diff --git a/agents/aggregation_agent.py b/agents/aggregation_agent.py
index 5d40724..22836be 100644
--- a/agents/aggregation_agent.py
+++ b/agents/aggregation_agent.py
@@ -13,6 +13,11 @@
 logger = logging.getLogger("MLEvolve")
 
 
+def _join_branch_summaries(summaries: List[str]) -> str:
+    separator = "\n" + "-" * 80 + "\n"
+    return separator.join(summaries)
+
+
 def _collect_branch_representatives(agent) -> List[SearchNode]:
     representatives = []
 
@@ -111,7 +116,7 @@ def run(
             )
             reference_summaries.append(branch_info)
 
-    reference_experiences = "\n" + "-" * 80 + "\n".join(reference_summaries)
+    reference_experiences = _join_branch_summaries(reference_summaries)
 
     prompt: Any = {
         "Introduction": introduction,
diff --git a/tests/test_aggregation_agent.py b/tests/test_aggregation_agent.py
new file mode 100644
index 0000000..72b4bcf
--- /dev/null
+++ b/tests/test_aggregation_agent.py
@@ -0,0 +1,22 @@
+import unittest
+
+from agents.aggregation_agent import _join_branch_summaries
+
+
+class JoinBranchSummariesTest(unittest.TestCase):
+    def test_places_separator_between_branch_summaries(self):
+        separator = "\n" + "-" * 80 + "\n"
+
+        result = _join_branch_summaries(["branch one", "branch two", "branch three"])
+
+        self.assertEqual(
+            result,
+            f"branch one{separator}branch two{separator}branch three",
+        )
+
+    def test_preserves_a_single_branch_summary(self):
+        self.assertEqual(_join_branch_summaries(["branch one"]), "branch one")
+
+
+if __name__ == "__main__":
+    unittest.main()