Setting parse-numbers: false caused number options to become strings as well. Since we seem to have little control over positional arguments, let's make it an option instead.

lib/cli/device/index.js

@@ -1,4 +1,4 @@
-module.exports.command = 'device <serial>'
+module.exports.command = 'device'
 
 module.exports.builder = function(yargs) {
   return yargs
@@ -101,6 +101,11 @@ module.exports.builder = function(yargs) {
     , type: 'string'
     , default: 'ws://${publicIp}:${publicPort}'
     })
+    .option('serial', {
+      describe: 'The USB serial number of the device.'
+    , type: 'string'
+    , demand: true
+    })
     .option('storage-url', {
       alias: 'r'
     , describe: 'The URL to the storage unit.'

lib/cli/provider/index.js

@@ -159,7 +159,8 @@ module.exports.handler = function(argv) {
       var fork = require('child_process').fork
 
       var args = [
-        'device', device.id
+        'device'
+      , '--serial', device.id
       , '--provider', argv.name
       , '--screen-port', ports.shift()
       , '--connect-port', ports.shift()

package.json

@@ -25,6 +25,9 @@
   "bin": {
     "stf": "./bin/stf"
   },
+  "yargs": {
+    "duplicate-arguments-array": false
+  },
   "scripts": {
     "test": "gulp test",
     "prepublish": "bower install && not-in-install && gulp build || in-install"