diff --git a/swh/core/api/serializers.py b/swh/core/api/serializers.py
--- a/swh/core/api/serializers.py
+++ b/swh/core/api/serializers.py
@@ -201,10 +201,23 @@
         decoders = {**decoders, **extra_decoders}
 
     def decode_types(obj):
+        # Support for current encodings
         if set(obj.keys()) == {b'd', b'swhtype'}:
             decoder = decoders.get(obj[b'swhtype'])
             if decoder:
                 return decoder(obj[b'd'])
+
+        # Support for legacy encodings
+        if b'__datetime__' in obj and obj[b'__datetime__']:
+            return iso8601.parse_date(obj[b's'], default_timezone=None)
+        if b'__uuid__' in obj and obj[b'__uuid__']:
+            return UUID(obj[b's'])
+        if b'__timedelta__' in obj and obj[b'__timedelta__']:
+            return datetime.timedelta(**obj[b's'])
+        if b'__arrow__' in obj and obj[b'__arrow__']:
+            return arrow.get(obj[b's'])
+
+        # Fallthrough
         return obj
 
     try:
diff --git a/swh/core/api/tests/test_serializers.py b/swh/core/api/tests/test_serializers.py
--- a/swh/core/api/tests/test_serializers.py
+++ b/swh/core/api/tests/test_serializers.py
@@ -84,6 +84,37 @@
                      'd': 'cdd8f804-9db6-40c3-93ab-5955d3836234'},
         }
 
+        self.legacy_msgpack = {
+            'bytes': b'\xc4\x0e123456789\x99\xaf\xff\x00\x12',
+            'datetime_naive': (
+                b'\x82\xc4\x0c__datetime__\xc3\xc4\x01s\xba'
+                b'2015-01-01T12:04:42.231455'
+            ),
+            'datetime_tz': (
+                b'\x82\xc4\x0c__datetime__\xc3\xc4\x01s\xd9 '
+                b'2015-03-04T18:25:13.001234+01:58'
+            ),
+            'datetime_utc': (
+                b'\x82\xc4\x0c__datetime__\xc3\xc4\x01s\xd9 '
+                b'2015-03-04T18:25:13.001234+00:00'
+            ),
+            'datetime_delta': (
+                b'\x82\xc4\r__timedelta__\xc3\xc4\x01s\x83\xa4'
+                b'days@\xa7seconds\x00\xacmicroseconds\x00'
+            ),
+            'arrow_date': (
+                b'\x82\xc4\t__arrow__\xc3\xc4\x01s\xd9 '
+                b'2018-04-25T16:17:53.533672+00:00'
+            ),
+            'swhtype': b'\xa4fake',
+            'swh_dict': b'\x82\xa7swhtype*\xa1d\xa4test',
+            'random_dict': b'\x81\xa7swhtype+',
+            'uuid': (
+                b'\x82\xc4\x08__uuid__\xc3\xc4\x01s\xd9$'
+                b'cdd8f804-9db6-40c3-93ab-5955d3836234'
+            ),
+        }
+
         self.generator = (i for i in range(5))
         self.gen_lst = list(range(5))
 
@@ -135,3 +166,7 @@
                           headers={'content-type': 'application/json'})
         response = requests.get('https://example.org/test/data')
         assert decode_response(response) == self.data
+
+    def test_decode_legacy_msgpack(self):
+        for k, v in self.legacy_msgpack.items():
+            assert msgpack_loads(v) == self.data[k]