diff --git a/CHANGES.md b/CHANGES.md
index 0932d8ed8c34..e04d6c5852b6 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -68,7 +68,7 @@
 
 ## Breaking Changes
 
-* X behavior was changed ([#X](https://github.com/apache/beam/issues/X)).
+* Python SDK CoGroupByKey outputs an iterable allowing for arbitrarily large results. [#21556](https://github.com/apache/beam/issues/21556) Beam users may see an error on transforms downstream from CoGroupByKey. Users must change methods expecting a List to expect an Iterable going forward. See [document](https://docs.google.com/document/d/1RIzm8-g-0CyVsPb6yasjwokJQFoKHG4NjRUcKHKINu0) for information and fixes.
 
 ## Deprecations
 
diff --git a/sdks/python/apache_beam/transforms/util.py b/sdks/python/apache_beam/transforms/util.py
index 13d68df3715f..cb4b86245e00 100644
--- a/sdks/python/apache_beam/transforms/util.py
+++ b/sdks/python/apache_beam/transforms/util.py
@@ -198,8 +198,7 @@ def expand(self, pcolls):
       input_value_types.append(value_type)
     output_key_type = typehints.Union[tuple(input_key_types)]
     iterable_input_value_types = tuple(
-        # TODO: Change List[t] to Iterable[t]
-        typehints.List[t] for t in input_value_types)
+        typehints.Iterable[t] for t in input_value_types)
 
     output_value_type = typehints.Dict[
         str, typehints.Union[iterable_input_value_types or [typehints.Any]]]