custom post type taxonomy "tag" archive : no post found

Question

I've registered a custom post type "object" and registered the taxonomy "post_tag" so that these objects can be tagged.

register_taxonomy_for_object_type('post_tag', 'object');

When I visit domain.com/tag/{object-slug} it does not find any post, although there are.

Here is how my CPT was declared:

register_post_type( 'objet', 
        array('labels' => array(/* removed for clarity */),
            'description' => __( 'blabla' ), 
            'public' => true,
            'publicly_queryable' => true,
            'exclude_from_search' => false,
            'show_ui' => true,
            'query_var' => true,
            'menu_position' => 3, 
            'menu_icon' => get_stylesheet_directory_uri() . '/library/images/custom-post-icon.png', 
            'rewrite'   => array( 'slug' => 'stock', 'with_front' => false ), 
            'has_archive' => 'stock',
            'capability_type' => 'post',
            'taxonomies' => array('post_tag'),
            'hierarchical' => false,
            'supports' => array( 'title', 'editor', 'thumbnail', 'revisions', 'sticky')
        ) 
    );

Answer 1

Tag and Category archive queries default to querying only the post post type, to add your custom post type to those queries, you can use the pre_get_posts action:

function wpa_cpt_tags( $query ) {
    if ( $query->is_tag() && $query->is_main_query() ) {
        $query->set( 'post_type', array( 'post', 'object' ) );
    }
}
add_action( 'pre_get_posts', 'wpa_cpt_tags' );