## Rationale
`activeStreamId` is the mutex that guarantees one live stream per thread
— but claiming it is a plain read-then-update. The resolver checks it,
then `streamAgentChat` enqueues the job **before** writing the claim.
Two racing sends both pass the check, both start jobs, and each job's
`resetStreamState` wipes the other's Redis chunk list — tokens from two
answers interleave into the visible message. The same window exists for
retry vs. send, the queue drain vs. send, and `stopAgentChatStream`,
which cleared the claim **unguarded** (`{ id, userWorkspaceId }`) and
could wipe a newer stream's claim entirely.
## Why this is the root cause, not a symptom patch
Ownership must live in the `activeStreamId` column regardless of any
locking mechanism — the queue-behind gate, the thread DTO, and stop all
read it. So the correct primitive is a single-row compare-and-set on
that column: `UPDATE … WHERE "activeStreamId" IS NULL` checked via
affected rows, claim **before** enqueue, release on enqueue failure.
Every mutation of the claim is now guarded on the observed value.
Alternatives evaluated and rejected:
- **BullMQ jobId dedup by threadId**: the driver appends a `-${v4()}`
suffix to custom ids and dedups via a non-atomic `getJobs(['waiting'])`
scan that ignores active jobs — two racing sends still run concurrently,
and it does nothing for stop/retry races.
- **`SELECT FOR UPDATE` / Redis SETNX / advisory locks**: all add a
second mechanism (transaction plumbing or a second source of truth) to
protect a single-row write that Postgres can already do atomically.
Path-specific claim predicates fall out naturally: send/drain claim with
`pendingQuestionMessageId IS NULL`, retry claims with `lastStreamError
IS NOT NULL` (and restores the error if its enqueue fails) — closing the
double-retry race for free.
## User impact
Double-send (impatient double-click, two tabs, retry racing a queued
drain) can currently garble the assistant's answer with interleaved
tokens from two model runs and strand one stream's claim. All of these
become deterministic: exactly one winner streams; the loser queues
politely.
## Test plan
- [x] New claim spec: conditional claim before enqueue, race-loser
queues, halted-backlog send queues at the back and kicks the drain
front-first, enqueue-failure releases the claim
- [x] Retry spec updated: rollback restores the prior `lastStreamError`;
guarded shapes asserted
- [ ] CI green
https://claude.ai/code/session_01Lyi6zTema2FMVVh8MD6c38
---
_Generated by [Claude
Code](https://claude.ai/code/session_01Lyi6zTema2FMVVh8MD6c38)_
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Félix Malfait
9ef1af9799
