Log WordPress REST API Errors

wp-rest-api-error-log.php

<?php
/**
 * Log REST API errors
 *
 * @param WP_REST_Response $result  Result that will be sent to the client.
 * @param WP_REST_Server   $server  The API server instance.
 * @param WP_REST_Request  $request The request used to generate the response.
 */
function log_rest_api_errors( $result, $server, $request ) {
	if ( $result->is_error() ) {
		error_log( sprintf(
			"REST request: %s: %s",
			$request->get_route(),
			print_r( $request->get_params(), true )
		) );
		error_log( sprintf(
			"REST result: %s: %s",
			$result->get_matched_route(),
			print_r( $result->get_data(), true )
		) );
	}
	return $result;
}
add_filter( 'rest_post_dispatch', 'log_rest_api_errors', 10, 3 );