diff --git a/python/ycm/tests/event_notification_test.py b/python/ycm/tests/event_notification_test.py
new file mode 100644
index 00000000..f7ee868f
--- /dev/null
+++ b/python/ycm/tests/event_notification_test.py
@@ -0,0 +1,286 @@
+#
+# Copyright (C) 2015 YouCompleteMe contributors
+#
+# This file is part of YouCompleteMe.
+#
+# YouCompleteMe is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# YouCompleteMe is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with YouCompleteMe.  If not, see <http://www.gnu.org/licenses/>.
+
+from ycm.test_utils import MockVimModule
+MockVimModule()
+
+import contextlib, os
+
+from ycm.youcompleteme import YouCompleteMe
+from ycmd import user_options_store
+from ycmd.responses import UnknownExtraConf
+
+from mock import call, MagicMock, patch
+
+
+# The default options which are only relevant to the client, not the server and
+# thus are not part of default_options.json, but are required for a working
+# YouCompleteMe object.
+DEFAULT_CLIENT_OPTIONS = {
+  'server_log_level': 'info',
+  'extra_conf_vim_data': [],
+}
+
+def PostVimMessage_Call( message ):
+  """Return a mock.call object for a call to vimsupport.PostVimMesasge with the
+  supplied message"""
+  return call( 'redraw | echohl WarningMsg | echom \''
+               + message +
+               '\' | echohl None' )
+
+
+def PresentDialog_Confirm_Call( message ):
+  """Return a mock.call object for a call to vimsupport.PresentDialog, as called
+  why vimsupport.Confirm with the supplied confirmation message"""
+  return call( message, [ 'Ok', 'Cancel' ] )
+
+
+@contextlib.contextmanager
+def MockArbitraryBuffer( filetype, native_available = True ):
+  """Used via the with statement, set up mocked versions of the vim module such
+  that a single buffer is open with an arbitrary name and arbirary contents. Its
+  filetype is set to the supplied filetype"""
+  with patch( 'vim.current' ) as vim_current:
+    def VimEval( value ):
+      """Local mock of the vim.eval() function, used to ensure we get the
+      correct behvaiour"""
+
+      if value == '&omnifunc':
+        # The omnicompleter is not required here
+        return ''
+
+      if value == 'getbufvar(0, "&mod")':
+        # Ensure that we actually send the even to the server
+        return 1
+
+      if value == 'getbufvar(0, "&ft")' or value == '&filetype':
+        return filetype
+
+      raise ValueError( 'Unexpected evaluation' )
+
+    # Arbitrary, but valid, cursor position
+    vim_current.window.cursor = (1,2)
+
+    # Arbitrary, but valid, single buffer open
+    current_buffer = MagicMock()
+    current_buffer.number = 0
+    current_buffer.filename = os.path.realpath( 'TEST_BUFFER' )
+    current_buffer.name = 'TEST_BUFFER'
+
+    # The rest just mock up the Vim module so that our single arbitrary buffer
+    # makes sense to vimsupport module.
+    with patch( 'vim.buffers', [ current_buffer ] ):
+      with patch( 'vim.current.buffer', current_buffer ):
+        with patch( 'vim.eval', side_effect=VimEval ):
+          yield
+
+
+@contextlib.contextmanager
+def MockEventNotification( response_method, native_filetype_completer = True ):
+  """Mock out the EventNotification client request object, replacing the
+  Response handler's JsonFromFuture with the supplied |response_method|.
+  Additionally mock out YouCompleteMe's FiletypeCompleterExistsForFiletype
+  method to return the supplied |native_filetype_completer| parameter, rather
+  than querying the server"""
+
+  # We don't want the event to actually be sent to the server, just have it
+  # return success
+  with patch( 'ycm.client.base_request.BaseRequest.PostDataToHandlerAsync',
+              return_value = MagicMock( return_value=True ) ):
+
+    # We set up a fake a Response (as called by EventNotification.Response)
+    # which calls the supplied callback method. Generally this callback just
+    # raises an apropriate exception, otherwise it would have to return a mock
+    # future object.
+    #
+    # Note: JsonFromFuture is actually part of ycm.client.base_request, but we
+    # must patch where an object is looked up, not where it is defined.
+    # See https://docs.python.org/dev/library/unittest.mock.html#where-to-patch
+    # for details.
+    with patch( 'ycm.client.event_notification.JsonFromFuture',
+                side_effect = response_method ):
+
+      # Filetype available information comes from the server, so rather than
+      # relying on that request, we mock out the check. The caller decides if
+      # filetype completion is available
+      with patch(
+        'ycm.youcompleteme.YouCompleteMe.FiletypeCompleterExistsForFiletype',
+        return_value = native_filetype_completer ):
+
+        yield
+
+
+class EventNotification_test( object ):
+
+  def setUp( self ):
+    options = dict( user_options_store.DefaultOptions() )
+    options.update ( DEFAULT_CLIENT_OPTIONS )
+    user_options_store.SetAll( options )
+
+    self.server_state = YouCompleteMe( user_options_store.GetAll() )
+    pass
+
+
+  def tearDown( self ):
+    if self.server_state:
+      self.server_state.OnVimLeave()
+
+
+  @patch( 'vim.command' )
+  def FileReadyToParse_NonDiagnostic_Error_test( self, vim_command ):
+    # This test validates the behaviour of YouCompleteMe.ValidateParseRequest in
+    # combination with YouCompleteMe.OnFileReadyToParse when the completer
+    # raises an exception handling FileReadyToParse event notification
+    ERROR_TEXT = 'Some completer response text'
+
+    def ErrorResponse( *args ):
+      raise RuntimeError( ERROR_TEXT )
+
+    with MockArbitraryBuffer( 'javascript' ):
+      with MockEventNotification( ErrorResponse ):
+        self.server_state.OnFileReadyToParse()
+        assert self.server_state.DiagnosticsForCurrentFileReady()
+        self.server_state.ValidateParseRequest()
+
+        # The first call raises a warning
+        vim_command.assert_has_calls( [
+          PostVimMessage_Call( ERROR_TEXT ),
+        ] )
+
+        # Subsequent calls don't re-raise the warning
+        self.server_state.ValidateParseRequest()
+        vim_command.assert_has_calls( [
+          PostVimMessage_Call( ERROR_TEXT ),
+        ] )
+
+        # But it does if a subsequent event raises again
+        self.server_state.OnFileReadyToParse()
+        assert self.server_state.DiagnosticsForCurrentFileReady()
+        self.server_state.ValidateParseRequest()
+        vim_command.assert_has_calls( [
+          PostVimMessage_Call( ERROR_TEXT ),
+          PostVimMessage_Call( ERROR_TEXT ),
+        ] )
+
+
+  @patch( 'vim.command' )
+  def FileReadyToParse_NonDiagnostic_Error_NonNative_test( self, vim_command ):
+    with MockArbitraryBuffer( 'javascript' ):
+      with MockEventNotification( None, False ):
+        self.server_state.OnFileReadyToParse()
+        self.server_state.ValidateParseRequest()
+        vim_command.assert_not_called()
+
+
+  @patch( 'ycm.client.event_notification._LoadExtraConfFile' )
+  @patch( 'ycm.client.event_notification._IgnoreExtraConfFile' )
+  def FileReadyToParse_NonDiagnostic_ConfirmExtraConf_test(
+      self,
+      ignore_extra_conf,
+      load_extra_conf,
+      *args ):
+
+    # This test validates the behaviour of YouCompleteMe.ValidateParseRequest in
+    # combination with YouCompleteMe.OnFileReadyToParse when the completer
+    # raises the (special) UnknownExtraConf exception
+
+    FILE_NAME = 'a_file'
+    MESSAGE = ( 'Found ' + FILE_NAME + '. Load? \n\n(Question can be '
+                'turned off with options, see YCM docs)' )
+
+    def UnknownExtraConfResponse( *args ):
+      raise UnknownExtraConf( FILE_NAME )
+
+    with MockArbitraryBuffer( 'javascript' ):
+      with MockEventNotification( UnknownExtraConfResponse ):
+
+        # When the user accepts the extra conf, we load it
+        with patch( 'ycm.vimsupport.PresentDialog',
+                    return_value = 0 ) as present_dialog:
+          self.server_state.OnFileReadyToParse()
+          assert self.server_state.DiagnosticsForCurrentFileReady()
+          self.server_state.ValidateParseRequest()
+
+          present_dialog.assert_has_calls( [
+            PresentDialog_Confirm_Call( MESSAGE ),
+          ] )
+          load_extra_conf.assert_has_calls( [
+            call( FILE_NAME ),
+          ] )
+
+          # Subsequent calls don't re-raise the warning
+          self.server_state.ValidateParseRequest()
+
+          present_dialog.assert_has_calls( [
+            PresentDialog_Confirm_Call( MESSAGE )
+          ] )
+          load_extra_conf.assert_has_calls( [
+            call( FILE_NAME ),
+          ] )
+
+          # But it does if a subsequent event raises again
+          self.server_state.OnFileReadyToParse()
+          assert self.server_state.DiagnosticsForCurrentFileReady()
+          self.server_state.ValidateParseRequest()
+
+          present_dialog.assert_has_calls( [
+            PresentDialog_Confirm_Call( MESSAGE ),
+            PresentDialog_Confirm_Call( MESSAGE ),
+          ] )
+          load_extra_conf.assert_has_calls( [
+            call( FILE_NAME ),
+            call( FILE_NAME ),
+          ] )
+
+        # When the user rejects the extra conf, we reject it
+        with patch( 'ycm.vimsupport.PresentDialog',
+                    return_value = 1 ) as present_dialog:
+          self.server_state.OnFileReadyToParse()
+          assert self.server_state.DiagnosticsForCurrentFileReady()
+          self.server_state.ValidateParseRequest()
+
+          present_dialog.assert_has_calls( [
+            PresentDialog_Confirm_Call( MESSAGE ),
+          ] )
+          ignore_extra_conf.assert_has_calls( [
+            call( FILE_NAME ),
+          ] )
+
+          # Subsequent calls don't re-raise the warning
+          self.server_state.ValidateParseRequest()
+
+          present_dialog.assert_has_calls( [
+            PresentDialog_Confirm_Call( MESSAGE )
+          ] )
+          ignore_extra_conf.assert_has_calls( [
+            call( FILE_NAME ),
+          ] )
+
+          # But it does if a subsequent event raises again
+          self.server_state.OnFileReadyToParse()
+          assert self.server_state.DiagnosticsForCurrentFileReady()
+          self.server_state.ValidateParseRequest()
+
+          present_dialog.assert_has_calls( [
+            PresentDialog_Confirm_Call( MESSAGE ),
+            PresentDialog_Confirm_Call( MESSAGE ),
+          ] )
+          ignore_extra_conf.assert_has_calls( [
+            call( FILE_NAME ),
+            call( FILE_NAME ),
+          ] )
diff --git a/python/ycm/tests/postcomplete_tests.py b/python/ycm/tests/postcomplete_tests.py
index 004dd482..0efda8fb 100644
--- a/python/ycm/tests/postcomplete_tests.py
+++ b/python/ycm/tests/postcomplete_tests.py
@@ -17,7 +17,7 @@
 # You should have received a copy of the GNU General Public License
 # along with YouCompleteMe.  If not, see <http://www.gnu.org/licenses/>.
 
-from ..test_utils import MockVimModule
+from ycm.test_utils import MockVimModule
 MockVimModule()
 
 from mock import ( MagicMock, DEFAULT, patch )
diff --git a/python/ycm/youcompleteme.py b/python/ycm/youcompleteme.py
index b5a7f6a9..6e86646f 100644
--- a/python/ycm/youcompleteme.py
+++ b/python/ycm/youcompleteme.py
@@ -488,11 +488,14 @@ class YouCompleteMe( object ):
     if ( self.DiagnosticsForCurrentFileReady() and
          self.NativeFiletypeCompletionUsable() ):
 
+      # YCM client has a hard-coded list of filetypes which are known to support
+      # diagnostics. These are found in autoload/youcompleteme.vim in
+      # s:diagnostic_ui_filetypes.
+      #
       # For filetypes which don't support diagnostics, we just want to check the
       # _latest_file_parse_request for any exception or UnknownExtraConf
-      # response. We don't actually use the results (which are diagnositcs)
-      # otherwise. FIXME: should the client even _know_ which filetypes support
-      # diagnostics? Even if it should, it should be told by the sever.
+      # response, to allow the server to raise configuration warnings, etc.
+      # to the user. We ignore any other supplied data.
       self._latest_file_parse_request.Response()
 
       # We set the diagnostics request to None because we want to prevent