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
@@ -5,6 +5,7 @@
 
 import base64
 import datetime
+from enum import Enum
 import json
 import traceback
 import types
@@ -45,6 +46,10 @@
 }
 
 
+class MsgpackExtTypeCodes(Enum):
+    LONG_INT = 1
+
+
 def encode_data_client(data: Any, extra_encoders=None) -> bytes:
     try:
         return msgpack_dumps(data, extra_encoders=extra_encoders)
@@ -175,6 +180,15 @@
         encoders += extra_encoders
 
     def encode_types(obj):
+        if isinstance(obj, int):
+            # integer overflowed while packing. Handle it as an extended type
+            length, rem = divmod(obj.bit_length(), 8)
+            if rem:
+                length += 1
+            return msgpack.ExtType(
+                MsgpackExtTypeCodes.LONG_INT.value, int.to_bytes(obj, length, "big")
+            )
+
         if isinstance(obj, types.GeneratorType):
             return list(obj)
 
@@ -200,6 +214,11 @@
     if extra_decoders:
         decoders = {**decoders, **extra_decoders}
 
+    def ext_hook(code, data):
+        if code == MsgpackExtTypeCodes.LONG_INT.value:
+            return int.from_bytes(data, "big")
+        raise ValueError("Unknown msgpack extended code %s" % code)
+
     def decode_types(obj):
         # Support for current encodings
         if set(obj.keys()) == {b"d", b"swhtype"}:
@@ -223,12 +242,20 @@
     try:
         try:
             return msgpack.unpackb(
-                data, raw=False, object_hook=decode_types, strict_map_key=False
+                data,
+                raw=False,
+                object_hook=decode_types,
+                ext_hook=ext_hook,
+                strict_map_key=False,
             )
         except TypeError:  # msgpack < 0.6.0
-            return msgpack.unpackb(data, raw=False, object_hook=decode_types)
+            return msgpack.unpackb(
+                data, raw=False, object_hook=decode_types, ext_hook=ext_hook
+            )
     except TypeError:  # msgpack < 0.5.2
-        return msgpack.unpackb(data, encoding="utf-8", object_hook=decode_types)
+        return msgpack.unpackb(
+            data, encoding="utf-8", object_hook=decode_types, ext_hook=ext_hook
+        )
 
 
 def exception_to_dict(exception):
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
@@ -150,6 +150,7 @@
         original_data = {
             **self.data,
             "none_dict_key": {None: 42},
+            "long_int_is_loooong": 10000000000000000000000000000000,
         }
         data = msgpack_dumps(original_data)
         self.assertEqual(original_data, msgpack_loads(data))