The reverse lookup is also available as JSON at /tools/geocoder/reverse with lat and lon query parameters (WGS84 degrees). Optional max_distance sets the search radius in metres (default 50). The endpoint requires no authentication; there are no rate limits so far.

$ curl 'https://nafn.de/tools/geocoder/reverse?lat=52.52&lon=13.405'
{"city":"Berlin","distance_m":46,"postcode":"10178","street":"Spandauer Straße","housenumber":"4"}