diff --git a/python/packages/devui/agent_framework_devui/_mapper.py b/python/packages/devui/agent_framework_devui/_mapper.py
index 771a571e2aa..d3288463247 100644
--- a/python/packages/devui/agent_framework_devui/_mapper.py
+++ b/python/packages/devui/agent_framework_devui/_mapper.py
@@ -1432,7 +1432,7 @@ async def _map_reasoning_content(self, content: Any, context: dict[str, Any]) ->
         """Map TextReasoningContent to ResponseReasoningTextDeltaEvent."""
         return ResponseReasoningTextDeltaEvent(
             type="response.reasoning_text.delta",
-            delta=content.text,
+            delta=content.text or "",
             item_id=context["item_id"],
             output_index=context["output_index"],
             content_index=context["content_index"],
diff --git a/python/packages/devui/tests/devui/test_mapper.py b/python/packages/devui/tests/devui/test_mapper.py
index d71a32c88ad..0eab8ffa6dc 100644
--- a/python/packages/devui/tests/devui/test_mapper.py
+++ b/python/packages/devui/tests/devui/test_mapper.py
@@ -1179,3 +1179,34 @@ async def test_superstep_completed_event(mapper: MessageMapper, test_request: Ag
     # superstep_completed event (type='superstep_completed') may not emit events (internal workflow signal)
     # Just ensure it doesn't crash
     assert isinstance(events, list)
+
+
+# =============================================================================
+# Reasoning Content Tests
+# =============================================================================
+
+
+async def test_map_reasoning_content_with_none_text(mapper: MessageMapper) -> None:
+    """Test that TextReasoningContent with None text falls back to empty string without raising validation error."""
+    from unittest.mock import MagicMock
+
+    context = {"item_id": "item-1", "output_index": 0, "content_index": 0, "sequence_counter": 0}
+    content = MagicMock()
+    content.text = None
+
+    event = await mapper._map_reasoning_content(content, context)
+    assert event.type == "response.reasoning_text.delta"
+    assert event.delta == ""
+
+
+async def test_map_reasoning_content_with_valid_text(mapper: MessageMapper) -> None:
+    """Test that TextReasoningContent with valid text maps delta properly."""
+    from unittest.mock import MagicMock
+
+    context = {"item_id": "item-1", "output_index": 0, "content_index": 0, "sequence_counter": 0}
+    content = MagicMock()
+    content.text = "Thinking through the steps..."
+
+    event = await mapper._map_reasoning_content(content, context)
+    assert event.type == "response.reasoning_text.delta"
+    assert event.delta == "Thinking through the steps..."