Cancelling a running turn briefly collapsed the already-streamed reasoning,
tool cards, and text to a bare "Task cancelled" marker before a follow-up
GET /api/session repopulated them — a visible race between the terminal
cancel SSE event and the refetch (#4076, #1361).
The terminal cancel event now carries a canonical, redacted session snapshot
captured under the per-session agent lock right after the cancelled turn is
persisted (including the recovered partial-assistant message). The frontend
applies that embedded snapshot immediately via _applyCancelSessionPayload and
skips the GET round-trip. The snapshot is a fully detached deep copy (redaction
rebuilds every container) so it cannot alias or be staled by concurrent writes;
it is only applied when its session_id matches the active view; and a
missing/failed snapshot degrades cleanly to the existing GET fallback.
The seven other cancel call sites are standardized via _cancel_event_payload()
(message + type + status), backward-compatible (no session key when none).
Co-authored-by: franksong2702 <franksong2702@users.noreply.github.com>
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