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
@@ -9,7 +9,7 @@
 import json
 import traceback
 import types
-from typing import Any, Dict, Tuple, Union
+from typing import Any, Callable, Dict, List, Optional, Tuple, Type, Union
 from uuid import UUID
 
 import arrow
@@ -88,6 +88,22 @@
 }
 
 
+def get_encoders(
+    extra_encoders: Optional[List[Tuple[Type, str, Callable]]]
+) -> List[Tuple[Type, str, Callable]]:
+    if extra_encoders is not None:
+        return [*ENCODERS, *extra_encoders]
+    else:
+        return ENCODERS
+
+
+def get_decoders(extra_decoders: Optional[Dict[str, Callable]]) -> Dict[str, Callable]:
+    if extra_decoders is not None:
+        return {**DECODERS, **extra_decoders}
+    else:
+        return DECODERS
+
+
 class MsgpackExtTypeCodes(Enum):
     LONG_INT = 1
     LONG_NEG_INT = 2
@@ -141,9 +157,7 @@
 
     def __init__(self, extra_encoders=None, **kwargs):
         super().__init__(**kwargs)
-        self.encoders = ENCODERS
-        if extra_encoders:
-            self.encoders += extra_encoders
+        self.encoders = get_encoders(extra_encoders)
 
     def default(self, o: Any) -> Union[Dict[str, Union[Dict[str, int], str]], list]:
         for (type_, type_name, encoder) in self.encoders:
@@ -185,9 +199,7 @@
 
     def __init__(self, extra_decoders=None, **kwargs):
         super().__init__(**kwargs)
-        self.decoders = DECODERS
-        if extra_decoders:
-            self.decoders = {**self.decoders, **extra_decoders}
+        self.decoders = get_decoders(extra_decoders)
 
     def decode_data(self, o: Any) -> Any:
         if isinstance(o, dict):
@@ -218,9 +230,7 @@
 
 def msgpack_dumps(data: Any, extra_encoders=None) -> bytes:
     """Write data as a msgpack stream"""
-    encoders = ENCODERS
-    if extra_encoders:
-        encoders += extra_encoders
+    encoders = get_encoders(extra_encoders)
 
     def encode_types(obj):
         if isinstance(obj, int):
@@ -256,9 +266,7 @@
        This function is used by swh.journal to decode the contents of the
        journal. This function **must** be kept backwards-compatible.
     """
-    decoders = DECODERS
-    if extra_decoders:
-        decoders = {**decoders, **extra_decoders}
+    decoders = get_decoders(extra_decoders)
 
     def ext_hook(code, data):
         if code == MsgpackExtTypeCodes.LONG_INT.value:
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
@@ -16,6 +16,7 @@
 
 from swh.core.api.classes import PagedResult
 from swh.core.api.serializers import (
+    ENCODERS,
     SWHJSONDecoder,
     SWHJSONEncoder,
     decode_response,
@@ -270,3 +271,15 @@
         )
         == expected_dt
     )
+
+
+def test_msgpack_extra_encoders_mutation():
+    data = msgpack_dumps({}, extra_encoders=extra_encoders)
+    assert data is not None
+    assert ENCODERS[-1][0] != ExtraType
+
+
+def test_json_extra_encoders_mutation():
+    data = json.dumps({}, cls=SWHJSONEncoder, extra_encoders=extra_encoders)
+    assert data is not None
+    assert ENCODERS[-1][0] != ExtraType