diff --git a/swh/storage/in_memory.py b/swh/storage/in_memory.py
--- a/swh/storage/in_memory.py
+++ b/swh/storage/in_memory.py
@@ -17,7 +17,7 @@
 
 import attr
 
-from swh.model.model import Content, Directory, Revision, Release
+from swh.model.model import Content, Directory, Revision, Release, Snapshot
 from swh.model.hashutil import DEFAULT_ALGORITHMS
 from swh.objstorage import get_objstorage
 from swh.objstorage.exc import ObjNotFoundError
@@ -762,19 +762,17 @@
 
         """
         count = 0
+        snapshots = (Snapshot.from_dict(d) for d in snapshots)
+        snapshots = (snap for snap in snapshots
+                     if snap.id not in self._snapshots)
         for snapshot in snapshots:
-            snapshot_id = snapshot['id']
-            if snapshot_id not in self._snapshots:
-                if self.journal_writer:
-                    self.journal_writer.write_addition('snapshot', snapshot)
-
-                self._snapshots[snapshot_id] = {
-                    'id': snapshot_id,
-                    'branches': copy.deepcopy(snapshot['branches']),
-                    '_sorted_branch_names': sorted(snapshot['branches'])
-                    }
-                self._objects[snapshot_id].append(('snapshot', snapshot_id))
-                count += 1
+            if self.journal_writer:
+                self.journal_writer.write_addition('snapshot', snapshot)
+
+            sorted_branch_names = sorted(snapshot.branches)
+            self._snapshots[snapshot.id] = (snapshot, sorted_branch_names)
+            self._objects[snapshot.id].append(('snapshot', snapshot.id))
+            count += 1
 
         return {'snapshot:add': count}
 
@@ -893,9 +891,9 @@
             dict: A dict whose keys are the target types of branches and
             values their corresponding amount
         """
-        branches = list(self._snapshots[snapshot_id]['branches'].values())
-        return collections.Counter(branch['target_type'] if branch else None
-                                   for branch in branches)
+        (snapshot, _) = self._snapshots[snapshot_id]
+        return collections.Counter(branch.target_type.value if branch else None
+                                   for branch in snapshot.branches.values())
 
     def snapshot_get_branches(self, snapshot_id, branches_from=b'',
                               branches_count=1000, target_types=None):
@@ -924,18 +922,18 @@
                   or :const:`None` if the snapshot has less than
                   `branches_count` branches after `branches_from` included.
         """
-        snapshot = self._snapshots.get(snapshot_id)
-        if snapshot is None:
+        res = self._snapshots.get(snapshot_id)
+        if res is None:
             return None
-        sorted_branch_names = snapshot['_sorted_branch_names']
+        (snapshot, sorted_branch_names) = res
         from_index = bisect.bisect_left(
                 sorted_branch_names, branches_from)
         if target_types:
             next_branch = None
             branches = {}
             for branch_name in sorted_branch_names[from_index:]:
-                branch = snapshot['branches'][branch_name]
-                if branch and branch['target_type'] in target_types:
+                branch = snapshot.branches[branch_name]
+                if branch and branch.target_type.value in target_types:
                     if len(branches) < branches_count:
                         branches[branch_name] = branch
                     else:
@@ -945,12 +943,16 @@
             # As there is no 'target_types', we can do that much faster
             to_index = from_index + branches_count
             returned_branch_names = sorted_branch_names[from_index:to_index]
-            branches = {branch_name: snapshot['branches'][branch_name]
+            branches = {branch_name: snapshot.branches[branch_name]
                         for branch_name in returned_branch_names}
             if to_index >= len(sorted_branch_names):
                 next_branch = None
             else:
                 next_branch = sorted_branch_names[to_index]
+
+        branches = {name: branch.to_dict() if branch else None
+                    for (name, branch) in branches.items()}
+
         return {
                 'id': snapshot_id,
                 'branches': branches,
diff --git a/swh/storage/tests/test_storage.py b/swh/storage/tests/test_storage.py
--- a/swh/storage/tests/test_storage.py
+++ b/swh/storage/tests/test_storage.py
@@ -2453,6 +2453,19 @@
             {**self.snapshot, 'next_branch': None},
             self.storage.snapshot_get(self.snapshot['id']))
 
+    def test_snapshot_add_validation(self):
+        snap = copy.deepcopy(self.snapshot)
+        snap['branches'][b'foo'] = {'target_type': 'revision'}
+
+        with self.assertRaisesRegex(KeyError, 'target'):
+            self.storage.snapshot_add([snap])
+
+        snap = copy.deepcopy(self.snapshot)
+        snap['branches'][b'foo'] = {'target': b'\x42'*20}
+
+        with self.assertRaisesRegex(KeyError, 'target_type'):
+            self.storage.snapshot_add([snap])
+
     def test_snapshot_add_count_branches(self):
         origin_id = self.storage.origin_add_one(self.origin)
         origin_visit1 = self.storage.origin_visit_add(origin_id,