In Next dev (Turbopack), each route compiles as its own entry — a
module-level Map in turn-liveness.ts was a DIFFERENT instance in the page
render, GET /api/chats/[id], and POST /api/chat. The chat route held the
live turn (hence the 409), but the page render read an empty registry:
initiallyInFlight=false → mount continuation fired a send → 409 → the
'Something went wrong' banner on returning mid-turn. The registry now
lives on globalThis (the standard Next singleton channel) — one Map per
process regardless of module instances. Semantics unchanged (TTL leak
guard, begin/end round trip).
Browser-verified end-to-end (headless tester): answer quiz → navigate
away → return mid-turn → graded card (no re-prompt), 'Thinking…' shimmer,
composer gated, no error banner; teacher response auto-delivered when the
background turn finished. Polling GET /api/chats/[id] observed working.
The bug: answering a quiz and navigating away mid-turn, then returning,
re-prompted the same question. Root causes: (1) the answer was durable only
at onFinish — every mid-turn view read the stale pre-answer record;
(2) the server had no notion of a turn in flight; (3) remount treated the
stale record as current, with no re-attach path. Worse, the stale dock let
the quiz be answered again, racing two turns into a last-writer-wins
saveMessages clobber.
Fix (four pieces):
- Write-ahead persist in chat-handler: the request body IS the client's
canonical state — saved to disk BEFORE the stream starts. The answer is
durable the instant the request arrives; crashes recover via the existing
mount continuation.
- Turn liveness registry (src/server/turn-liveness.ts): in-memory chatId →
in-flight map with a TTL leak guard; beginTurn rejects concurrent turns
(409) so one turn ever owns a chat. endTurn runs in onFinish AFTER
persistTurn, so pollers that see not-in-flight always read a finished
record; a turn-start failure clears the registry (no lockout).
- GET /api/chats/[id] reports turnInFlight; the chat page passes
initiallyInFlight to the client.
- Client re-attach (components/chat.tsx): while in flight, busy shimmer +
composer gated + mount continuation suppressed; poll /api/chats/[id] until
the turn finishes, then syncFromServer adopts the completed transcript.
Stop stays local-stream only (a server turn has no client request to
abort).
Verified by scripted-model tests: concurrent turn 409 + registry lifecycle,
write-ahead durability mid-turn, failed-turn lockout guard.
Amos