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
@@ -38,6 +38,8 @@
     def __init__(self, journal_writer=None):
         self._contents = {}
         self._content_indexes = defaultdict(lambda: defaultdict(set))
+        self._skipped_contents = {}
+        self._skipped_content_indexes = defaultdict(lambda: defaultdict(set))
 
         self.reset()
 
@@ -77,10 +79,38 @@
                     del content['data']
                 self.journal_writer.write_addition('content', content)
 
-        count_contents = 0
+        content_with_data = []
+        content_without_data = []
+        for content in contents:
+            if 'status' not in content:
+                content['status'] = 'visible'
+            if 'length' not in content:
+                content['length'] = -1
+            if content['status'] == 'visible':
+                content_with_data.append(content)
+            elif content['status'] == 'absent':
+                content_without_data.append(content)
+
+        count_content_added, count_content_bytes_added = \
+            self._content_add_present(content_with_data, with_data)
+
+        count_skipped_content_added = self._content_add_absent(
+            content_without_data
+        )
+
+        summary = {
+            'content:add': count_content_added,
+            'skipped_content:add': count_skipped_content_added,
+        }
+
+        if with_data:
+            summary['content:add:bytes'] = count_content_bytes_added
+
+        return summary
+
+    def _content_add_present(self, contents, with_data):
         count_content_added = 0
         count_content_bytes_added = 0
-
         for content in contents:
             key = self._content_key(content)
             if key in self._contents:
@@ -96,23 +126,25 @@
                 ('content', content['sha1']))
             self._contents[key] = copy.deepcopy(content)
             bisect.insort(self._sorted_sha1s, content['sha1'])
-            count_contents += 1
-            if self._contents[key]['status'] == 'visible':
-                count_content_added += 1
-                if with_data:
-                    content_data = self._contents[key].pop('data')
-                    count_content_bytes_added += len(content_data)
-                    self.objstorage.add(content_data, content['sha1'])
+            count_content_added += 1
+            if with_data:
+                content_data = self._contents[key].pop('data')
+                count_content_bytes_added += len(content_data)
+                self.objstorage.add(content_data, content['sha1'])
 
-        summary = {
-            'content:add': count_content_added,
-            'skipped_content:add': count_contents - count_content_added,
-        }
+        return (count_content_added, count_content_bytes_added)
 
-        if with_data:
-            summary['content:add:bytes'] = count_content_bytes_added
+    def _content_add_absent(self, contents):
+        count = 0
+        skipped_content_missing = self.skipped_content_missing(contents)
+        for content in skipped_content_missing:
+            key = self._content_key(content)
+            for algo in DEFAULT_ALGORITHMS:
+                self._skipped_content_indexes[algo][content[algo]].add(key)
+            self._skipped_contents[key] = copy.deepcopy(content)
+            count += 1
 
-        return summary
+        return count
 
     def content_add(self, content):
         """Add content blobs to the storage
@@ -352,6 +384,26 @@
             if content not in self._content_indexes['sha1']:
                 yield content
 
+    def skipped_content_missing(self, contents):
+        """List all skipped_content missing from storage
+
+        Args:
+            contents: Iterable of sha1 to check for skipped content entry
+
+        Returns:
+            iterable: dict of skipped content entry
+        """
+
+        for content in contents:
+            for (key, algorithm) in self._content_key_algorithm(content):
+                if algorithm == 'blake2s256':
+                    continue
+                if key not in self._skipped_content_indexes[algorithm]:
+                    # index must contain hashes of algos except blake2s256
+                    # else the content is considered skipped
+                    yield content
+                    break
+
     def directory_add(self, directories):
         """Add directories to the storage
 
@@ -1660,6 +1712,12 @@
         return tuple(content.get(key) for key in sorted(DEFAULT_ALGORITHMS))
 
     @staticmethod
+    def _content_key_algorithm(content):
+        """ A stable key and the algorithm for a content"""
+        return tuple((content.get(key), key)
+                     for key in sorted(DEFAULT_ALGORITHMS))
+
+    @staticmethod
     def _tool_key(tool):
         return '%r %r %r' % (tool['name'], tool['version'],
                              tuple(sorted(tool['configuration'].items())))
diff --git a/swh/storage/tests/test_in_memory.py b/swh/storage/tests/test_in_memory.py
--- a/swh/storage/tests/test_in_memory.py
+++ b/swh/storage/tests/test_in_memory.py
@@ -37,11 +37,6 @@
     def test_content_add_metadata_db(self):
         pass
 
-    @pytest.mark.skip(
-        'not implemented, see https://forge.softwareheritage.org/T1633')
-    def test_skipped_content_add(self):
-        pass
-
     if not _test_origin_ids:
         @pytest.mark.skip('requires origin ids')
         def test_origin_metadata_add(self):