diff --git a/nanobot/utils/helpers.py b/nanobot/utils/helpers.py
index a10a4f18b..a7c2c2574 100644
--- a/nanobot/utils/helpers.py
+++ b/nanobot/utils/helpers.py
@@ -124,8 +124,8 @@ def build_assistant_message(
     msg: dict[str, Any] = {"role": "assistant", "content": content}
     if tool_calls:
         msg["tool_calls"] = tool_calls
-    if reasoning_content is not None:
-        msg["reasoning_content"] = reasoning_content
+    if reasoning_content is not None or thinking_blocks:
+        msg["reasoning_content"] = reasoning_content if reasoning_content is not None else ""
     if thinking_blocks:
         msg["thinking_blocks"] = thinking_blocks
     return msg