diff --git a/lib/crewai/src/crewai/tools/structured_tool.py b/lib/crewai/src/crewai/tools/structured_tool.py
index 8ecba85496..946b203e11 100644
--- a/lib/crewai/src/crewai/tools/structured_tool.py
+++ b/lib/crewai/src/crewai/tools/structured_tool.py
@@ -58,6 +58,11 @@ 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)):
+            try:
+                return json.dumps(raw_result, ensure_ascii=False)
+            except Exception:
+                return str(raw_result)
         return str(raw_result)
 
     try:
@@ -70,6 +75,23 @@ def _format_tool_output_for_agent(tool: Any, raw_result: Any) -> str:
         validated = result_schema.model_validate(validation_input)
         return validated.model_dump_json()
     except Exception as exc:
+        if isinstance(raw_result, (dict, list)):
+            try:
+                serialized = json.dumps(raw_result, ensure_ascii=False)
+                warnings.warn(
+                    (
+                        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 JSON-serialized raw_result."
+                    ),
+                    RuntimeWarning,
+                    stacklevel=2,
+                )
+                return serialized
+            except Exception:
+                pass
+
         warnings.warn(
             (
                 f"Failed to validate or serialize output from tool "
diff --git a/lib/crewai/tests/tools/test_structured_tool.py b/lib/crewai/tests/tools/test_structured_tool.py
index 0241abbcff..90beeb590b 100644
--- a/lib/crewai/tests/tools/test_structured_tool.py
+++ b/lib/crewai/tests/tools/test_structured_tool.py
@@ -519,3 +519,31 @@ def failing_function(should_fail: bool) -> str:
         tool.invoke({"should_fail": True})
 
     assert call_count == 1
+
+
+def test_format_output_for_agent_with_dict_and_list():
+    """Test that format_output_for_agent automatically serializes dict and list outputs to JSON."""
+    def dict_tool() -> dict:
+        return {"nested": {"key": "value"}}
+
+    tool = CrewStructuredTool.from_function(
+        func=dict_tool,
+        name="dict_tool",
+        description="A tool returning a dict",
+    )
+    
+    result = tool.format_output_for_agent({"nested": {"key": "value"}})
+    assert json.loads(result) == {"nested": {"key": "value"}}
+
+    def list_tool() -> list:
+        return [{"nested": {"key": "value"}}]
+
+    tool_list = CrewStructuredTool.from_function(
+        func=list_tool,
+        name="list_tool",
+        description="A tool returning a list",
+    )
+
+    result_list = tool_list.format_output_for_agent([{"nested": {"key": "value"}}])
+    assert json.loads(result_list) == [{"nested": {"key": "value"}}]
+