diff -r 116da6bb7e3b -r f4a540c203d7 mercurial/bundle2.py
--- a/mercurial/bundle2.py	Tue May 23 01:28:56 2023 +0200
+++ b/mercurial/bundle2.py	Tue May 23 01:39:47 2023 +0200
@@ -1736,6 +1736,8 @@
         caps[b'obsmarkers'] = (b'V1',)
     if opts.get(b'streamv2'):
         caps[b'stream'] = [b'v2']
+    elif opts.get(b'streamv3-exp'):
+        caps[b'stream'] = [b'v3-exp']
     bundle = bundle20(ui, caps)
     bundle.setcompression(compression, compopts)
     _addpartsfromopts(ui, repo, bundle, source, outgoing, opts)
@@ -1781,6 +1783,9 @@
     if opts.get(b'streamv2', False):
         addpartbundlestream2(bundler, repo, stream=True)
 
+    if opts.get(b'streamv3-exp', False):
+        addpartbundlestream2(bundler, repo, stream=True)
+
     if opts.get(b'tagsfnodescache', True):
         addparttagsfnodescache(repo, bundler, outgoing)