Serialize date metadata values

changelog.d/fix-metadata-date-json.fixed.md

@@ -0,0 +1 @@
+Serialize date values in country metadata responses.

policyengine_api/country.py

@@ -17,6 +17,7 @@
 import dpath
 import math
 import pandas as pd
+from datetime import date, datetime
 from pathlib import Path
 from policyengine_api.data.congressional_districts import (
     build_congressional_district_metadata,
@@ -62,6 +63,8 @@ def build_metadata(self):
     def _json_safe(self, value):
         if isinstance(value, Path):
             return str(value)
+        if isinstance(value, (date, datetime)):
+            return value.isoformat()
         if isinstance(value, dict):
             return {
                 key: self._json_safe(nested_value)

tests/unit/test_country.py

@@ -1,11 +1,29 @@
 import pytest
 import pandas as pd
+from datetime import date, datetime
 from pathlib import Path
 from types import SimpleNamespace
 
 from policyengine_api.country import COUNTRIES, PolicyEngineCountry
 
 
+class TestCountryJsonSafety:
+    def test__json_safe_serializes_dates(self):
+        country = PolicyEngineCountry.__new__(PolicyEngineCountry)
+
+        result = country._json_safe(
+            {
+                "release_date": date(2026, 5, 12),
+                "nested": [datetime(2026, 5, 12, 8, 30, 45)],
+            }
+        )
+
+        assert result == {
+            "release_date": "2026-05-12",
+            "nested": ["2026-05-12T08:30:45"],
+        }
+
+
 class TestUKCountryMetadata:
     """Tests for UK country metadata, specifically local authority loading."""