Nodejs REST resouce not "reusable" to start mocha tests programmatically

I started to use mocha programmatically, which means I run the test suite every time when I request the resouce localhost:3000/tests/start.

app.get('/tests/start', (req, res) => {
  var mocha = new Mocha({
    reporter: 'json'
  });

  mocha.useColors(false);
  config.tests.forEach(test => {
    if(test.enabled) 
      mocha.addFile('./test/cases/' + test.filename + '.js')
  });

  mocha.run(function(failures){
    console.log(failures);
    res.send();
  });
});

If I start node and call localhost:3000/tests/start everything works fine and my tests pass. But when I call the same resouce later again, I get only that feedback:

{
  "stats": {
    "suites": 0,
    "tests": 0,
    "passes": 0,
    "pending": 0,
    "failures": 0,
    "start": "2017-04-10T14:54:14.322Z",
    "end": "2017-04-10T14:54:14.322Z",
    "duration": 0
  },
  "tests": [],
  "pending": [],
  "failures": [],
  "passes": []
}

I thought REST is stateles... Do anyone know more about this "issue" or a good way to debug my application? Thanks!

via Fabian