TIL: Any quoted string is a valid JSON text since RFC-7159 (2014).
A JSON text is a serialized value. Note that certain previous specifications of JSON constrained a JSON text to be an object or an array. […] A JSON value MUST be an object, array, number, or string, or one of the following three literal names: false null true
The problem is that some parsers still only accept objects or arrays, so people might complain even if your API returns valid JSON.