Serialized settings in rest api

I have registered settings and need it to be showed in REST API.

$args = [
    'show_in_rest' => true
];
register_setting('default_sidebars', 'default_sidebars', $args);

When I save a single value and then request data on endpoint /wp-json/wp/v2/settings, everything works perfectly.

But the problem is that I save serialized data like this.

a:2:{s:4:"post";s:7:"general";s:4:"blog";s:9:"sidebar_1";}

Now I would expect in response something like this:

default_sidebars: {
  post: "general",
  blog: "sidebar_1"
}

But instead I got default_sidebars: null.

What should I do to get my data in REST API?

JSON Schema

It’s supported if you explicitly register the object JSON schema as:

$args = array(
    'show_in_rest' => array(
        'schema' => array(
            'type'       => 'object',
            'properties' => array(
                'post' => array(
                    'type' => 'string',
                ),
                'blog' => array(
                    'type' => 'string',
                ),
            )
        ),
    ),
);

register_setting( 'default_sidebars', 'default_sidebars', $args );

Resulting in the following object:

default_sidebars: {
  post: "general",
  blog: "sidebar_1"
}

in /wp-json/wp/v2/settings for the serialized option data:

a:2:{s:4:"post";s:7:"general";s:4:"blog";s:9:"sidebar_1";}

See similar object schema support for register_meta() in 5.3:

WP 5.3 Supports Object and Array Meta Types in the REST API