REST request trace files are created for requests where the request URI appears to be that used for REST requests. Since tracing is started before WordPress has determined that it’s processing a REST_REQUEST, oik-bwtrace checks for /wp-json/wp/v2 or /index.php?rest_route= in the $_SERVER['REQUEST_URI'].

At the end of REST processing, the filter rest_pre_echo_response allows us to see what’s being returned to the client. It traces $HTTP_RAW_POST_DATA and the 3 parameters passed to the filter result, server and request.