diff --git a/src/google/adk/tools/agent_tool.py b/src/google/adk/tools/agent_tool.py
index 46d8616619..e08b71d159 100644
--- a/src/google/adk/tools/agent_tool.py
+++ b/src/google/adk/tools/agent_tool.py
@@ -186,9 +186,12 @@ async def run_async(
     # to avoid "Attempted to exit cancel scope in a different task" errors
     await runner.close()
 
-    if not last_content:
+    if not last_content or not last_content.parts:
       return ''
     merged_text = '\n'.join(p.text for p in last_content.parts if p.text)
+    # no text present -> no json, return empty string
+    if not merged_text:
+      return ''
     if isinstance(self.agent, LlmAgent) and self.agent.output_schema:
       tool_result = self.agent.output_schema.model_validate_json(
           merged_text