diff --git a/diplomacy/client/notification_managers.py b/diplomacy/client/notification_managers.py
index 05e6ab74c..98d17acd3 100644
--- a/diplomacy/client/notification_managers.py
+++ b/diplomacy/client/notification_managers.py
@@ -115,7 +115,8 @@ def on_game_message_received(game, notification):
         :type game: diplomacy.client.network_game.NetworkGame
         :type notification: diplomacy.communication.notifications.GameMessageReceived
     """
-    Game.add_message(game, notification.message)
+    if notification.message.time_sent != 0:
+        Game.add_message(game, notification.message)
 
 def on_game_processed(game, notification):
     """ Manage notification GamePhaseUpdate (for omniscient and observer games).
diff --git a/diplomacy/client/response_managers.py b/diplomacy/client/response_managers.py
index 5464cc81b..c32ae5414 100644
--- a/diplomacy/client/response_managers.py
+++ b/diplomacy/client/response_managers.py
@@ -232,7 +232,10 @@ def on_send_game_message(context, response):
     request = context.request  # type: requests.SendGameMessage
     message = request.message
     message.time_sent = response.data
-    Game.add_message(context.game, message)
+    if message.time_sent == 0:
+        return 0
+    
+    return Game.add_message(context.game, message)
 
 def on_set_game_state(context, response):
     """ Manage response for request SetGameState.
diff --git a/diplomacy/engine/game.py b/diplomacy/engine/game.py
index 158694de2..1ba03b6d5 100644
--- a/diplomacy/engine/game.py
+++ b/diplomacy/engine/game.py
@@ -880,7 +880,7 @@ def new_log_data(self, body, recipient="OMNISCIENT"):
         assert self.is_player_game()
         return Log(phase=self.current_short_phase, sender=self.role, recipient=recipient, message=body)
 
-    def new_power_message(self, recipient, body):
+    def new_power_message(self, recipient, body, thread_idx=None):
         """ Create a undated (without timestamp) power message to be sent from a power to another via server.
             Server will answer with timestamp, and message will be updated
             and added to local game messages.
@@ -893,7 +893,7 @@ def new_power_message(self, recipient, body):
         assert self.is_player_game()
         if not self.has_power(recipient):
             raise exceptions.MapPowerException(recipient)
-        return Message(phase=self.current_short_phase, sender=self.role, recipient=recipient, message=body)
+        return Message(phase=self.current_short_phase, sender=self.role, recipient=recipient, message=body, thread_idx=thread_idx)
 
     def new_global_message(self, body):
         """ Create an undated (without timestamp) global message to be sent from a power via server.
@@ -932,6 +932,7 @@ def add_log(self, log):
 
         self.logs.put(log.time_sent, log)
         return log.time_sent
+    
     def add_message(self, message):
         """ Add message to current game data.
             Only a server game can add a message with no timestamp:
@@ -957,6 +958,21 @@ def add_message(self, message):
             time.sleep(1e-6)
             message.time_sent = common.timestamp_microseconds()
 
+        if (message.thread_idx is not None) and (not self.is_player_game()):
+            def _is_current_thread(a, b):
+                if (a.phase != b.phase):                                       return False
+                if (a.sender == b.sender)    and (a.recipient == b.recipient): return True
+                if (a.sender == b.recipient) and (a.recipient == b.sender):    return True
+                return False
+
+            expected_thread_idx = len([msg for msg in self.messages.values() if _is_current_thread(msg, message)])
+            if message.thread_idx != expected_thread_idx:
+                print('rejected', self.role, message)
+                return 0
+        
+        if not self.is_player_game():
+            print('accepted', self.role, message)
+        
         self.messages.put(message.time_sent, message)
         return message.time_sent
 
diff --git a/diplomacy/engine/message.py b/diplomacy/engine/message.py
index 8094bfeb7..720b9c753 100644
--- a/diplomacy/engine/message.py
+++ b/diplomacy/engine/message.py
@@ -65,13 +65,14 @@ class Message(Jsonable):
         are stored on server. Therefore, message timestamp is the time when server stores the message, not the time
         when message was sent by any client.
     """
-    __slots__ = ['sender', 'recipient', 'time_sent', 'phase', 'message']
+    __slots__ = ['sender', 'recipient', 'time_sent', 'phase', 'message', 'thread_idx']
     model = {
         strings.SENDER: str,                                # either SYSTEM or a power name.
         strings.RECIPIENT: str,                             # either GLOBAL, OBSERVER, OMNISCIENT or a power name.
         strings.TIME_SENT: parsing.OptionalValueType(int),  # given by server.
         strings.PHASE: str,                                 # phase short name (e.g. 'S1901M' or 'COMPLETED')
         strings.MESSAGE: str,
+        strings.THREAD_IDX: parsing.OptionalValueType(int),
     }
 
     def __init__(self, **kwargs):
@@ -80,10 +81,11 @@ def __init__(self, **kwargs):
         self.time_sent = None               # type: int
         self.phase = None                   # type: str
         self.message = None                 # type: str
+        self.thread_idx = None              # type: int
         super(Message, self).__init__(**kwargs)
 
     def __str__(self):
-        return '[%d/%s/%s->%s](%s)' % (self.time_sent or 0, self.phase, self.sender, self.recipient, self.message)
+        return '[%d/%s/%s->%s/tid=%d](%s)' % (self.time_sent or 0, self.phase, self.sender, self.recipient, self.thread_idx if self.thread_idx is not None else -1, self.message)
 
     def __hash__(self):
         return hash(self.time_sent)
diff --git a/diplomacy/utils/strings.py b/diplomacy/utils/strings.py
index 7fdfa985d..6e55c5c3b 100644
--- a/diplomacy/utils/strings.py
+++ b/diplomacy/utils/strings.py
@@ -192,6 +192,7 @@
 STATES = 'states'
 STATUS = 'status'
 SUPPLY_CENTERS = 'supply_centers'
+THREAD_IDX = 'thread_idx'
 TIME_SENT = 'time_sent'
 TIMESTAMP = 'timestamp'
 TIMESTAMP_CREATED = 'timestamp_created'