diff --git a/agents/paradigm_agent.py b/agents/paradigm_agent.py
index 30c7dfc..1f785c1 100644
--- a/agents/paradigm_agent.py
+++ b/agents/paradigm_agent.py
@@ -263,7 +263,9 @@ def _build_structure_prompt(signals: dict) -> str:
             continue
         sections.append(f"\n## {title}")
         for row in rows[:10]:
-            sections.append(f"- {json.dumps(row, ensure_ascii=True)[:220]}")
+            # Postgres rows carry datetime objects (SQLite returns strings);
+            # default=str keeps prompt building backend-agnostic.
+            sections.append(f"- {json.dumps(row, ensure_ascii=True, default=str)[:220]}")
 
     return "\n".join(sections)
 
diff --git a/tests/test_paradigm_prompt.py b/tests/test_paradigm_prompt.py
new file mode 100644
index 0000000..e514e86
--- /dev/null
+++ b/tests/test_paradigm_prompt.py
@@ -0,0 +1,39 @@
+"""_build_structure_prompt must handle DB rows from both backends.
+
+Postgres returns datetime objects in signal rows where SQLite returns
+strings; prompt building must not crash on either."""
+
+from __future__ import annotations
+
+import os
+import unittest
+from datetime import datetime
+
+os.environ["DEEPGRAPH_DATABASE_URL"] = ""  # force SQLite tmpdir; never touch a real DB from the environment
+
+
+class StructurePromptTest(unittest.TestCase):
+    def test_rows_with_datetime_values(self):
+        from agents.paradigm_agent import _build_structure_prompt
+
+        signals = {
+            "entity_overlaps": [],
+            "pattern_matches": [],
+            "contradiction_clusters": [],
+            "taxonomy_map": [],
+            "hidden_variable_bridges": [
+                {
+                    "entity": "gaussian splatting",
+                    "node_a": "cv.3d",
+                    "node_b": "cv.sfm",
+                    "created_at": datetime(2026, 6, 12, 12, 0, 0),
+                }
+            ],
+        }
+        prompt = _build_structure_prompt(signals)
+        self.assertIn("gaussian splatting", prompt)
+        self.assertIn("2026-06-12", prompt)
+
+
+if __name__ == "__main__":
+    unittest.main()