Summary

Solid, well-tested fix for the #1580 follow-up: a chat turn becomes a first-class RunManager/Run that outlives the SSE connection, so navigating away no longer kills the agent, and the sidebar + re-attach give users a clear "still running" signal. The refactor is clean — extracting consumeSSEResponse so /send and /attach parse events identically is the right call, and the RunManager unit tests cover the genuinely tricky paths (survives subscriber disconnect, late-subscriber replay, overlap-409, cancel). The single thing worth confirming before merge is the delete-while-running race: delete_session removes the DB session before run_manager.cancel, and cancel is a best-effort flag the producer only observes at its next step boundary — so an in-flight run can still persist an assistant message to a session that no longer exists.

No critical or security issues. The threading-atomicity reasoning in run_manager.py (synchronous emit/subscribe/finish, no await between read and write of shared state) is sound for the single-threaded event loop.

Issues

🟡 Important

Delete-while-running can persist to a dead session (sessions.py:139-146, run_manager.py:1056)
delete_session deletes the session row first, then calls run_manager.cancel. Two gaps let a write slip through to the now-deleted session:

1. cancel() only sets the flag if run.handler is not None. handler is assigned partway into _stream_chat_impl, so a run that's registered but hasn't reached that line yet returns cancel() → True while setting nothing — the run proceeds uncancelled.
2. Even when the flag is set, the producer only observes it "at its next step boundary," so a run mid-final-generation can still db.add_message(session_id, "assistant", …) after the row is gone (orphan row, or FK error if enforced).

For the common path this is narrow, but data integrity on delete is worth hardening. Options: have the producer re-check session existence before its final persist, or gate add_message on the run not being cancelled. At minimum, confirm the orphaned-row behavior is harmless in your schema and note it. Not exploitable, just a correctness edge.

🟢 Minor

• Inconsistent import placement for run_manager (sessions.py:144). chat.py:25 imports it at module level; here it's a function-local import. run_manager.py only pulls stdlib, so there's no circular-import reason for the local form — move it to the top for consistency.
• Replay buffer is unbounded for the run's lifetime (run_manager.py:944). Bounded by run duration and dropped on completion, so fine in practice; just flagging that a pathologically long agent run holds every SSE string in memory until it finishes.
• getActiveRuns/RunManager assume a single backend process. Consistent with the existing module-global _active_sse_handlers, and the Agent UI is a local single-process app — so this is correct here. Worth a one-line comment that the registry is process-local if multi-worker is ever on the table.
• ChatView re-attach effect keys on [sessionId] only (ChatView.tsx). If a run starts on the session you're already viewing (e.g. triggered from another device), the global poll updates the spinner but this effect won't re-fire to auto-attach. Out of scope for the revisit flow this PR targets — noting for completeness.

Strengths

• consumeSSEResponse extraction removes a ~90-line duplicated SSE parse loop and guarantees /send and /attach stay in lockstep — exactly the right refactor to land alongside the new endpoint.
• Test coverage targets the hard parts, not just the happy path: test_run_survives_subscriber_disconnect, test_late_subscriber_gets_full_replay, test_subscribe_after_done_terminates_immediately, and the overlap/cancel cases all assert the registry's real contract. The endpoint tests also cover send-409-while-running and delete-cancels-run.
• The 409 guard extension is the correct invariant (chat.py:100): session_lock.locked() or run_manager.is_running(sid) closes the window where the HTTP lock is released but the background run is still mutating the cached agent's conversation state. The comment explains why clearly.
• Reference-stable setRunningSessions (chatStore.ts:150) avoids re-rendering the sidebar on every 2.6s poll tick — and is backed by an explicit test.

Verdict

Approve with suggestions. No blocking issues. Please confirm (or guard) the delete-while-running persistence race before merge — if the orphaned-row outcome is benign in your schema, a one-line note suffices; otherwise a re-check before the producer's final add_message would close it. The minor items are cleanup-at-leisure.