diff --git a/src/api/session.js b/src/api/session.js
index e5ff93359..c58fbbbbe 100644
--- a/src/api/session.js
+++ b/src/api/session.js
@@ -1,4 +1,4 @@
-import {deprecate} from 'util'
+import { deprecate, getUserPublicProperties } from 'util'
 
 import {InvalidCredential, AlreadyAuthenticated} from '../api-errors'
 
@@ -15,7 +15,7 @@ export async function signIn (credentials) {
   }
   this.session.set('user_id', user.id)
 
-  return this.getUserPublicProperties(user)
+  return getUserPublicProperties(user)
 }
 
 signIn.description = 'sign in'
@@ -55,7 +55,7 @@ export async function getUser () {
 
   return userId === undefined
     ? null
-    : this.getUserPublicProperties(await this.getUser(userId))
+    : getUserPublicProperties(await this.getUser(userId))
 }
 
 getUser.description = 'return the currently connected user'
diff --git a/src/api/user.js b/src/api/user.js
index 3f70b8ccc..541724f97 100644
--- a/src/api/user.js
+++ b/src/api/user.js
@@ -1,5 +1,5 @@
 import {InvalidParameters} from '../api-errors'
-import { mapToArray } from '../utils'
+import { getUserPublicProperties, mapToArray } from '../utils'
 
 // ===================================================================
 
@@ -48,7 +48,7 @@ export async function getAll () {
   const users = await this.getAllUsers()
 
   // Filters out private properties.
-  return mapToArray(users, this.getUserPublicProperties)
+  return mapToArray(users, getUserPublicProperties)
 }
 
 getAll.description = 'returns all the existing users'
diff --git a/src/api/vm.coffee b/src/api/vm.coffee
index db98c4ae7..1f90a963a 100644
--- a/src/api/vm.coffee
+++ b/src/api/vm.coffee
@@ -819,8 +819,7 @@ stop = $coroutine ({vm, force}) ->
     yield xapi.call 'VM.clean_shutdown', vm._xapiRef
   catch error
     if error.code is 'VM_MISSING_PV_DRIVERS' or error.code is 'VM_LACKS_FEATURE_SHUTDOWN'
-      # TODO: Improve reporting: this message is unclear.
-      @throw 'INVALID_PARAMS'
+      throw new InvalidParameters('clean shutdown requires PV drivers')
     else
       throw error
 
@@ -1116,10 +1115,7 @@ setBootOrder = $coroutine ({vm, order}) ->
     yield xapi.call 'VM.set_HVM_boot_params', vm._xapiRef, order
     return true
 
-  @throw(
-    'INVALID_PARAMS'
-    'You can only set the boot order on a HVM guest'
-  )
+  throw new InvalidParameters('You can only set the boot order on a HVM guest')
 
 setBootOrder.params = {
   vm: { type: 'string' },
diff --git a/src/index.js b/src/index.js
index 9bd79b1a6..c6a11ac05 100644
--- a/src/index.js
+++ b/src/index.js
@@ -9,7 +9,6 @@ import eventToPromise from 'event-to-promise'
 import has from 'lodash/has'
 import helmet from 'helmet'
 import includes from 'lodash/includes'
-import pick from 'lodash/pick'
 import proxyConsole from './proxy-console'
 import serveStatic from 'serve-static'
 import startsWith from 'lodash/startsWith'
@@ -18,14 +17,8 @@ import { compile as compilePug } from 'pug'
 import { createServer as createProxyServer } from 'http-proxy'
 import { join as joinPath } from 'path'
 
-import {
-  AlreadyAuthenticated,
-  InvalidCredential,
-  InvalidParameters,
-  NoSuchObject,
-  NotImplemented
-} from './api-errors'
 import JsonRpcPeer from 'json-rpc-peer'
+import { InvalidCredential } from './api-errors'
 import {
   readFile,
   readdir
@@ -407,27 +400,6 @@ const setUpStaticFiles = (express, opts) => {
 
 // ===================================================================
 
-const errorClasses = {
-  ALREADY_AUTHENTICATED: AlreadyAuthenticated,
-  INVALID_CREDENTIAL: InvalidCredential,
-  INVALID_PARAMS: InvalidParameters,
-  NO_SUCH_OBJECT: NoSuchObject,
-  NOT_IMPLEMENTED: NotImplemented
-}
-
-const apiHelpers = {
-  getUserPublicProperties (user) {
-    // Handles both properties and wrapped models.
-    const properties = user.properties || user
-
-    return pick(properties, 'id', 'email', 'groups', 'permission', 'preferences', 'provider')
-  },
-
-  throw (errorId, data) {
-    throw new (errorClasses[errorId])(data)
-  }
-}
-
 const setUpApi = (webServer, xo, verboseLogsOnErrors) => {
   const webSocketServer = new WebSocket.Server({
     server: webServer,
@@ -437,7 +409,7 @@ const setUpApi = (webServer, xo, verboseLogsOnErrors) => {
 
   // FIXME: it can cause issues if there any property assignments in
   // XO methods called from the API.
-  const context = { __proto__: xo, ...apiHelpers }
+  const context = { __proto__: xo }
 
   const api = new Api({
     context,
diff --git a/src/utils.js b/src/utils.js
index 0e7102404..d015158f8 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -11,6 +11,7 @@ import isString from 'lodash/isString'
 import keys from 'lodash/keys'
 import kindOf from 'kindof'
 import multiKeyHashInt from 'multikey-hash'
+import pick from 'lodash/pick'
 import xml2js from 'xml2js'
 
 // Moment timezone can be loaded only one time, it's a workaround to load
@@ -177,6 +178,13 @@ export function extractProperty (obj, prop) {
 
 // -------------------------------------------------------------------
 
+export const getUserPublicProperties = user => pick(
+  user.properties || user,
+  'id', 'email', 'groups', 'permission', 'preferences', 'provider'
+)
+
+// -------------------------------------------------------------------
+
 export const getPseudoRandomBytes = n => {
   const bytes = new Buffer(n)