If using a @ApplicationPath("") or @ApplicationPath("/") the JSAPIWriter appends a trailing slash onto the uri. The endpoint is then also appended in the generated javascript client which results in a double slash in the final url.

For example from generated client javascript:

REST.apiURL = 'https://localhost:17080/api/';
var ListApi = {};
// POST /list/contact
ListApi.contact = function(_params){
 var params = _params ? _params : {};
 var request = new REST.Request();
 request.setMethod('POST');
 var uri = params.$apiURL ? params.$apiURL : REST.apiURL;
 uri += '/list/contact';

The end url is: https://localhost:17080/api//list/contact which fails due to the double slash.

The buggy code in question is in JSAPIWriter at line 51:

      for (Map.Entry<String, ServiceRegistry> entry : serviceRegistries.entrySet())
      {
         String uri = base;
         if (entry.getKey() != null) uri += entry.getKey();

For a empty @ApplicationPath(""), the service registry will have an entry.getKey() of "/" which erroneously gets appended to the uri.

Fix could maybe be:

if (entry.getKey() != null && !entry.getKey().equals("/")) uri += entry.getKey();