diff --git a/python/copilot/tools.py b/python/copilot/tools.py
index 762b79c45b..de81fe7fd8 100644
--- a/python/copilot/tools.py
+++ b/python/copilot/tools.py
@@ -351,7 +351,7 @@ def _normalize_result(result: Any) -> ToolResult:
     # Everything else gets JSON-serialized (with Pydantic model support)
     def default(obj: Any) -> Any:
         if isinstance(obj, BaseModel):
-            return obj.model_dump()
+            return obj.model_dump(mode="json")
         raise TypeError(f"Object of type {type(obj).__name__} is not JSON serializable")
 
     try:
diff --git a/python/test_tools.py b/python/test_tools.py
index 646f17c03a..97de41df42 100644
--- a/python/test_tools.py
+++ b/python/test_tools.py
@@ -389,6 +389,44 @@ class Item(BaseModel):
         assert parsed == [{"name": "a", "value": 1}, {"name": "b", "value": 2}]
         assert result.result_type == "success"
 
+    def test_pydantic_model_with_non_primitive_fields_is_serialized(self):
+        from datetime import date, datetime
+        from decimal import Decimal
+        from enum import Enum
+        from uuid import UUID
+
+        class Status(Enum):
+            ACTIVE = "active"
+
+        class Record(BaseModel):
+            id: UUID
+            created: datetime
+            day: date
+            score: Decimal
+            status: Status
+            tags: set[str]
+
+        record = Record(
+            id=UUID("12345678-1234-5678-1234-567812345678"),
+            created=datetime(2026, 1, 15, 10, 30, 0),
+            day=date(2026, 1, 15),
+            score=Decimal("99.5"),
+            status=Status.ACTIVE,
+            tags={"python", "sdk"},
+        )
+        result = _normalize_result(record)
+        parsed = json.loads(result.text_result_for_llm)
+        assert parsed == {
+            "id": "12345678-1234-5678-1234-567812345678",
+            "created": "2026-01-15T10:30:00",
+            "day": "2026-01-15",
+            "score": "99.5",
+            "status": "active",
+            "tags": parsed["tags"],
+        }
+        assert set(parsed["tags"]) == {"python", "sdk"}
+        assert result.result_type == "success"
+
     def test_raises_for_unserializable_value(self):
         # Functions cannot be JSON serialized
         with pytest.raises(TypeError, match="Failed to serialize"):