## Summary
Sending a chat message immediately after an action (for example an undo)
could make the message's response vanish from the UI. The transport
opened a response stream that closed on the *earlier* turn's completion
instead of waiting for the send's own turn. The agent still produced and
persisted the answer, so it reappeared on refresh. Same "disappearing
message" class as
[#4176](https://github.com/triggerdotdev/trigger.dev/pull/4176),
different cause.
## Fix
A send's response stream had no way to tell whether a `turn-complete`
belonged to its turn. `POST /realtime/v1/sessions/:id/in/append` now
returns the appended record's sequence number, and the transport skips
any turn-complete whose `session-in-event-id` (the agent's committed
`.in` cursor) is below that seq, closing only on its own turn. Older
webapps omit the seq, in which case the transport falls back to the
previous behavior, so the SDK and server can ship independently.
Because the fix spans the SDK and the server, both a webapp deploy and
an SDK release are needed for the full effect.
Verified end to end with the ai-chat reference app:
undo-then-immediate-send loses the follow-up's answer before the fix and
streams it inline after, with a revert-the-guard run reproducing the
loss on the same script. Unit tests cover the skip and the no-seq
fallback.
Eric Allam
9b3a7bd7b2