https://gerrit.wikimedia.org/r/#/c/111255/1 is work in progress for a new API. It works: api.php?action=query&prop=flow&titles=Talk:Main_Page returns something. However, because this API module doesn't implement getAllowedParams() and getParamDescription(), Special:ApiSandbox has errors if you try to invoke it. If I choose action=query, prop=flow, "Loading..." spins forever, the [Make Request] button remains disabled, and the browser console shows the error
TypeError: this.params is undefined http://localhost/wiki/extensions/ApiSandbox/resources/UiBuilder.js Line 62
The fix is for ApiSandbox's JS to handle an absent parameters list. It would be nice if it warned "<modulename> does not supply allowed parameters".