fix: don't revert replace/outlet content on chat save (#25485)

update_chat_by_id re-derived assistant content from `output` on every
save (serialize_output) so frontend edits to output items reflect in
content. But it ran unconditionally, so content set independently of
output — an __event_emitter__ {"type":"replace"} from an Action, or an
outlet filter footer — was reverted to the original output-derived text
on the next save. The reload reads chat.chat directly, so the change
vanished after navigating away (regression vs 0.9.2, which predates the
output mechanism).

Re-derive only when the message's `output` actually changed versus what's
stored, which still reflects genuine output edits but leaves
independently-set content intact.

Fixes #24585

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/open_webui/routers/chats.py

@@ -968,12 +968,15 @@ async def update_chat_by_id(
     if chat:
         updated_chat = {**chat.chat, **form_data.chat}
 
-        # Re-derive content from output for assistant messages so that
-        # frontend edits to output items are always reflected in content.
-        # serialize_output() is the single source of truth for this conversion.
-        for msg in updated_chat.get('history', {}).get('messages', {}).values():
+        # Re-derive content from output for assistant messages so that frontend
+        # edits to output items are reflected in content. Only when output
+        # actually changed — otherwise content set independently of output
+        # (e.g. a `replace` event or an outlet filter footer) would be reverted.
+        existing_messages = (chat.chat.get('history') or {}).get('messages') or {}
+        for msg_id, msg in updated_chat.get('history', {}).get('messages', {}).items():
             if msg.get('role') == 'assistant' and msg.get('output'):
-                msg['content'] = serialize_output(msg['output'])
+                if msg.get('output') != existing_messages.get(msg_id, {}).get('output'):
+                    msg['content'] = serialize_output(msg['output'])
 
         chat = await Chats.update_chat_by_id(id, updated_chat, db=db)