fix: session transcript disappears mid-stream when live cache only has the active turn#1616
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… the active turn The live transcript cache (fed by SSE delta events) was given absolute priority over the snapshot. When a long session streams a new turn, the cache often contains only the current user+assistant pair while the snapshot still holds the full history. The old deriveRenderedSessionMessages returned whichever was non-empty first, so the user saw only their last message until the session went idle and a full re-seed restored everything. Extract message-merge helpers that union snapshot history with live streaming parts, preferring longer cached text for in-progress parts. deriveRenderedSessionMessages now merges both sources when the live cache is missing messages the snapshot knows about, and appends live-only messages (the active turn) when streaming. seedSessionState in session-sync.ts reuses the same merge helpers, replacing the inline O(N*M) merge that could also drop older cached messages when a busy snapshot arrived with fewer entries.
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benjaminshafii
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May 15, 2026
… the active turn (#1616) The live transcript cache (fed by SSE delta events) was given absolute priority over the snapshot. When a long session streams a new turn, the cache often contains only the current user+assistant pair while the snapshot still holds the full history. The old deriveRenderedSessionMessages returned whichever was non-empty first, so the user saw only their last message until the session went idle and a full re-seed restored everything. Extract message-merge helpers that union snapshot history with live streaming parts, preferring longer cached text for in-progress parts. deriveRenderedSessionMessages now merges both sources when the live cache is missing messages the snapshot knows about, and appends live-only messages (the active turn) when streaming. seedSessionState in session-sync.ts reuses the same merge helpers, replacing the inline O(N*M) merge that could also drop older cached messages when a busy snapshot arrived with fewer entries.
benjaminshafii
added a commit
that referenced
this pull request
May 15, 2026
… the active turn (#1616) The live transcript cache (fed by SSE delta events) was given absolute priority over the snapshot. When a long session streams a new turn, the cache often contains only the current user+assistant pair while the snapshot still holds the full history. The old deriveRenderedSessionMessages returned whichever was non-empty first, so the user saw only their last message until the session went idle and a full re-seed restored everything. Extract message-merge helpers that union snapshot history with live streaming parts, preferring longer cached text for in-progress parts. deriveRenderedSessionMessages now merges both sources when the live cache is missing messages the snapshot knows about, and appends live-only messages (the active turn) when streaming. seedSessionState in session-sync.ts reuses the same merge helpers, replacing the inline O(N*M) merge that could also drop older cached messages when a busy snapshot arrived with fewer entries.
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Summary
deriveRenderedSessionMessagesgave absolute priority to the live transcript cache (fed by SSE deltas). When a new turn starts streaming, the cache often only contains the current user+assistant pair while the snapshot still holds the full history. The old logic returned whichever source was non-empty first, dropping the snapshot history.message-merge.tshelpers that union snapshot history with live streaming parts.deriveRenderedSessionMessagesnow detects when the live cache is missing messages the snapshot knows about and merges both, appending live-only messages (the active turn) when streaming.seedSessionStatereuses the same merge helpers, replacing the inline O(N*M) merge that could also drop older cached messages when a busy snapshot arrived with fewer entries.Changes
apps/app/src/react-app/domains/session/sync/message-merge.tsapps/app/src/react-app/domains/session/surface/session-render-state.tsapps/app/src/react-app/domains/session/surface/session-surface.tsxincludeLiveOnlyMessagesflag during streamingapps/app/src/react-app/domains/session/sync/session-sync.tsseedSessionStatedelegates to shared merge helpersapps/app/scripts/session-render-state.test.tsVerification
bun test apps/app/scripts/session-render-state.test.ts— 7/7 passpnpm --filter @openwork/app typecheck— clean