Fix

backend/tests/test_json_serialization.py

@@ -0,0 +1,27 @@
+"""Regression coverage for API JSON encoding of PostgreSQL value types."""
+
+from __future__ import annotations
+
+import json
+import unittest
+from datetime import date, datetime, timezone
+
+from app.main import _json_default
+
+
+class JsonSerializationTests(unittest.TestCase):
+    def test_json_default_serializes_postgres_datetime_and_date_values(self) -> None:
+        payload = {
+            "started_at": datetime(2026, 5, 21, 10, 11, 12, tzinfo=timezone.utc),
+            "day": date(2026, 5, 21),
+        }
+
+        encoded = json.loads(json.dumps(payload, default=_json_default))
+
+        self.assertEqual(
+            encoded,
+            {
+                "started_at": "2026-05-21T10:11:12+00:00",
+                "day": "2026-05-21",
+            },
+        )