diff --git a/backend/open_webui/utils/middleware.py b/backend/open_webui/utils/middleware.py
index 9cb936d3ec..847b714cb0 100644
--- a/backend/open_webui/utils/middleware.py
+++ b/backend/open_webui/utils/middleware.py
@@ -233,6 +233,9 @@ def _split_tool_calls(
     """
 
     def split_json_objects(raw: str) -> list[str]:
+        if not isinstance(raw, str):
+            raw = '' if raw is None else json.dumps(raw)
+
         decoder = json.JSONDecoder()
         results = []
         position = 0
@@ -253,7 +256,11 @@ def _split_tool_calls(
 
     expanded = []
     for tool_call in tool_calls:
-        arguments = tool_call.get('function', {}).get('arguments', '')
+        function = tool_call.setdefault('function', {})
+        arguments = function.get('arguments')
+        if not isinstance(arguments, str):
+            arguments = '' if arguments is None else json.dumps(arguments)
+            function['arguments'] = arguments
         split_arguments = split_json_objects(arguments)
 
         if len(split_arguments) <= 1:
@@ -4397,7 +4404,13 @@ async def streaming_chat_response_handler(response, ctx):
                                                     # Add the new tool call
                                                     delta_tool_call.setdefault('function', {})
                                                     delta_tool_call['function'].setdefault('name', '')
-                                                    delta_tool_call['function'].setdefault('arguments', '')
+                                                    delta_arguments = delta_tool_call['function'].get('arguments')
+                                                    if not isinstance(delta_arguments, str):
+                                                        delta_tool_call['function']['arguments'] = (
+                                                            ''
+                                                            if delta_arguments is None
+                                                            else json.dumps(delta_arguments)
+                                                        )
                                                     response_tool_calls.append(delta_tool_call)
                                                 else:
                                                     # Update the existing tool call
@@ -4409,10 +4422,18 @@ async def streaming_chat_response_handler(response, ctx):
                                                     if delta_name:
                                                         current_response_tool_call['function']['name'] = delta_name
 
-                                                    if delta_arguments:
-                                                        current_response_tool_call['function']['arguments'] += (
-                                                            delta_arguments
-                                                        )
+                                                    if delta_arguments is not None:
+                                                        if not isinstance(delta_arguments, str):
+                                                            delta_arguments = json.dumps(delta_arguments)
+                                                        current_response_tool_call.setdefault('function', {})
+                                                        if not isinstance(
+                                                            current_response_tool_call['function'].get('arguments'),
+                                                            str,
+                                                        ):
+                                                            current_response_tool_call['function']['arguments'] = ''
+                                                        current_response_tool_call['function'][
+                                                            'arguments'
+                                                        ] += delta_arguments
 
                                         # Emit pending tool calls in real-time
                                         if response_tool_calls: