diff --git a/ably/realtime/channel.py b/ably/realtime/channel.py
index 768eeb7d..33e338d6 100644
--- a/ably/realtime/channel.py
+++ b/ably/realtime/channel.py
@@ -526,6 +526,8 @@ async def _send_update(
             serial=message.serial,
             action=action,
             version=version,
+            extras=message.extras,
+            annotations=message.annotations,
         )
 
         # Encrypt if needed
diff --git a/ably/rest/channel.py b/ably/rest/channel.py
index f6b118b7..32cc7e7e 100644
--- a/ably/rest/channel.py
+++ b/ably/rest/channel.py
@@ -194,6 +194,8 @@ async def _send_update(
             serial=message.serial,
             action=action,
             version=version,
+            extras=message.extras,
+            annotations=message.annotations,
         )
 
         # Encrypt if needed
diff --git a/test/ably/realtime/realtimechannelmutablemessages_test.py b/test/ably/realtime/realtimechannelmutablemessages_test.py
index 047ea3b6..a5866936 100644
--- a/test/ably/realtime/realtimechannelmutablemessages_test.py
+++ b/test/ably/realtime/realtimechannelmutablemessages_test.py
@@ -263,6 +263,43 @@ def on_message(message):
         assert appended_message.version.description == 'Appended to message'
         assert appended_message.serial == serial
 
+    # RTL32b, TM2i
+    async def test_update_message_preserves_extras(self):
+        """Test that extras are preserved when updating a message"""
+        channel = self.ably.channels[self.get_channel_name('mutable:update_extras')]
+
+        # Publish a message
+        result = await channel.publish('test-event', 'original data')
+        assert len(result.serials) > 0
+        serial = result.serials[0]
+
+        messages_received = []
+        update_received = WaitableEvent()
+
+        def on_message(message):
+            if message.action == MessageAction.MESSAGE_UPDATE:
+                messages_received.append(message)
+                update_received.finish()
+
+        await channel.subscribe(on_message)
+
+        # Update with extras
+        message = Message(
+            data='updated data',
+            serial=serial,
+            extras={'headers': {'status': 'complete'}},
+        )
+
+        update_result = await channel.update_message(message)
+        assert update_result is not None
+
+        await update_received.wait()
+
+        assert len(messages_received) > 0
+        received = messages_received[0]
+        assert received.extras is not None
+        assert received.extras['headers']['status'] == 'complete'
+
     async def wait_until_message_with_action_appears(self, channel, serial, action):
         message: Message | None = None
         async def check_message_action():
diff --git a/test/ably/rest/restchannelmutablemessages_test.py b/test/ably/rest/restchannelmutablemessages_test.py
index 7b144ab0..b4f32ef4 100644
--- a/test/ably/rest/restchannelmutablemessages_test.py
+++ b/test/ably/rest/restchannelmutablemessages_test.py
@@ -270,6 +270,33 @@ async def test_append_message_with_string_data(self):
         assert appended_message.version.description == 'Appended to message'
         assert appended_message.serial == serial
 
+    # RSL15b, TM2i
+    async def test_update_message_preserves_extras(self):
+        """Test that extras are preserved when updating a message"""
+        channel = self.ably.channels[self.get_channel_name('mutable:update_extras')]
+
+        # Publish a message
+        result = await channel.publish('test-event', 'original data')
+        assert len(result.serials) > 0
+        serial = result.serials[0]
+
+        # Update with extras
+        message = Message(
+            data='updated data',
+            serial=serial,
+            extras={'headers': {'status': 'complete'}},
+        )
+
+        update_result = await channel.update_message(message)
+        assert update_result is not None
+
+        updated_message = await self.wait_until_message_with_action_appears(
+            channel, serial, MessageAction.MESSAGE_UPDATE
+        )
+        assert updated_message.data == 'updated data'
+        assert updated_message.extras is not None
+        assert updated_message.extras['headers']['status'] == 'complete'
+
     async def wait_until_message_with_action_appears(self, channel, serial, action):
         message: Message | None = None
         async def check_message_action():
diff --git a/test/unit/mutable_message_test.py b/test/unit/mutable_message_test.py
index 6f5afc92..8ce603b9 100644
--- a/test/unit/mutable_message_test.py
+++ b/test/unit/mutable_message_test.py
@@ -96,6 +96,62 @@ def test_message_version_serialization():
     assert reconstructed.description == version.description
     assert reconstructed.metadata == version.metadata
 
+# RSL15b, RTL32b, TM2i
+def test_message_extras_preserved_in_as_dict():
+    """Test that extras are included when a Message with extras is serialized.
+
+    Regression test: _send_update() in both RestChannel and RealtimeChannel
+    constructed a new Message without copying extras or annotations from the
+    user-supplied message, violating RSL15b/RTL32b which require "whatever
+    fields were in the user-supplied Message" to be sent.
+    See commits 1723f5d (REST) and 0b93c10 (Realtime).
+    """
+    extras = {'headers': {'status': 'complete'}}
+    message = Message(
+        name='test',
+        data='updated data',
+        serial='abc123',
+        action=MessageAction.MESSAGE_UPDATE,
+        extras=extras,
+    )
+
+    msg_dict = message.as_dict()
+    assert msg_dict['extras'] == extras
+    assert msg_dict['extras']['headers']['status'] == 'complete'
+
+
+# RSL15b, RTL32b, TM2i
+def test_message_extras_none_excluded_from_as_dict():
+    """Test that extras=None does not appear in as_dict output."""
+    message = Message(
+        name='test',
+        data='data',
+        serial='abc123',
+        action=MessageAction.MESSAGE_UPDATE,
+    )
+
+    msg_dict = message.as_dict()
+    assert 'extras' not in msg_dict
+
+
+# RSL15b, RTL32b, TM2u
+def test_message_annotations_preserved_in_as_dict():
+    """Test that annotations are included when a Message with annotations is serialized."""
+    from ably.types.message import MessageAnnotations
+    annotations = MessageAnnotations(summary={'reaction:distinct.v1': {'thumbsup': 5}})
+    message = Message(
+        name='test',
+        data='data',
+        serial='abc123',
+        action=MessageAction.MESSAGE_UPDATE,
+        annotations=annotations,
+    )
+
+    msg_dict = message.as_dict()
+    assert msg_dict['annotations'] is not None
+    assert msg_dict['annotations']['summary']['reaction:distinct.v1'] == {'thumbsup': 5}
+
+
 def test_message_operation_serialization():
     """Test MessageOperation can be serialized and deserialized"""
     operation = MessageOperation(