diff --git a/workflowai/core/domain/run.py b/workflowai/core/domain/run.py
index 0b4d522..62f15af 100644
--- a/workflowai/core/domain/run.py
+++ b/workflowai/core/domain/run.py
@@ -110,7 +110,8 @@ def format_output(self) -> str:
         # Format the output string
         output: list[str] = []
         # In case of partial validation, it is possible that the output is an empty model
-        if dumped_output := self.output.model_dump():
+        if dumped_output := self.output.model_dump(mode="json"):
+            # Use model_dump_json which handles datetime serialization correctly
             output += [
                 "\nOutput:",
                 "=" * 50,
diff --git a/workflowai/core/domain/run_test.py b/workflowai/core/domain/run_test.py
index 0a3b413..e85d408 100644
--- a/workflowai/core/domain/run_test.py
+++ b/workflowai/core/domain/run_test.py
@@ -1,3 +1,4 @@
+from datetime import datetime, timezone
 from unittest.mock import Mock, patch
 
 import pytest
@@ -17,6 +18,11 @@ class _TestOutput(BaseModel):
     message: str
 
 
+class _TestOutputWithDatetime(BaseModel):
+    timestamp: datetime
+    message: str
+
+
 @pytest.fixture
 def mock_agent() -> Mock:
     mock = Mock(spec=_AgentBase)
@@ -169,6 +175,29 @@ def test_format_output_tool_call_requests(self):
 URL: https://workflowai.hello/_/agents/agent-id/runs/run-id"""
         )
 
+    def test_format_output_with_datetime(self):
+        """Test that datetimes in the output model are correctly serialized to ISO strings."""
+        test_dt = datetime(2024, 1, 1, 12, 30, 0, tzinfo=timezone.utc)
+        run = Run[_TestOutputWithDatetime](
+            id="run-dt-id",
+            agent_id="agent-dt-id",
+            schema_id=2,
+            output=_TestOutputWithDatetime(timestamp=test_dt, message="datetime test"),
+            duration_seconds=0.5,
+            cost_usd=0.0001,
+        )
+
+        expected_json_part = '{\n  "timestamp": "2024-01-01T12:30:00Z",\n  "message": "datetime test"\n}'
+        expected = f"""\nOutput:
+==================================================
+{expected_json_part}
+==================================================
+Cost: $ 0.00010
+Latency: 0.50s
+URL: https://workflowai.hello/_/agents/agent-dt-id/runs/run-dt-id"""
+
+        assert run.format_output() == expected
+
 
 class TestRunURL:
     # The @patch decorator from unittest.mock temporarily replaces the value of an attribute