--- a/mercurial/manifest.py	Wed Feb 19 23:18:17 2025 +0100
+++ b/mercurial/manifest.py	Tue Dec 17 19:29:08 2024 -0500
@@ -1345,7 +1345,7 @@
 
     def fastdelta(
         self, base: ByteString, changes: Iterable[Tuple[bytes, bool]]
-    ) -> ByteString:
+    ) -> tuple[ByteString, ByteString]:
         raise FastdeltaUnavailable()
 
     def diff(