diff --git a/swh/core/api/__init__.py b/swh/core/api/__init__.py
--- a/swh/core/api/__init__.py
+++ b/swh/core/api/__init__.py
@@ -327,11 +327,14 @@
         from flask import request
 
         @self.route('/'+meth._endpoint_path, methods=['POST'])
+        @negotiate(MsgpackFormatter)
+        @negotiate(JSONFormatter)
         @functools.wraps(meth)  # Copy signature and doc
         def _f():
             # Call the actual code
             obj_meth = getattr(backend_factory(), meth_name)
-            return encode_data_server(obj_meth(**decode_request(request)))
+            kw = decode_request(request)
+            return obj_meth(**kw)
 
 
 @deprecated(version='0.0.64',
diff --git a/swh/core/api/tests/test_api.py b/swh/core/api/tests/test_api.py
--- a/swh/core/api/tests/test_api.py
+++ b/swh/core/api/tests/test_api.py
@@ -37,9 +37,11 @@
             return error_handler(exception, encode_data_server)
 
         client = WerkzeugTestClient(app, BaseResponse)
-        res = client.post('/test_endpoint_url',
-                          headers={'Content-Type': 'application/x-msgpack'},
-                          data=b'\x81\xa9test_data\xa4spam')
+        res = client.post(
+            '/test_endpoint_url',
+            headers=[('Content-Type', 'application/x-msgpack'),
+                     ('Accept', 'application/x-msgpack')],
+            data=b'\x81\xa9test_data\xa4spam')
 
         self.assertEqual(nb_endpoint_calls, 1)
         self.assertEqual(b''.join(res.response), b'\xa3egg')