While some JSON parsers may consider it invalid, both PHP and Javascript treat a simple true response as valid JSON:
true
In PHP, the following code snippets demonstrate how "true" is considered valid JSON:
PHP-
echo json_encode( true ); // outputs: true
echo json_decode( true ); // outputs: 1
echo gettype(json_decode( true )); // outputs: boolean
Similarly in jQuery:
JSON.stringify( true ); // outputs: true
jQuery.parseJSON( true ); // outputs: true
typeof jQuery.parseJSON( true ); // outputs: boolean
Despite what validators may suggest, it appears that sending a true response formatted as JSON is just fine. Perhaps the validators need to reconsider their criteria.