How to add a filter to a custom post type to get adjacent custom posts via the REST API

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That filter is actually dynamic, so you’ll just need to append your post type:

add_filter( 'rest_prepare_projects', 'chrest_adjacent_posts', 10, 3 );

https://github.com/WP-API/WP-API/blob/develop/lib/endpoints/class-wp-rest-posts-controller.php#L1355

Hope that helps!!!