diff --git a/.librarian/config.yaml b/.librarian/config.yaml
index 4e6a62c4c085..1620e14aa0da 100644
--- a/.librarian/config.yaml
+++ b/.librarian/config.yaml
@@ -1,3 +1,11 @@
+# This repo is now in legacylibrarian "release-only mode"
+# as part of the migration to librarian.
+#
+# Attempting to regenerate using legacylibrarian will fail,
+# and releasing will not expect commits to be generated by
+# legacylibrarian.
+release_only_mode: true
+
 global_files_allowlist:
   # Allow the container to read and write the root `CHANGELOG.md`
   # file during the `release` step to update the latest client library