The WordPress Abilities API, from registration to an MCP call.

<?php
// wp-content/mu-plugins/count-drafts.php
add_action( 'wp_abilities_api_categories_init', function () {
  wp_register_ability_category( 'demo-site', array(
    'label'       => 'Demo site facts',
    'description' => 'Read-only facts about this site.',
  ) );
} );

add_action( 'wp_abilities_api_init', function () {
  wp_register_ability( 'demo/count-drafts', array(
    'label'               => 'Count drafts',
    'description'         => 'Counts draft posts of one post type.',
    'category'            => 'demo-site',
    'input_schema'        => array(
      'type'                 => 'object',
      'properties'           => array( 'post_type' => array( 'type' => 'string', 'enum' => array( 'post', 'page' ), 'default' => 'post' ) ),
      'additionalProperties' => false,
    ),
    'output_schema'       => array(
      'type'       => 'object',
      'properties' => array(
        'post_type' => array( 'type' => 'string' ),
        'drafts'    => array( 'type' => 'integer' ),
      ),
    ),
    'execute_callback'    => function ( $input ) {
      $type = sanitize_key( $input['post_type'] ?? 'post' );
      return array( 'post_type' => $type, 'drafts' => (int) wp_count_posts( $type )->draft );
    },
    'permission_callback' => function ( $input ) {
      $type = sanitize_key( $input['post_type'] ?? 'post' );
      if ( ! in_array( $type, array( 'post', 'page' ), true ) ) {
        return false;
      }
      $object = get_post_type_object( $type );
      return $object && current_user_can( $object->cap->edit_posts );
    },
    'meta'                => array(
      'show_in_rest' => true,
      'annotations'  => array( 'readonly' => true, 'destructive' => false, 'idempotent' => true ),
      'mcp'          => array( 'public' => true ),
    ),
  ) );
} );