fix(observability): handle datetime serialization in tool results (#2248)

Fixes #2219

Adds default=str to json.dumps() calls to handle non-JSON-serializable
types like datetime objects in tool function results.

Co-authored-by: kishikawa-hayato <84244732+HerBest-max@users.noreply.github.com>

python/packages/core/agent_framework/observability.py

@@ -1474,10 +1474,10 @@ def _to_otel_part(content: "Contents") -> dict[str, Any] | None:
                         elif isinstance(item, BaseModel):
                             res.append(item.model_dump(exclude_none=True))
                         else:
-                            res.append(json.dumps(item))
-                    response = json.dumps(res)
+                            res.append(json.dumps(item, default=str))
+                    response = json.dumps(res, default=str)
                 else:
-                    response = json.dumps(content.result)
+                    response = json.dumps(content.result, default=str)
             return {"type": "tool_call_response", "id": content.call_id, "response": response}
         case _:
             # GenericPart in otel output messages json spec.

python/packages/core/tests/test_observability_datetime.py

@@ -0,0 +1,26 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+"""Test datetime serialization in observability telemetry."""
+
+import json
+from datetime import datetime
+
+from agent_framework._types import FunctionResultContent
+from agent_framework.observability import _to_otel_part
+
+
+def test_datetime_in_tool_results() -> None:
+    """Test that tool results with datetime values are serialized.
+
+    Reproduces issue #2219 where datetime objects caused TypeError.
+    """
+    content = FunctionResultContent(
+        call_id="test-call",
+        result={"timestamp": datetime(2025, 11, 16, 10, 30, 0)},
+    )
+
+    result = _to_otel_part(content)
+    parsed = json.loads(result["response"])
+
+    # Datetime should be converted to string
+    assert isinstance(parsed["timestamp"], str)