diff --git a/backend/open_webui/utils/middleware.py b/backend/open_webui/utils/middleware.py
index ebb51cb559..06b420f6c7 100644
--- a/backend/open_webui/utils/middleware.py
+++ b/backend/open_webui/utils/middleware.py
@@ -303,6 +303,15 @@ def tool_result_content(tool_result: Any) -> str:
     return str(tool_result)
 
 
+def append_text(container: dict, key: str, value: str) -> None:
+    text = container.get(key, '')
+    container[key] = ''  # dropping the dict's reference lets CPython extend the str in place
+    try:
+        text += value
+    finally:  # a non-str value raises above and would leave the slot cleared
+        container[key] = text
+
+
 def merge_streamed_reasoning_details(target: list, details) -> None:
     items = details if isinstance(details, list) else [details]
     for item in items:
@@ -321,7 +330,7 @@ def merge_streamed_reasoning_details(target: list, details) -> None:
 
         for key, value in item.items():
             if key in ('text', 'summary') and isinstance(value, str) and isinstance(existing.get(key), str):
-                existing[key] += value
+                append_text(existing, key, value)
             else:
                 existing[key] = value
 
@@ -5136,7 +5145,7 @@ async def streaming_chat_response_handler(response, ctx):
                                             # Append to reasoning content
                                             parts = reasoning_item.get('content', [])
                                             if parts and parts[-1].get('type') == 'output_text':
-                                                parts[-1]['text'] += reasoning_content
+                                                append_text(parts[-1], 'text', reasoning_content)
                                             else:
                                                 reasoning_item['content'] = [
                                                     {
@@ -5234,11 +5243,11 @@ async def streaming_chat_response_handler(response, ctx):
                                         if inside_tag_block:
                                             # Append to the existing tag-based item
                                             if last_item_type == 'open_webui:code_interpreter':
-                                                last_item['code'] = last_item.get('code', '') + value
+                                                append_text(last_item, 'code', value)
                                             elif last_item_type == 'reasoning':
                                                 parts = last_item.get('content', [])
                                                 if parts and parts[-1].get('type') == 'output_text':
-                                                    parts[-1]['text'] += value
+                                                    append_text(parts[-1], 'text', value)
                                                 else:
                                                     last_item['content'] = [
                                                         {
@@ -5250,7 +5259,7 @@ async def streaming_chat_response_handler(response, ctx):
                                                 # solution or other _tag_type message
                                                 msg_parts = last_item.get('content', [])
                                                 if msg_parts and msg_parts[-1].get('type') == 'output_text':
-                                                    msg_parts[-1]['text'] += value
+                                                    append_text(msg_parts[-1], 'text', value)
                                                 else:
                                                     last_item['content'] = [
                                                         {
@@ -5278,7 +5287,7 @@ async def streaming_chat_response_handler(response, ctx):
                                             # Append value to last message item's text
                                             msg_parts = output[-1].get('content', [])
                                             if msg_parts and msg_parts[-1].get('type') == 'output_text':
-                                                msg_parts[-1]['text'] += value
+                                                append_text(msg_parts[-1], 'text', value)
                                             else:
                                                 output[-1]['content'] = [
                                                     {