diff --git a/swh/storage/cassandra/storage.py b/swh/storage/cassandra/storage.py
--- a/swh/storage/cassandra/storage.py
+++ b/swh/storage/cassandra/storage.py
@@ -260,7 +260,7 @@
             if not res:
                 yield content[key_hash]
 
-    def content_missing_per_sha1(self, contents):
+    def content_missing_per_sha1(self, contents: List[bytes]) -> Iterable[bytes]:
         return self.content_missing([{"sha1": c for c in contents}])
 
     def content_missing_per_sha1_git(self, contents):
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
@@ -361,7 +361,7 @@
                     yield content[key_hash]
                     break
 
-    def content_missing_per_sha1(self, contents):
+    def content_missing_per_sha1(self, contents: List[bytes]) -> Iterable[bytes]:
         for content in contents:
             if content not in self._content_indexes["sha1"]:
                 yield content
diff --git a/swh/storage/interface.py b/swh/storage/interface.py
--- a/swh/storage/interface.py
+++ b/swh/storage/interface.py
@@ -269,18 +269,18 @@
         ...
 
     @remote_api_endpoint("content/missing/sha1")
-    def content_missing_per_sha1(self, contents):
+    def content_missing_per_sha1(self, contents: List[bytes]) -> Iterable[bytes]:
         """List content missing from storage based only on sha1.
 
         Args:
             contents: List of sha1 to check for absence.
 
-        Returns:
-            iterable: missing ids
-
         Raises:
             TODO: an exception when we get a hash collision.
 
+        Returns:
+            Iterable of missing content ids (sha1)
+
         """
         ...
 
@@ -293,6 +293,7 @@
 
         Yields:
             missing contents sha1_git
+
         """
         ...
 
diff --git a/swh/storage/storage.py b/swh/storage/storage.py
--- a/swh/storage/storage.py
+++ b/swh/storage/storage.py
@@ -354,7 +354,9 @@
 
     @timed
     @db_transaction_generator()
-    def content_missing_per_sha1(self, contents, db=None, cur=None):
+    def content_missing_per_sha1(
+        self, contents: List[bytes], db=None, cur=None
+    ) -> Iterable[bytes]:
         for obj in db.content_missing_per_sha1(contents, cur):
             yield obj[0]