diff --git a/python/packages/autogen-core/src/autogen_core/tools/_base.py b/python/packages/autogen-core/src/autogen_core/tools/_base.py
index d2ea76e21da1..7385f48b7fc7 100644
--- a/python/packages/autogen-core/src/autogen_core/tools/_base.py
+++ b/python/packages/autogen-core/src/autogen_core/tools/_base.py
@@ -171,6 +171,16 @@ def return_value_as_string(self, value: Any) -> str:
                 return json.dumps(dumped)
             return str(dumped)
 
+        if isinstance(value, (dict, list)):
+            # Serialize structured return values as valid JSON instead of their
+            # Python repr (e.g. ``str``) so downstream consumers receive
+            # double-quoted JSON rather than single-quoted repr strings. Fall
+            # back to ``str`` for values that are not JSON serializable.
+            try:
+                return json.dumps(value)
+            except (TypeError, ValueError):
+                return str(value)
+
         return str(value)
 
     @abstractmethod