## Summary
Sending a message to a chat whose run had ended could make the message
vanish: the continuation run replayed already-answered messages, never
processed the new one, and a page refresh lost it entirely. Chasing that
report surfaced four composing message-loss bugs in the chat session
runtime; this PR fixes all of them, each with a regression test.
## The fixes
1. **Stale resume cursor.** Records delivered while a run was suspended
(the waitpoint path) advanced the SSE resume counter but not the
committed-consume cursor, so the `session-in-event-id` header stamped on
turn-completes went stale by one record per suspended turn. Continuation
boots seed from that header, which is what made them replay
already-processed messages. `session.in.wait()` now advances both
cursors.
2. **Only the first buffered message dispatched.** Messages arriving
during a turn are consumed into a buffer whose end-of-turn pickup
dispatched only the first entry; the buffer was recreated each turn, so
the rest were discarded, and since consuming a record commits the cursor
the loss was permanent. A continuation boot's replay delivers several
records back-to-back, which put the user's new message at index 1 or
later. The buffer now outlives the turn and drains one message per turn
in both `chat.agent` and `chat.createSession` (whose equivalent buffer
was never read at all).
3. **Post-stop window in `chat.createSession`.** The turn's message
listener stayed attached through the stopped turn's post-stream work, so
a message sent shortly after stopping a turn was consumed into the dead
steering queue and lost. The listener now detaches when the stream
settles, matching the `chat.agent` loop.
4. **Handler leak on errored turns.** A turn that threw outside the
streaming section (for example from an `onTurnStart` hook) leaked its
message listener. Previously that silently lost mid-turn messages; with
the loop-level buffer it would have duplicated them instead. The
subscription handle is now detached by the turn's catch/finally, and
`chat.createSession` defensively detaches its prior turn's listener when
user code exits a turn without `complete()`/`done()`.
## Verification
Reproduced end-to-end with the ai-chat reference project before the fix
(message consumed but never answered, two replayed turns, gone on
refresh) and verified after (single clean turn, survives refresh,
turn-complete cursors strictly advancing). Regression tests in
`packages/trigger-sdk/test/pending-message-drain.test.ts` cover all
four, each verified red against the unfixed behavior. A smoke sweep of
the standard chat scenarios (basic send, multi-turn, suspend/resume,
mid-stream refresh, stop, steering, cancel + continue, and the
`createSession` variant) passes on the final branch state.
Eric Allam
add0a7da0a