diff --git a/swh/storage/api/client.py b/swh/storage/api/client.py
--- a/swh/storage/api/client.py
+++ b/swh/storage/api/client.py
@@ -3,9 +3,10 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
-from swh.core.api import RPCClient
+from swh.core.api import RPCClient, RemoteException
 
-from ..exc import StorageAPIError
+from .. import HashCollision
+from ..exc import StorageAPIError, StorageArgumentException
 from ..interface import StorageInterface
 
 
@@ -14,6 +15,25 @@
     api_exception = StorageAPIError
     backend_class = StorageInterface
 
+    def raise_for_status(self, response) -> None:
+        try:
+            super().raise_for_status(response)
+        except RemoteException as e:
+            if e.response and e.response.status_code == 400:
+                assert e.args[0]['type'] \
+                    == 'StorageArgumentException'
+                raise StorageArgumentException(
+                    *e.args[0]['args'])
+            elif e.response and e.response.status_code == 500 and e.args \
+                    and e.args[0].get('type') == 'HashCollision':
+                # XXX: workaround until we fix these HashCollisions happening
+                # when they shouldn't
+                raise HashCollision(
+                    *e.args[0]['args'])
+            else:
+                print(e.args)
+                raise
+
     def reset(self):
         return self.post('reset', {})
 
diff --git a/swh/storage/api/server.py b/swh/storage/api/server.py
--- a/swh/storage/api/server.py
+++ b/swh/storage/api/server.py
@@ -14,6 +14,7 @@
 
 from ..interface import StorageInterface
 from ..metrics import timed
+from ..exc import StorageArgumentException
 
 
 def get_storage():
@@ -30,6 +31,11 @@
 storage = None
 
 
+@app.errorhandler(StorageArgumentException)
+def argument_error_handler(exception):
+    return error_handler(exception, encode_data, status_code=400)
+
+
 @app.errorhandler(Exception)
 def my_error_handler(exception):
     return error_handler(exception, encode_data)