I got the push but the chat was empty

Recurring complaint from myself: the assistant pushes a notification that it's done, I open the app, the conversation is there, the assistant bubble is empty. Pull-to-refresh fills it in. Background sync never does.

What was happening

The assistant streams its response over SSE while a long-running tool job churns in the background. When the iOS app gets backgrounded mid-stream, it disconnects from SSE and falls back to polling /conversations/{id}/poll. That endpoint returns:

changed = (conv.updated_at > since)

If changed is false, the client doesn't re-fetch /messages. The push notification arrives at job-end via APNs, the user opens the app, but updated_at looks the same as it did when the user closed the app — so the client never refetches and the bubble stays empty.

What I found

HistoryStore.append_to_message was updating the message row's content field on every chunk, but it never touched the conversation's META row. updated_at only got bumped by add_message (called once at the start of the turn, with empty content) and by the end-of-job touch().

So during the actual streaming window — when the bubble is filling up — the conversation's updated_at was frozen. The poll endpoint correctly reported "nothing changed" because the field it was checking truly hadn't moved.

The fix

One DynamoDB UpdateItem per chunk. Cheap.

async def append_to_message(self, pk: str, msg_id: str, chunk: str):
    # existing UpdateItem on the message row
    await self._update_message_content(pk, msg_id, chunk)
    # new: bump the META row so pollers learn there's new content
    await self.touch(pk.removeprefix("CONV#"))

touch() was already there for the end-of-job bump; it sets updated_at and gsi1sk together so the sidebar's recency sort also moves the conversation to the top during streaming.

What I'd do differently

The SSE handler and the poll handler are two different code paths agreeing on a shared invariant — "any visible change bumps updated_at." Nothing in the codebase enforced that invariant. I'd add a test that streams a chunk and then asserts the conversation META row's updated_at moved, so the next person who writes a new append path doesn't drop the touch and reintroduce the same ghost bubble.