sorry for the late reply and many thanks for your help
I managed to filter $_post['post_meta'] directly inside json_prepare_post
reason I had to do this was to completely rewrite my ['post_meta'] data
I'm using Advanced Custom Fields and arrays are natively stored in strings, whereas I needed arrays in the json object
do you have any hints regarding this? using common WordPress conditionals for query (is_singular, is_archive...) is useless, since the posts looped in the /posts/ route are all individual... I should filter the /posts/ route or should I create an entirely new route?
I'm a bit confused since I'm new in working with APIs and WP-API...