Jenkins "Failed to load" Plugin at Startup - Fix Disabled Plugins

During boot Jenkins tried to initialize a plugin and could not, so it disabled it and showed a warning. The plugin file may be corrupt, half-upgraded, or missing a dependency - and everything that plugin provides is now unavailable.

What this error means

The startup banner or Manage Jenkins shows Failed to load: <plugin> with one or more disabled plugins. Jobs that depend on those plugins break, and the plugin appears disabled in the Plugin Manager.

jenkins.log

SEVERE: Failed Loading plugin Foo Plugin v1.2 (foo)
java.io.IOException: Failed to initialize foo
Caused by: java.lang.NoClassDefFoundError: org/example/Bar
# or
WARNING: Plugin foo.jpi failed to load and was disabled

Common causes

Corrupt or partially-written plugin file

An interrupted download/upgrade leaves a truncated .hpi/.jpi (or a stray .jpi.bak/.disabled), which Jenkins cannot load.

Unmet dependency at load time

A class the plugin needs is missing (NoClassDefFoundError) because a dependency plugin failed to load first or is absent.

How to fix it

Reinstall the plugin cleanly

Stop Jenkins.
Remove the bad plugin artifacts from $JENKINS_HOME/plugins/ (the .jpi, its expanded dir, and any .jpi.bak/.disabled).
Reinstall the plugin (and its dependencies) at a compatible version, then start Jenkins.

Resolve the missing dependency

If the log shows a NoClassDefFoundError or a failed dependency, install/upgrade the dependency plugin so the load order can complete.

Terminal

# rebuild the plugin set from a pinned list
jenkins-plugin-cli --plugin-file plugins.txt

How to prevent it

Install plugins from a pinned plugins.txt so the set is reproducible.
Back up $JENKINS_HOME (including plugins) before upgrades.
Verify a full, clean restart in staging after plugin changes.