → Nodes → <agent> → Log to see why the launch failed.
For SSH agents, verify host, port, credentials, and that the key is authorized.
For inbound (JNLP) agents, confirm the agent port is open and the secret matches.

Install a compatible Java and clear disk pressure

Terminal (agent)

# on the agent host
java -version        # must satisfy the controller's required Java
df -h /              # free space if a disk monitor took it offline

How to prevent it

Provision agents from a known-good image with the correct Java preinstalled.
Alert on the node disk/response monitors before they force agents offline.
Manage agent connection secrets/credentials so they do not drift.