How do I get the site url from a Block (within the block editor)

Perhaps this is because I’ve got WP set up as multisite.

Yes, that’s right, WordPress disables the siteurl (and also admin_email) settings on a Multisite — see register_initial_settings() for the source.

And I don’t know why they disable those (two) settings, but you can explicitly enable them like so for the url/siteurl setting, which for example, would go in the theme’s functions.php file:

// I copied this from the source..
register_setting(
    'general',
    'siteurl',
    array(
        'show_in_rest' => array(
            'name'   => 'url',
            'schema' => array(
                'format' => 'uri',
            ),
        ),
        'type'         => 'string',
        'description'  => __( 'Site URL.' ),
    )
);

Tried & tested working with WordPress 5.6.1 Multisite.

And also, wp.data.select( 'core' ).getSite() uses the Settings endpoint, so it’s normal that the url property is missing by default on a Multisite.

Apart from that, getSite() caches the API results, so you’d need to reload the post editing screen/page in order for getSite() to give you the correct result, i.e. with the url property.

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