What is a must-use plugin (mu-plugin) in WordPress?

A must-use plugin (mu-plugin) is a PHP file placed in wp-content/mu-plugins. WordPress loads it automatically, before the normal plugins. You cannot deactivate it from the Plugins screen, and it has no activation or update steps.

<?php
/**
 * Plugin Name: TC Must Use Demo
 */
add_filter( 'the_title', function ( $title ) {
	return $title . ' *';
} );
Terminal output

$ wp plugin list –status=must-use –fields=name,status
name status
tc-mu must-use
$ curl -s http://example.com/hello-world/ | grep -o "<title>[^<]*"
<title>Hello world! &#8211; Demo Site

  • Files in sub folders are not loaded automatically, you need a loader file.
  • Use it for code that must always run: site wide rules, security checks or hosting features.
  • The folder does not exist by default, you create it.

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