fix(activity): make new session boundaries exact

src/main/services/team/TeamDataService.ts

@@ -2936,9 +2936,11 @@ export class TeamDataService {
     }
     const leadName =
       transcriptContext.config.members?.find((m) => isLeadMember(m))?.name ?? 'team-lead';
-    const sessionIds = Array.from(
+    const knownLeadSessionIds = this.getRecentLeadSessionIds(config);
-      new Set([...this.getRecentLeadSessionIds(config), ...transcriptContext.sessionIds])
+    if (knownLeadSessionIds.length === 0) {
-    );
+      return [];
+    }
+    const sessionIds = knownLeadSessionIds;
     if (sessionIds.length === 0) {
       return [];
     }

src/renderer/components/team/activity/ActivityTimeline.tsx

@@ -48,12 +48,6 @@ interface ActivityTimelineProps {
   expandOverrides?: Set<string>;
   /** Called when user toggles expand/collapse override on a specific message. */
   onToggleExpandOverride?: (key: string) => void;
-  /**
-   * All session IDs belonging to this team (current + history).
-   * Used together with currentLeadSessionId to suppress only the reconnect boundary
-   * from the current live session back into the team's previous session history.
-   */
-  teamSessionIds?: Set<string>;
   /** Current lead session ID for the active team, if known. */
   currentLeadSessionId?: string;
   /** Whether the current team is alive. */
@@ -281,7 +275,6 @@ export const ActivityTimeline = React.memo(function ActivityTimeline({
   allCollapsed,
   expandOverrides,
   onToggleExpandOverride,
-  teamSessionIds,
   currentLeadSessionId,
   isTeamAlive,
   leadActivity,
@@ -425,10 +418,13 @@ export const ActivityTimeline = React.memo(function ActivityTimeline({
     setVisibleCount(Infinity);
   };
 
-  const getItemSessionId = (item: TimelineItem): string | undefined =>
+  const getItemSessionAnchorId = (item: TimelineItem): string | undefined => {
-    item.type === 'lead-thoughts'
+    if (item.type === 'lead-thoughts') {
-      ? item.group.thoughts[0].leadSessionId
+      return item.group.thoughts[0]?.leadSessionId;
-      : item.message.leadSessionId;
+    }
+
+    return undefined;
+  };
 
   // Pin the newest thought group (if first) so it stays at the top and doesn't jump.
   const pinnedThoughtGroup = timelineItems[0]?.type === 'lead-thoughts' ? timelineItems[0] : null;
@@ -535,18 +531,16 @@ export const ActivityTimeline = React.memo(function ActivityTimeline({
         // Session boundary separator (messages sorted desc — new on top)
         let sessionSeparator: React.JSX.Element | null = null;
         if (realIndex > 0) {
-          const prevSessionId = getItemSessionId(timelineItems[realIndex - 1]);
+          const currSessionId = getItemSessionAnchorId(item);
-          const currSessionId = getItemSessionId(item);
+          let prevSessionId: string | undefined;
+          for (let searchIndex = realIndex - 1; searchIndex >= 0; searchIndex -= 1) {
+            const candidateSessionId = getItemSessionAnchorId(timelineItems[searchIndex]);
+            if (candidateSessionId) {
+              prevSessionId = candidateSessionId;
+              break;
+            }
+          }
           if (prevSessionId && currSessionId && prevSessionId !== currSessionId) {
-            // Suppress only the boundary between the current live session and the team's
-            // older session history. Older historical session boundaries should still render.
-            const isReconnectBoundary =
-              !!currentLeadSessionId &&
-              teamSessionIds &&
-              teamSessionIds.has(prevSessionId) &&
-              teamSessionIds.has(currSessionId) &&
-              (prevSessionId === currentLeadSessionId || currSessionId === currentLeadSessionId);
-            if (!isReconnectBoundary) {
             sessionSeparator = (
               <div
                 className="flex items-center gap-3"
@@ -561,7 +555,6 @@ export const ActivityTimeline = React.memo(function ActivityTimeline({
             );
           }
         }
-        }
 
         if (item.type === 'lead-thoughts') {
           const { group } = item;

src/renderer/components/team/messages/MessagesPanel.tsx

@@ -80,8 +80,6 @@ interface MessagesPanelProps {
   leadContextUpdatedAt?: string;
   /** Time window for filtering. */
   timeWindow: TimeWindow | null;
-  /** Team session IDs for timeline. */
-  teamSessionIds: Set<string>;
   /** Current lead session ID. */
   currentLeadSessionId?: string;
   /** Pending replies tracker (shared with parent for MemberList). */
@@ -114,7 +112,6 @@ export const MessagesPanel = memo(function MessagesPanel({
   leadActivity,
   leadContextUpdatedAt,
   timeWindow,
-  teamSessionIds,
   currentLeadSessionId,
   pendingRepliesByMember,
   onPendingReplyChange,
@@ -658,7 +655,6 @@ export const MessagesPanel = memo(function MessagesPanel({
         allCollapsed={messagesCollapsed}
         expandOverrides={expandedSet}
         onToggleExpandOverride={toggleExpandOverride}
-        teamSessionIds={teamSessionIds}
         currentLeadSessionId={currentLeadSessionId}
         isTeamAlive={isTeamAlive}
         leadActivity={leadActivity}
@@ -844,7 +840,6 @@ export const MessagesPanel = memo(function MessagesPanel({
             allCollapsed={messagesCollapsed}
             expandOverrides={expandedSet}
             onToggleExpandOverride={toggleExpandOverride}
-            teamSessionIds={teamSessionIds}
             currentLeadSessionId={currentLeadSessionId}
             isTeamAlive={isTeamAlive}
             leadActivity={leadActivity}
@@ -1131,7 +1126,6 @@ export const MessagesPanel = memo(function MessagesPanel({
                   allCollapsed={messagesCollapsed}
                   expandOverrides={expandedSet}
                   onToggleExpandOverride={toggleExpandOverride}
-                  teamSessionIds={teamSessionIds}
                   currentLeadSessionId={currentLeadSessionId}
                   isTeamAlive={isTeamAlive}
                   leadActivity={leadActivity}

test/main/services/team/TeamDataService.test.ts

@@ -3404,7 +3404,7 @@ describe('TeamDataService', () => {
     expect(persistedConfig.projectPath).toBe(fixture.staleProjectPath);
   });
 
-  it('uses resolver-discovered session ids when config has no leadSessionId or sessionHistory', async () => {
+  it('does not guess lead_session messages from resolver-discovered session ids when config has no leadSessionId or sessionHistory', async () => {
     const fixture = await createResolverBackedLeadFixture({
       leadSessionId: undefined,
       sessionFileId: 'lead-discovered',
@@ -3413,13 +3413,48 @@ describe('TeamDataService', () => {
 
     const page = await service.getMessagesPage(fixture.teamName, { limit: 10 });
 
+    expect(page.messages.some((message) => message.source === 'lead_session')).toBe(false);
+  });
+
+  it('does not mix resolver-discovered non-lead session ids into durable lead_session messages when config already knows the lead session', async () => {
+    const fixture = await createResolverBackedLeadFixture();
+    await fs.writeFile(
+      path.join(fixture.actualProjectDir, 'member-1.jsonl'),
+      `${JSON.stringify({
+        teamName: fixture.teamName,
+        type: 'assistant',
+        timestamp: '2026-04-18T10:05:00.000Z',
+        cwd: fixture.actualProjectPath,
+        message: {
+          role: 'assistant',
+          content: [
+            {
+              type: 'text',
+              text: 'Member bootstrap noise that should never appear as a lead_session thought in the team activity timeline.',
+            },
+          ],
+        },
+      })}\n`,
+      'utf8'
+    );
+    const service = createResolverBackedService();
+
+    const page = await service.getMessagesPage(fixture.teamName, { limit: 20 });
+    const leadSessionMessages = page.messages.filter((message) => message.source === 'lead_session');
+
     expect(
-      page.messages.find(
+      leadSessionMessages.some((message) =>
-        (message) =>
-          message.source === 'lead_session' &&
         message.text.includes('recovered through the transcript resolver')
       )
-    ).toBeTruthy();
+    ).toBe(true);
+    expect(
+      leadSessionMessages.some((message) =>
+        message.text.includes('Member bootstrap noise that should never appear')
+      )
+    ).toBe(false);
+    expect(new Set(leadSessionMessages.map((message) => message.leadSessionId))).toEqual(
+      new Set(['lead-1'])
+    );
   });
 
   it('fails fast when config is missing before any read-phase step starts', async () => {

test/renderer/components/team/activity/ActivityTimeline.test.ts

@@ -0,0 +1,156 @@
+import React from 'react';
+import { act } from 'react';
+import { createRoot } from 'react-dom/client';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+import type { InboxMessage } from '@shared/types';
+
+vi.mock('@renderer/components/team/activity/ActivityItem', () => ({
+  ActivityItem: ({ message }: { message: InboxMessage }) =>
+    React.createElement('div', { 'data-testid': 'activity-item' }, message.text),
+  isNoiseMessage: () => false,
+}));
+
+vi.mock('@renderer/components/team/activity/AnimatedHeightReveal', () => ({
+  ENTRY_REVEAL_ANIMATION_MS: 220,
+  AnimatedHeightReveal: ({ children }: { children: React.ReactNode }) =>
+    React.createElement(React.Fragment, null, children),
+}));
+
+vi.mock('@renderer/components/team/activity/useNewItemKeys', () => ({
+  useNewItemKeys: () => new Set<string>(),
+}));
+
+import { ActivityTimeline } from '@renderer/components/team/activity/ActivityTimeline';
+
+function makeMessage(overrides: Partial<InboxMessage> = {}): InboxMessage {
+  return {
+    from: 'team-lead',
+    text: 'message',
+    timestamp: '2026-04-18T13:00:00.000Z',
+    read: true,
+    source: 'inbox',
+    messageId: 'message-id',
+    leadSessionId: 'lead-session-1',
+    ...overrides,
+  };
+}
+
+describe('ActivityTimeline session separators', () => {
+  let container: HTMLDivElement;
+
+  beforeEach(() => {
+    vi.stubGlobal('IS_REACT_ACT_ENVIRONMENT', true);
+    container = document.createElement('div');
+    document.body.appendChild(container);
+  });
+
+  afterEach(() => {
+    container.remove();
+    document.body.innerHTML = '';
+    vi.unstubAllGlobals();
+  });
+
+  it('does not render New session for regular message rows even when their session ids differ', async () => {
+    const root = createRoot(container);
+    const messages: InboxMessage[] = [
+      makeMessage({
+        messageId: 'member-newest',
+        text: 'member newest',
+        leadSessionId: 'member-session-2',
+        from: 'alice',
+        source: 'inbox',
+      }),
+      makeMessage({
+        messageId: 'member-older',
+        text: 'member older',
+        leadSessionId: 'member-session-1',
+        from: 'alice',
+        source: 'inbox',
+      }),
+    ];
+
+    await act(async () => {
+      root.render(React.createElement(ActivityTimeline, { messages, teamName: 'demo-team' }));
+    });
+
+    expect(container.textContent).not.toContain('New session');
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
+
+  it('renders New session between lead thought groups from different sessions', async () => {
+    const root = createRoot(container);
+    const messages: InboxMessage[] = [
+      makeMessage({
+        messageId: 'thought-newest',
+        text: 'lead thought newest',
+        leadSessionId: 'lead-session-2',
+        from: 'team-lead',
+        source: 'lead_session',
+      }),
+      makeMessage({
+        messageId: 'regular-between',
+        text: 'regular message between sessions',
+        leadSessionId: 'member-session-1',
+        from: 'alice',
+        source: 'inbox',
+      }),
+      makeMessage({
+        messageId: 'thought-older',
+        text: 'lead thought older',
+        leadSessionId: 'lead-session-1',
+        from: 'team-lead',
+        source: 'lead_session',
+      }),
+    ];
+
+    await act(async () => {
+      root.render(React.createElement(ActivityTimeline, { messages, teamName: 'demo-team' }));
+    });
+
+    expect(container.textContent).toContain('New session');
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
+
+  it('still renders New session when the newest thought belongs to currentLeadSessionId', async () => {
+    const root = createRoot(container);
+    const messages: InboxMessage[] = [
+      makeMessage({
+        messageId: 'thought-current',
+        text: 'current lead thought',
+        leadSessionId: 'lead-session-current',
+        from: 'team-lead',
+        source: 'lead_session',
+      }),
+      makeMessage({
+        messageId: 'thought-history',
+        text: 'historical lead thought',
+        leadSessionId: 'lead-session-history',
+        from: 'team-lead',
+        source: 'lead_session',
+      }),
+    ];
+
+    await act(async () => {
+      root.render(
+        React.createElement(ActivityTimeline, {
+          messages,
+          teamName: 'demo-team',
+          currentLeadSessionId: 'lead-session-current',
+        })
+      );
+    });
+
+    expect(container.textContent).toContain('New session');
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
+});

test/renderer/components/team/messages/MessagesPanel.test.ts

@@ -184,7 +184,6 @@ describe('MessagesPanel idle summary invariants', () => {
           tasks: [],
           messages,
           timeWindow: null,
-          teamSessionIds: new Set<string>(),
           pendingRepliesByMember: {},
           onPendingReplyChange: vi.fn(),
         })
@@ -235,7 +234,6 @@ describe('MessagesPanel idle summary invariants', () => {
           tasks: [],
           messages,
           timeWindow: null,
-          teamSessionIds: new Set<string>(),
           pendingRepliesByMember: { alice: pendingSentAtMs },
           onPendingReplyChange,
         })
@@ -280,7 +278,6 @@ describe('MessagesPanel idle summary invariants', () => {
           tasks: [],
           messages,
           timeWindow: null,
-          teamSessionIds: new Set<string>(),
           pendingRepliesByMember: { alice: pendingSentAtMs },
           onPendingReplyChange,
         })
@@ -319,7 +316,6 @@ describe('MessagesPanel idle summary invariants', () => {
           tasks: [],
           messages: [makeMessage()],
           timeWindow: null,
-          teamSessionIds: new Set<string>(),
           pendingRepliesByMember: {},
           onPendingReplyChange: vi.fn(),
         })