diff --git a/backend/open_webui/utils/middleware.py b/backend/open_webui/utils/middleware.py
index 19e23cf73..6db969261 100644
--- a/backend/open_webui/utils/middleware.py
+++ b/backend/open_webui/utils/middleware.py
@@ -455,7 +455,7 @@ def serialize_output(output: list) -> str:
     Convert OR-aligned output items to HTML for display.
     For LLM consumption, use convert_output_to_messages() instead.
     """
-    content = ''
+    parts: list[str] = []
 
     # First pass: collect function_call_output items by call_id for lookup
     tool_outputs = {}
@@ -472,53 +472,48 @@ def serialize_output(output: list) -> str:
                 if 'text' in content_part:
                     text = content_part.get('text', '').strip()
                     if text:
-                        content = f'{content}{text}\n'
+                        parts.append(text)
 
         elif item_type == 'function_call':
-            # Render tool call inline with its result (if available)
-            if content and not content.endswith('\n'):
-                content += '\n'
-
             call_id = item.get('call_id', '')
             name = item.get('name', '')
             arguments = item.get('arguments', '')
 
             result_item = tool_outputs.get(call_id)
             if result_item:
-                result_text = ''
+                result_parts: list[str] = []
                 for result_output in result_item.get('output', []):
                     if 'text' in result_output:
                         output_text = result_output.get('text', '')
-                        result_text += str(output_text) if not isinstance(output_text, str) else output_text
+                        result_parts.append(str(output_text) if not isinstance(output_text, str) else output_text)
+                result_text = ''.join(result_parts)
                 files = result_item.get('files')
                 embeds = result_item.get('embeds', '')
 
-                content += f'<details type="tool_calls" done="true" id="{call_id}" name="{name}" arguments="{html.escape(json.dumps(arguments))}" files="{html.escape(json.dumps(files)) if files else ""}" embeds="{html.escape(json.dumps(embeds))}">\n<summary>Tool Executed</summary>\n{html.escape(json.dumps(result_text, ensure_ascii=False))}\n</details>\n'
+                parts.append(f'<details type="tool_calls" done="true" id="{call_id}" name="{name}" arguments="{html.escape(json.dumps(arguments))}" files="{html.escape(json.dumps(files)) if files else ""}" embeds="{html.escape(json.dumps(embeds))}">\n<summary>Tool Executed</summary>\n{html.escape(json.dumps(result_text, ensure_ascii=False))}\n</details>')
             else:
-                content += f'<details type="tool_calls" done="false" id="{call_id}" name="{name}" arguments="{html.escape(json.dumps(arguments))}">\n<summary>Executing...</summary>\n</details>\n'
+                parts.append(f'<details type="tool_calls" done="false" id="{call_id}" name="{name}" arguments="{html.escape(json.dumps(arguments))}">\n<summary>Executing...</summary>\n</details>')
 
         elif item_type == 'function_call_output':
             # Already handled inline with function_call above
             pass
 
         elif item_type in _OPENAI_TOOL_DISPLAY_NAMES:
-            if content and not content.endswith('\n'):
-                content += '\n'
             status = item.get('status', 'in_progress')
             done = status in ('completed', 'failed', 'incomplete') or idx != len(output) - 1
-            content += _render_openai_tool_call_handler(item, done)
+            parts.append(_render_openai_tool_call_handler(item, done).rstrip('\n'))
 
         elif item_type == 'reasoning':
-            reasoning_content = ''
+            reasoning_parts: list[str] = []
             # Check for 'summary' (new structure) or 'content' (legacy/fallback)
             source_list = item.get('summary', []) or item.get('content', [])
             for content_part in source_list:
                 if 'text' in content_part:
-                    reasoning_content += content_part.get('text', '')
+                    reasoning_parts.append(content_part.get('text', ''))
                 elif 'summary' in content_part:  # Handle potential nested logic if any
                     pass
 
-            reasoning_content = reasoning_content.strip()
+            reasoning_content = ''.join(reasoning_parts).strip()
 
             duration = item.get('duration')
             status = item.get('status', 'in_progress')
@@ -527,9 +522,6 @@ def serialize_output(output: list) -> str:
             # render as done (a subsequent item means reasoning is complete)
             is_last_item = idx == len(output) - 1
 
-            if content and not content.endswith('\n'):
-                content += '\n'
-
             display = html.escape(
                 '\n'.join(
                     (f'> {line}' if not line.startswith('>') else line) for line in reasoning_content.splitlines()
@@ -537,19 +529,22 @@ def serialize_output(output: list) -> str:
             )
 
             if status == 'completed' or duration is not None or not is_last_item:
-                content = f'{content}<details type="reasoning" done="true" duration="{duration or 0}">\n<summary>Thought for {duration or 0} seconds</summary>\n{display}\n</details>\n'
+                parts.append(f'<details type="reasoning" done="true" duration="{duration or 0}">\n<summary>Thought for {duration or 0} seconds</summary>\n{display}\n</details>')
             else:
-                content = f'{content}<details type="reasoning" done="false">\n<summary>Thinking…</summary>\n{display}\n</details>\n'
+                parts.append(f'<details type="reasoning" done="false">\n<summary>Thinking…</summary>\n{display}\n</details>')
 
         elif item_type == 'open_webui:code_interpreter':
+            # Code interpreter needs to inspect/mutate prior accumulated content
+            # to strip trailing unclosed code fences — materialize only here.
+            content = '\n'.join(parts)
             content_stripped, original_whitespace = split_content_and_whitespace(content)
             if is_opening_code_block(content_stripped):
                 content = content_stripped.rstrip('`').rstrip() + original_whitespace
             else:
                 content = content_stripped + original_whitespace
 
-            if content and not content.endswith('\n'):
-                content += '\n'
+            # Re-split back into parts list after mutation
+            parts = [content] if content else []
 
             # Render the code_interpreter item as a <details> block
             # so the frontend Collapsible renders "Analyzing..."/"Analyzed".
@@ -575,11 +570,11 @@ def serialize_output(output: list) -> str:
                 output_attr = f' output="{html.escape(output_json)}"'
 
             if status == 'completed' or duration is not None or not is_last_item:
-                content += f'<details type="code_interpreter" done="true" duration="{duration or 0}"{output_attr}>\n<summary>Analyzed</summary>\n{display}\n</details>\n'
+                parts.append(f'<details type="code_interpreter" done="true" duration="{duration or 0}"{output_attr}>\n<summary>Analyzed</summary>\n{display}\n</details>')
             else:
-                content += f'<details type="code_interpreter" done="false"{output_attr}>\n<summary>Analyzing…</summary>\n{display}\n</details>\n'
+                parts.append(f'<details type="code_interpreter" done="false"{output_attr}>\n<summary>Analyzing…</summary>\n{display}\n</details>')
 
-    return content.strip()
+    return '\n'.join(parts).strip()
 
 
 def deep_merge(target, source):