diff --git a/lib/crewai/src/crewai/tools/structured_tool.py b/lib/crewai/src/crewai/tools/structured_tool.py
index 8ecba85496..f9ed0dbd15 100644
--- a/lib/crewai/src/crewai/tools/structured_tool.py
+++ b/lib/crewai/src/crewai/tools/structured_tool.py
@@ -58,6 +58,8 @@ def _format_tool_output_for_agent(tool: Any, raw_result: Any) -> str:
 
     result_schema = getattr(tool, "result_schema", None)
     if not (isinstance(result_schema, type) and issubclass(result_schema, BaseModel)):
+        if isinstance(raw_result, (dict, list)):
+            return json.dumps(raw_result, default=str)
         return str(raw_result)
 
     try:
@@ -75,11 +77,13 @@ def _format_tool_output_for_agent(tool: Any, raw_result: Any) -> str:
                 f"Failed to validate or serialize output from tool "
                 f"'{getattr(tool, 'name', '<unknown>')}' using result_schema "
                 f"'{result_schema.__name__}': {exc.__class__.__name__}. "
-                "Falling back to str(raw_result)."
+                "Falling back to JSON for dict/list, otherwise str(raw_result)."
             ),
             RuntimeWarning,
             stacklevel=2,
         )
+        if isinstance(raw_result, (dict, list)):
+            return json.dumps(raw_result, default=str)
         return str(raw_result)