diff --git a/src/socketio/msgpack_packet.py b/src/socketio/msgpack_packet.py
index 6e86a72..846db2f 100644
--- a/src/socketio/msgpack_packet.py
+++ b/src/socketio/msgpack_packet.py
@@ -16,11 +16,11 @@ class MsgPackPacket(packet.Packet):
         dumps_default=None,
         ext_hook=None,
     ):
+        self.dumps_default = dumps_default
+        self.ext_hook = ext_hook
         super().__init__(
             packet_type, data, namespace, id, binary, encoded_packet
         )
-        self.dumps_default = dumps_default
-        self.ext_hook = ext_hook
 
     def encode(self):
         """Encode the packet for transmission."""
diff --git a/tests/common/test_msgpack_packet.py b/tests/common/test_msgpack_packet.py
index e0197a2..1079e01 100644
--- a/tests/common/test_msgpack_packet.py
+++ b/tests/common/test_msgpack_packet.py
@@ -1,3 +1,8 @@
+from datetime import datetime, timedelta, timezone
+
+import pytest
+import msgpack
+
 from socketio import msgpack_packet
 from socketio import packet
 
@@ -5,7 +10,8 @@ from socketio import packet
 class TestMsgPackPacket:
     def test_encode_decode(self):
         p = msgpack_packet.MsgPackPacket(
-            packet.CONNECT, data={'auth': {'token': '123'}}, namespace='/foo')
+            packet.CONNECT, data={'auth': {'token': '123'}}, namespace='/foo'
+        )
         p2 = msgpack_packet.MsgPackPacket(encoded_packet=p.encode())
         assert p.packet_type == p2.packet_type
         assert p.data == p2.data
@@ -14,7 +20,8 @@ class TestMsgPackPacket:
 
     def test_encode_decode_with_id(self):
         p = msgpack_packet.MsgPackPacket(
-            packet.EVENT, data=['ev', 42], id=123, namespace='/foo')
+            packet.EVENT, data=['ev', 42], id=123, namespace='/foo'
+        )
         p2 = msgpack_packet.MsgPackPacket(encoded_packet=p.encode())
         assert p.packet_type == p2.packet_type
         assert p.data == p2.data
@@ -32,3 +39,101 @@ class TestMsgPackPacket:
         assert p.packet_type == packet.ACK
         p2 = msgpack_packet.MsgPackPacket(encoded_packet=p.encode())
         assert p2.data == {'foo': b'bar'}
+
+    def test_encode_with_dumps_default(self):
+        def default(obj):
+            if isinstance(obj, datetime):
+                return obj.isoformat()
+            raise TypeError('Unknown type')
+
+        data = {
+            'current': datetime.now(tz=timezone(timedelta(0))),
+            'key': 'value',
+        }
+        p = msgpack_packet.MsgPackPacket(data=data, dumps_default=default)
+        p2 = msgpack_packet.MsgPackPacket(encoded_packet=p.encode())
+        assert p.packet_type == p2.packet_type
+        assert p.id == p2.id
+        assert p.namespace == p2.namespace
+        assert p.data != p2.data
+
+        assert isinstance(p2.data, dict)
+        assert 'current' in p2.data
+        assert isinstance(p2.data['current'], str)
+        assert default(data['current']) == p2.data['current']
+
+        data.pop('current')
+        p2_data_without_current = p2.data.copy()
+        p2_data_without_current.pop('current')
+        assert data == p2_data_without_current
+
+    def test_encode_without_dumps_default(self):
+        data = {
+            'current': datetime.now(tz=timezone(timedelta(0))),
+            'key': 'value',
+        }
+        p_without_default = msgpack_packet.MsgPackPacket(data=data)
+        with pytest.raises(
+            TypeError, match="can not serialize 'datetime.datetime' object"
+        ):
+            p_without_default.encode()
+
+    def test_encode_decode_with_ext_hook(self):
+        class Custom:
+            def __init__(self, value):
+                self.value = value
+
+            def __eq__(self, value: object) -> bool:
+                return isinstance(value, Custom) and self.value == value.value
+
+        def default(obj):
+            if isinstance(obj, Custom):
+                return msgpack.ExtType(1, obj.value)
+            raise TypeError('Unknown type')
+
+        def ext_hook(code, data):
+            if code == 1:
+                return Custom(data)
+            raise TypeError('Unknown ext type')
+
+        data = {'custom': Custom(b'custom_data'), 'key': 'value'}
+        p = msgpack_packet.MsgPackPacket(data=data, dumps_default=default)
+        p2 = msgpack_packet.MsgPackPacket(
+            encoded_packet=p.encode(), ext_hook=ext_hook
+        )
+        assert p.packet_type == p2.packet_type
+        assert p.id == p2.id
+        assert p.data == p2.data
+        assert p.namespace == p2.namespace
+
+    def test_encode_decode_without_ext_hook(self):
+        class Custom:
+            def __init__(self, value):
+                self.value = value
+
+            def __eq__(self, value: object) -> bool:
+                return isinstance(value, Custom) and self.value == value.value
+
+        def default(obj):
+            if isinstance(obj, Custom):
+                return msgpack.ExtType(1, obj.value)
+            raise TypeError('Unknown type')
+
+        data = {'custom': Custom(b'custom_data'), 'key': 'value'}
+        p = msgpack_packet.MsgPackPacket(data=data, dumps_default=default)
+        p2 = msgpack_packet.MsgPackPacket(encoded_packet=p.encode())
+        assert p.packet_type == p2.packet_type
+        assert p.id == p2.id
+        assert p.namespace == p2.namespace
+        assert p.data != p2.data
+
+        assert isinstance(p2.data, dict)
+        assert 'custom' in p2.data
+        assert isinstance(p2.data['custom'], msgpack.ExtType)
+        assert p2.data['custom'].code == 1
+        assert p2.data['custom'].data == b'custom_data'
+
+        data.pop('custom')
+        p2_data_without_custom = p2.data.copy()
+        p2_data_without_custom.pop('custom')
+        assert data == p2_data_without_custom