diff -r 116da6bb7e3b -r f4a540c203d7 mercurial/exchange.py
--- a/mercurial/exchange.py	Tue May 23 01:28:56 2023 +0200
+++ b/mercurial/exchange.py	Tue May 23 01:39:47 2023 +0200
@@ -146,6 +146,12 @@
                 splitted = requirements.split()
                 params = bundle2._formatrequirementsparams(splitted)
                 return b'none-v2;stream=v2;%s' % params
+            elif part.type == b'stream3-exp' and version is None:
+                # A stream3 part requires to be part of a v2 bundle
+                requirements = urlreq.unquote(part.params[b'requirements'])
+                splitted = requirements.split()
+                params = bundle2._formatrequirementsparams(splitted)
+                return b'none-v2;stream=v3-exp;%s' % params
             elif part.type == b'obsmarkers':
                 params[b'obsolescence'] = b'yes'
                 if not part.mandatory: