custom field value is blank or empty

Question

function add_custom_fields($post_id) {
    global $post;

    $metadescription = wp_trim_words( get_the_content(), 55 );
    add_post_meta( $post_id, 'meta_description', $metadescription, true );
}
    
add_action('wp_insert_post', 'add_custom_fields');

Answer 1

You need to pass the $post_id into the get_the_content() :

function add_custom_fields($post_id) {
    global $post;

    $metadescription = wp_trim_words( get_the_content($post_id), 55 );
    add_post_meta($post_id, 'meta_description', '' . $metadescription . '', true);
}
    
add_action('wp_insert_post', 'add_custom_fields');