When the same model is selected multiple times in a side-by-side chat,
each response is created with a distinct modelIdx (0,1,2,3) that
identifies its column. The backend now owns message persistence, but it
built the assistant placeholders without modelIdx, so the field was
never saved. On reload MultiResponseMessages groups responses by
modelIdx and falls back to grouping by model id when modelIdx is
missing; with duplicate models that fallback lumps every response into
each column, so all columns render the first response (and show a bogus
'1/N' pager).
Send modelIdx with each message_ids entry from the frontend and persist
it on the assistant placeholders in both the new-chat and existing-chat
paths. The message_ids list is now forwarded for every send (not just
multi-model ones) so single-column regenerations in a duplicate-model
chat also keep their column identity across reloads.
G30
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