fix: session branch visibility and parent-child tree display (#561)

- Fix branch button hidden when AI is responding by decoupling
  branchActionsDisabled from chatState (MessageList.tsx)
- Add sourceSessionId/sourceMessageId to SessionListItem type
  (both server and desktop)
- Scan forkedFrom in listSessions to expose parent-child relationships
- Preserve sourceSessionId/sourceMessageId in branchSession optimistic store
- Add buildSessionTree utility for tree-flattened session list with
  depth and isLastChild markers
- Render forked sessions indented with GitBranch icon in sidebar
- Add comprehensive tests (187 total: 81 desktop + 106 server)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

desktop/src/components/chat/MessageList.test.tsx

@@ -2725,27 +2725,36 @@ describe('MessageList nested tool calls', () => {
     })
   })
 
-  it('hides branch actions while the current session is still running', () => {
+  it('hides branch actions for the in-progress turn while preserving them for completed turns', () => {
     useChatStore.setState({
       sessions: {
         [ACTIVE_TAB]: makeSessionState({
           chatState: 'streaming',
           streamingText: 'partial',
           messages: [
+            // Completed turn — should still show branch buttons
             {
               id: 'local-user-1',
               transcriptMessageId: 'transcript-user-1',
               type: 'user_text',
-              content: '从这里开始',
+              content: 'Starting point',
               timestamp: 1,
             },
             {
               id: 'local-assistant-1',
               transcriptMessageId: 'transcript-assistant-1',
               type: 'assistant_text',
-              content: '这是完成的答复。',
+              content: 'This is a completed reply.',
               timestamp: 2,
             },
+            // In-progress turn (no response yet) — should NOT show branch buttons
+            {
+              id: 'local-user-2',
+              transcriptMessageId: 'transcript-user-2',
+              type: 'user_text',
+              content: 'A new question without a reply yet',
+              timestamp: 3,
+            },
           ],
         }),
       },
@@ -2753,7 +2762,19 @@ describe('MessageList nested tool calls', () => {
 
     render(<MessageList />)
 
-    expect(screen.queryByRole('button', { name: 'Fork a new conversation' })).toBeNull()
+    // Completed turn messages are still branchable
+    const branchButtons = screen.getAllByRole('button', { name: 'Fork a new conversation' })
+    expect(branchButtons).toHaveLength(2)
+
+    // The first user message should be branchable (has response)
+    expect(branchButtons[0]!.closest('[data-message-actions]')).toBeTruthy()
+    // The second user message should NOT appear in branch buttons
+    const secondUserBranchBtn = screen
+      .queryAllByRole('button', { name: 'Fork a new conversation' })
+      .filter((btn) =>
+        btn.closest('[data-message-actions]')?.textContent?.includes('A new question')
+      )
+    expect(secondUserBranchBtn).toHaveLength(0)
   })
 
   it('keeps historical sessions readable when turn checkpoint payloads are missing', async () => {

desktop/src/components/chat/MessageList.tsx

@@ -1217,13 +1217,7 @@ export function MessageList({ sessionId, compact = false }: MessageListProps = {
   })
   const [isVirtualScrollActive, setIsVirtualScrollActive] = useState(false)
   const [measuredItemsVersion, setMeasuredItemsVersion] = useState(0)
-  const branchActionsDisabled =
-    isMemberSession ||
-    chatState !== 'idle' ||
-    streamingText.trim().length > 0 ||
-    Boolean(activeThinkingId) ||
-    Boolean(sessionState?.activeToolUseId) ||
-    Boolean(sessionState?.activeToolName)
+  const branchActionsDisabled = isMemberSession
   const hasCompactingDivider = messages.some((message) =>
     message.type === 'compact_summary' && message.phase === 'compacting')
 

desktop/src/components/layout/Sidebar.tsx

@@ -5,6 +5,7 @@ import { useUIStore } from '../../stores/uiStore'
 import { useTranslation, type TranslationKey } from '../../i18n'
 import { ConfirmDialog } from '../shared/ConfirmDialog'
 import type { SessionListItem } from '../../types/session'
+import { buildSessionTree } from '../../lib/sessionTree'
 import { useTabStore, SETTINGS_TAB_ID, SCHEDULED_TAB_ID } from '../../stores/tabStore'
 import { useChatStore } from '../../stores/chatStore'
 import { useOpenTargetStore } from '../../stores/openTargetStore'
@@ -834,6 +835,7 @@ export function Sidebar({ isMobile = false, onRequestClose }: SidebarProps) {
                 const visibleItems = projectCollapsed
                   ? []
                   : getVisibleProjectSessions(project.sessions, sessionsExpanded, activeTabId)
+                const treeItems = buildSessionTree(visibleItems)
                 const hiddenCount = project.sessions.length - visibleItems.length
                 const groupIds = project.sessions.map((session) => session.id)
                 const groupSelectedCount = groupIds.filter((id) => selectedSessionIds.has(id)).length
@@ -934,8 +936,21 @@ export function Sidebar({ isMobile = false, onRequestClose }: SidebarProps) {
                           className={hasInternalScroll ? 'max-h-[420px] overflow-y-auto pr-1' : undefined}
                           data-testid={`sidebar-project-session-list-${domSafeProjectKey(project.key)}`}
                         >
-                          {visibleItems.map((session) => (
+                          {treeItems.map((treeSession) => {
+                            const { depth, isLastChild, ...session } = treeSession
+                            const indentLeft = `${Math.min(depth, 3) * 1}rem`
+                            return (
                             <div key={session.id} className="relative mb-0.5 last:mb-0">
+                              {depth > 0 && (
+                                <div
+                                  className="absolute top-0 border-l-2 border-[var(--color-border)]"
+                                  style={{
+                                    left: `calc(${indentLeft} - 0.5rem)`,
+                                    height: isLastChild ? '50%' : '100%',
+                                  }}
+                                  aria-hidden="true"
+                                />
+                              )}
                               {renamingId === session.id ? (
                                 <input
                                   autoFocus
@@ -950,6 +965,7 @@ export function Sidebar({ isMobile = false, onRequestClose }: SidebarProps) {
                                     }
                                   }}
                                   className="w-full rounded-[var(--radius-md)] border border-[var(--color-border-focus)] bg-[var(--color-surface)] px-3 py-2 text-sm text-[var(--color-text-primary)] outline-none"
+                                  style={{ paddingLeft: `calc(0.75rem + ${indentLeft})` }}
                                 />
                               ) : (
                                 <button
@@ -972,9 +988,17 @@ export function Sidebar({ isMobile = false, onRequestClose }: SidebarProps) {
                                       : 'sidebar-session-row--idle text-[var(--color-text-secondary)] hover:bg-[var(--color-sidebar-item-hover)] hover:text-[var(--color-text-primary)]'
                                     }
                                   `}
+                                  style={{ paddingLeft: `calc(0.625rem + ${indentLeft})` }}
                                   aria-pressed={isBatchMode ? selectedSessionIds.has(session.id) : undefined}
                                 >
                                   <span className="flex min-w-0 items-center gap-2">
+                                    {depth > 0 && (
+                                      <GitBranch
+                                        size={13}
+                                        className="flex-shrink-0 text-[var(--color-text-tertiary)] opacity-50"
+                                        aria-hidden="true"
+                                      />
+                                    )}
                                     {isBatchMode ? (
                                       <span
                                         className={`flex h-4 w-4 flex-shrink-0 items-center justify-center rounded-[5px] border transition-colors ${
@@ -1008,7 +1032,7 @@ export function Sidebar({ isMobile = false, onRequestClose }: SidebarProps) {
                                 </button>
                               )}
                             </div>
-                          ))}
+                          )})}
                         </div>
                         {(hiddenCount > 0 || sessionsExpanded) && (
                           <div className="mt-2 flex justify-start px-2.5">

desktop/src/components/layout/SidebarTreeView.test.tsx

@@ -0,0 +1,285 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { cleanup, render, screen, waitFor } from '@testing-library/react'
+import '@testing-library/jest-dom/vitest'
+
+// Mock localStorage with full API before any imports
+const localStorageStore = new Map<string, string>()
+vi.stubGlobal('localStorage', {
+  getItem: vi.fn((key: string) => localStorageStore.get(key) ?? null),
+  setItem: vi.fn((key: string, value: string) => { localStorageStore.set(key, value) }),
+  removeItem: vi.fn((key: string) => { localStorageStore.delete(key) }),
+  clear: vi.fn(() => { localStorageStore.clear() }),
+  get length() { return localStorageStore.size },
+  key: vi.fn((index: number) => [...localStorageStore.keys()][index] ?? null),
+})
+
+const desktopUiPreferencesApiMock = vi.hoisted(() => ({
+  getPreferences: vi.fn(),
+  updateSidebarPreferences: vi.fn(),
+}))
+
+vi.mock('../../api/desktopUiPreferences', () => ({
+  desktopUiPreferencesApi: desktopUiPreferencesApiMock,
+}))
+
+const openTargetStoreMock = vi.hoisted(() => ({
+  ensureTargets: vi.fn(),
+  openTarget: vi.fn(),
+  targets: [{ id: 'finder', kind: 'file_manager', label: 'Finder', platform: 'darwin' }],
+}))
+
+vi.mock('../../stores/openTargetStore', () => ({
+  useOpenTargetStore: {
+    getState: () => openTargetStoreMock,
+  },
+}))
+
+vi.mock('../../i18n', () => ({
+  useTranslation: () => (key: string, params?: Record<string, string | number>) => {
+    const translations: Record<string, string> = {
+      'sidebar.newSession': 'New Session',
+      'sidebar.settings': 'Settings',
+      'sidebar.searchPlaceholder': 'Search sessions',
+      'sidebar.noSessions': 'No sessions',
+      'sidebar.noMatching': 'No matching sessions',
+      'sidebar.sessionListFailed': 'Session list failed',
+      'sidebar.refreshSessions': 'Refresh sessions',
+      'sidebar.projects': 'Projects',
+      'sidebar.projectMenu': 'Project menu',
+      'sidebar.newProject': 'New project',
+      'sidebar.archiveAllChats': 'Archive all chats',
+      'sidebar.organizeSidebar': 'Organize sidebar',
+      'sidebar.sortCondition': 'Sort condition',
+      'sidebar.organizeByProject': 'By project',
+      'sidebar.organizeByRecentProject': 'Recent projects',
+      'sidebar.organizeByTime': 'By time',
+      'sidebar.sortByCreatedAt': 'Created time',
+      'sidebar.sortByUpdatedAt': 'Updated time',
+      'sidebar.newBlankProject': 'New blank project',
+      'sidebar.useExistingFolder': 'Use existing folder',
+      'sidebar.chooseProjectFolderUnavailable': 'Folder selection is only available in the desktop app.',
+      'sidebar.projectActions': 'Project actions for {project}',
+      'sidebar.pinProject': 'Pin Project',
+      'sidebar.unpinProject': 'Unpin Project',
+      'sidebar.openInFinder': 'Open in Finder',
+      'sidebar.openInFinderFailed': 'Could not open the project in Finder.',
+      'sidebar.openInFinderUnavailable': 'No file manager is available.',
+      'sidebar.hideProjectFromSidebar': 'Hide from Sidebar',
+      'sidebar.restoreProjectToSidebar': 'Restore to Sidebar',
+      'sidebar.restoreHiddenProjects': 'Restore hidden projects ({count})',
+      'sidebar.projectHidden': '{project} was hidden from the sidebar.',
+      'sidebar.newSessionInProject': 'New session in {project}',
+      'sidebar.showMoreSessions': 'Expand display',
+      'sidebar.showFewerSessions': 'Collapse display',
+      'sidebar.expandProject': 'Expand {project}',
+      'sidebar.collapseProject': 'Collapse {project}',
+      'sidebar.worktree': 'worktree',
+      'sidebar.sessionRunning': 'Session running',
+      'common.retry': 'Retry',
+      'common.loading': 'Loading...',
+      'common.cancel': 'Cancel',
+      'common.delete': 'Delete',
+      'common.rename': 'Rename',
+      'sidebar.timeGroup.today': 'Today',
+      'sidebar.timeGroup.yesterday': 'Yesterday',
+      'sidebar.timeGroup.last7days': 'Last 7 Days',
+      'sidebar.timeGroup.last30days': 'Last 30 Days',
+      'sidebar.timeGroup.older': 'Older',
+      'sidebar.missingDir': 'Missing',
+      'sidebar.confirmDelete': 'Delete this session?',
+      'sidebar.batchManage': 'Batch manage',
+      'sidebar.batchSelectedCount': '{count} selected',
+      'sidebar.batchSelectAll': 'Select all',
+      'sidebar.batchDeselectAll': 'Deselect all',
+      'sidebar.batchSelectGroup': 'Select {group}',
+      'sidebar.batchDeleteSelected': 'Delete selected ({count})',
+      'sidebar.batchDeleteConfirm': 'Delete {count} sessions?',
+      'sidebar.batchDeleteConfirmBody': 'The following sessions will be deleted:',
+      'sidebar.batchDeleteMore': '...and {count} more',
+      'sidebar.batchExit': 'Cancel batch mode',
+      'sidebar.batchDeleteSucceeded': 'Deleted {count} sessions.',
+      'sidebar.batchDeleteFailed': '{count} sessions could not be deleted.',
+      'sidebar.collapse': 'Collapse sidebar',
+      'sidebar.expand': 'Expand sidebar',
+      'session.lastUpdated': 'last updated {time}',
+      'session.timeJustNow': 'just now',
+      'session.timeMinutes': '{n}m ago',
+      'session.timeHours': '{n}h ago',
+      'session.timeDays': '{n}d ago',
+    }
+    let text = translations[key] ?? key
+    for (const [name, value] of Object.entries(params ?? {})) {
+      text = text.replace(new RegExp(`\\{${name}\\}`, 'g'), String(value))
+    }
+    return text
+  },
+}))
+
+import { Sidebar } from './Sidebar'
+import { useChatStore } from '../../stores/chatStore'
+import { useSessionStore } from '../../stores/sessionStore'
+import { useTabStore } from '../../stores/tabStore'
+import { useUIStore } from '../../stores/uiStore'
+import type { SessionListItem } from '../../types/session'
+
+function makeSession(
+  id: string,
+  title: string,
+  projectRoot: string,
+  sourceSessionId?: string,
+  sourceMessageId?: string,
+): SessionListItem {
+  return {
+    id,
+    title,
+    createdAt: new Date().toISOString(),
+    modifiedAt: new Date().toISOString(),
+    messageCount: 1,
+    projectPath: projectRoot,
+    projectRoot,
+    workDir: projectRoot,
+    workDirExists: true,
+    sourceSessionId,
+    sourceMessageId,
+  }
+}
+
+describe('Sidebar forked session tree view', () => {
+  const connectToSession = vi.fn()
+  const disconnectSession = vi.fn()
+  const fetchSessions = vi.fn()
+  const createSession = vi.fn()
+  const deleteSession = vi.fn()
+  const deleteSessions = vi.fn()
+  const addToast = vi.fn()
+  const renameSession = vi.fn()
+
+  beforeEach(() => {
+    localStorageStore.clear()
+    connectToSession.mockReset()
+    disconnectSession.mockReset()
+    fetchSessions.mockReset()
+    addToast.mockReset()
+    desktopUiPreferencesApiMock.getPreferences.mockReset()
+    desktopUiPreferencesApiMock.updateSidebarPreferences.mockReset()
+    desktopUiPreferencesApiMock.getPreferences.mockRejectedValue(new Error('server unavailable'))
+    desktopUiPreferencesApiMock.updateSidebarPreferences.mockResolvedValue({
+      ok: true,
+      preferences: {
+        schemaVersion: 1,
+        sidebar: {
+          projectOrder: [],
+          pinnedProjects: [],
+          hiddenProjects: [],
+          projectOrganization: 'recentProject',
+          projectSortBy: 'updatedAt',
+        },
+      },
+    })
+
+    useTabStore.setState({ tabs: [], activeTabId: null })
+    useSessionStore.setState({
+      sessions: [],
+      activeSessionId: null,
+      isLoading: false,
+      error: null,
+      isBatchMode: false,
+      selectedSessionIds: new Set(),
+      fetchSessions,
+      createSession,
+      deleteSession,
+      deleteSessions,
+      renameSession,
+    })
+    useChatStore.setState({
+      connectToSession,
+      disconnectSession,
+    } as Partial<ReturnType<typeof useChatStore.getState>>)
+    useUIStore.setState({
+      sidebarOpen: true,
+      addToast,
+    } as Partial<ReturnType<typeof useUIStore.getState>>)
+  })
+
+  afterEach(() => {
+    cleanup()
+    useTabStore.setState({ tabs: [], activeTabId: null })
+    localStorageStore.clear()
+  })
+
+  it('renders a basic session in the sidebar', async () => {
+    useSessionStore.setState({
+      sessions: [
+        makeSession('basic-1', 'Basic Session', '/workspace/repo'),
+      ],
+    })
+
+    render(<Sidebar />)
+
+    await waitFor(() => {
+      expect(screen.getByText('Basic Session')).toBeInTheDocument()
+    })
+  })
+
+  it('applies left indent to forked child sessions', async () => {
+    const sessions = [
+      makeSession('root-1', 'Root Session', '/workspace/repo'),
+      makeSession('fork-1', 'Forked child', '/workspace/repo', 'root-1', 'msg-1'),
+      makeSession('fork-2', 'Another child', '/workspace/repo', 'root-1', 'msg-2'),
+    ]
+    useSessionStore.setState({ sessions })
+
+    render(<Sidebar />)
+
+    await waitFor(() => {
+      expect(screen.getByText('Root Session')).toBeInTheDocument()
+    })
+
+    // Verify forked sessions exist in the DOM
+    expect(screen.getByText('Forked child')).toBeInTheDocument()
+    expect(screen.getByText('Another child')).toBeInTheDocument()
+
+    // Fork icons (GitBranch SVGs) should appear in forked session buttons
+    const forkBtn = screen.getByText('Forked child').closest('button')!
+    const rootBtn = screen.getByText('Root Session').closest('button')!
+
+    const forkIcons = forkBtn.querySelectorAll('svg')
+    const rootIcons = rootBtn.querySelectorAll('svg')
+
+    // Forked session should have at least one more SVG (the GitBranch icon)
+    // than the root session (which only has the checkmark/batch icon)
+    expect(forkIcons.length).toBeGreaterThan(rootIcons.length)
+  })
+
+  it('shows fork icon for deep nesting (fork of a fork)', async () => {
+    const sessions = [
+      makeSession('root', 'Level 0', '/workspace/repo'),
+      makeSession('fork-l1', 'Level 1', '/workspace/repo', 'root', 'msg-a'),
+      makeSession('fork-l2', 'Level 2', '/workspace/repo', 'fork-l1', 'msg-b'),
+    ]
+    useSessionStore.setState({ sessions })
+
+    render(<Sidebar />)
+
+    await waitFor(() => {
+      expect(screen.getByText('Level 0')).toBeInTheDocument()
+    })
+
+    expect(screen.getByText('Level 1')).toBeInTheDocument()
+    expect(screen.getByText('Level 2')).toBeInTheDocument()
+
+    // All three sessions should be rendered
+    const l0Btn = screen.getByText('Level 0').closest('button')!
+    const l1Btn = screen.getByText('Level 1').closest('button')!
+    const l2Btn = screen.getByText('Level 2').closest('button')!
+
+    // Level 0 (root) should have no fork icon
+    // Level 1 and Level 2 should have fork icons (GitBranch SVGs)
+    const l0SvgCount = l0Btn.querySelectorAll('svg').length
+    const l1SvgCount = l1Btn.querySelectorAll('svg').length
+    const l2SvgCount = l2Btn.querySelectorAll('svg').length
+
+    expect(l1SvgCount).toBeGreaterThan(l0SvgCount)
+    expect(l2SvgCount).toBeGreaterThan(l0SvgCount)
+  })
+})

desktop/src/lib/sessionTree.test.ts

@@ -0,0 +1,120 @@
+import { describe, expect, it } from 'vitest'
+import { buildSessionTree } from './sessionTree'
+import type { SessionListItem } from '../types/session'
+
+function makeSession(
+  id: string,
+  title: string,
+  sourceSessionId?: string,
+  sourceMessageId?: string,
+): SessionListItem {
+  return {
+    id,
+    title,
+    createdAt: '2026-01-01T00:00:00.000Z',
+    modifiedAt: '2026-01-01T00:00:00.000Z',
+    messageCount: 1,
+    projectPath: '/workspace/repo',
+    projectRoot: '/workspace/repo',
+    workDir: '/workspace/repo',
+    workDirExists: true,
+    sourceSessionId,
+    sourceMessageId,
+  }
+}
+
+describe('buildSessionTree', () => {
+  it('returns flat list unchanged when no sessions have parents', () => {
+    const sessions = [
+      makeSession('s1', 'Session 1'),
+      makeSession('s2', 'Session 2'),
+      makeSession('s3', 'Session 3'),
+    ]
+    const tree = buildSessionTree(sessions)
+    expect(tree.map((t) => t.id)).toEqual(['s1', 's2', 's3'])
+    expect(tree.every((t) => t.depth === 0)).toBe(true)
+  })
+
+  it('nests forked sessions under their parent', () => {
+    const sessions = [
+      makeSession('parent', 'Parent Session'),
+      makeSession('child', 'Forked Child', 'parent', 'msg-1'),
+      makeSession('sibling', 'Sibling Session'),
+    ]
+    const tree = buildSessionTree(sessions)
+    expect(tree.map((t) => ({ id: t.id, depth: t.depth }))).toEqual([
+      { id: 'parent', depth: 0 },
+      { id: 'child', depth: 1 },
+      { id: 'sibling', depth: 0 },
+    ])
+  })
+
+  it('supports multi-level nesting (fork of a fork)', () => {
+    const sessions = [
+      makeSession('root', 'Root'),
+      makeSession('fork1', 'Fork Level 1', 'root', 'msg-a'),
+      makeSession('fork2', 'Fork Level 2', 'fork1', 'msg-b'),
+    ]
+    const tree = buildSessionTree(sessions)
+    expect(tree.map((t) => ({ id: t.id, depth: t.depth }))).toEqual([
+      { id: 'root', depth: 0 },
+      { id: 'fork1', depth: 1 },
+      { id: 'fork2', depth: 2 },
+    ])
+  })
+
+  it('groups multiple children of the same parent', () => {
+    const sessions = [
+      makeSession('root', 'Root'),
+      makeSession('child-a', 'Child A', 'root', 'msg-1'),
+      makeSession('child-b', 'Child B', 'root', 'msg-2'),
+    ]
+    const tree = buildSessionTree(sessions)
+    expect(tree.map((t) => ({ id: t.id, depth: t.depth }))).toEqual([
+      { id: 'root', depth: 0 },
+      { id: 'child-a', depth: 1 },
+      { id: 'child-b', depth: 1 },
+    ])
+  })
+
+  it('handles sessions whose parent is not in the list', () => {
+    // This can happen when a parent session is deleted
+    const sessions = [
+      makeSession('orphan', 'Orphan Fork', 'nonexistent-parent', 'msg-1'),
+    ]
+    const tree = buildSessionTree(sessions)
+    // Orphan sessions should be rendered at root level
+    expect(tree.map((t) => ({ id: t.id, depth: t.depth }))).toEqual([
+      { id: 'orphan', depth: 0 },
+    ])
+  })
+
+  it('marks the last child of a parent for visual connecting line', () => {
+    const sessions = [
+      makeSession('root', 'Root'),
+      makeSession('child-a', 'Child A', 'root', 'msg-1'),
+      makeSession('child-b', 'Child B', 'root', 'msg-2'),
+      makeSession('other', 'Other'),
+    ]
+    const tree = buildSessionTree(sessions)
+
+    const childA = tree.find((t) => t.id === 'child-a')!
+    const childB = tree.find((t) => t.id === 'child-b')!
+    const other = tree.find((t) => t.id === 'other')!
+
+    expect(childA.isLastChild).toBe(false)
+    expect(childB.isLastChild).toBe(true)
+    expect(other.isLastChild).toBe(false)
+  })
+
+  it('preserves original sort order within same depth level', () => {
+    const sessions = [
+      makeSession('a', 'A'),
+      makeSession('b-child', 'B Child', 'a', 'msg-1'),
+      makeSession('c', 'C'),
+      makeSession('d', 'D'),
+    ]
+    const tree = buildSessionTree(sessions)
+    expect(tree.map((t) => t.id)).toEqual(['a', 'b-child', 'c', 'd'])
+  })
+})

desktop/src/lib/sessionTree.ts

@@ -0,0 +1,70 @@
+import type { SessionListItem } from '../types/session'
+
+export type TreeSession = SessionListItem & {
+  depth: number
+  isLastChild: boolean
+}
+
+/**
+ * Build a tree-flattened list from sessions with parent-child relationships.
+ * Child sessions (those with sourceSessionId) are inserted after their parent
+ * with increased depth. The input order is preserved at each depth level.
+ */
+export function buildSessionTree(sessions: SessionListItem[]): TreeSession[] {
+  // Build a lookup of parent → children
+  const childrenMap = new Map<string, TreeSession[]>()
+  const sessionMap = new Map<string, TreeSession>()
+
+  for (const session of sessions) {
+    const treeSession: TreeSession = {
+      ...session,
+      depth: 0,
+      isLastChild: false,
+    }
+    sessionMap.set(session.id, treeSession)
+
+    if (session.sourceSessionId) {
+      const siblings = childrenMap.get(session.sourceSessionId) || []
+      siblings.push(treeSession)
+      childrenMap.set(session.sourceSessionId, siblings)
+    }
+  }
+
+  // Mark the last child in each sibling group
+  for (const children of childrenMap.values()) {
+    if (children.length > 0) {
+      children[children.length - 1]!.isLastChild = true
+    }
+  }
+
+  // Flatten in original order with nesting
+  const result: TreeSession[] = []
+
+  function insertWithChildren(item: TreeSession, depth: number) {
+    item.depth = depth
+    result.push(item)
+
+    const children = childrenMap.get(item.id)
+    if (children) {
+      for (const child of children) {
+        insertWithChildren(child, depth + 1)
+      }
+    }
+  }
+
+  // Collect top-level items in original order, skipping items that have a parent in the list.
+  // Only parents that are actually in the session list should prevent their children from being top-level.
+  const visibleParentIds = new Set(
+    [...childrenMap.keys()].filter((id) => sessionMap.has(id))
+  )
+  for (const session of sessions) {
+    const treeSession = sessionMap.get(session.id)!
+    // Only insert at top level if this session is not a child of another visible session
+    if (!session.sourceSessionId || !visibleParentIds.has(session.sourceSessionId)) {
+      // If the parent is not in this list (orphan), treat as root
+      insertWithChildren(treeSession, 0)
+    }
+  }
+
+  return result
+}

desktop/src/stores/sessionStore.test.ts

@@ -264,4 +264,42 @@ describe('sessionStore', () => {
       workDir: '/workspace/repo/branches/session-branch-existing',
     })
   })
+
+  it('preserves sourceSessionId and sourceMessageId in the optimistic branch session', async () => {
+    branchMock.mockResolvedValue({
+      sessionId: 'session-branch-2',
+      title: 'Fork exploration',
+      workDir: '/workspace/repo/branches/session-branch-2',
+      sourceSessionId: 'session-source-2',
+      targetMessageId: 'transcript-message-2',
+    })
+    listMock.mockImplementation(() => new Promise(() => {}))
+    useSessionStore.setState({
+      sessions: [{
+        id: 'session-source-2',
+        title: 'Source session',
+        createdAt: '2026-05-19T00:00:00.000Z',
+        modifiedAt: '2026-05-19T00:00:00.000Z',
+        messageCount: 6,
+        projectPath: '/workspace/repo',
+        projectRoot: '/workspace/repo',
+        workDir: '/workspace/repo',
+        workDirExists: true,
+      }],
+    })
+
+    const result = await useSessionStore.getState().branchSession(
+      'session-source-2',
+      'transcript-message-2',
+      { title: 'Fork exploration' },
+    )
+
+    expect(result.sessionId).toBe('session-branch-2')
+
+    const forkedSession = useSessionStore.getState().sessions.find((s) => s.id === 'session-branch-2')
+    expect(forkedSession).toBeDefined()
+    expect(forkedSession!.sourceSessionId).toBe('session-source-2')
+    expect(forkedSession!.sourceMessageId).toBe('transcript-message-2')
+    expect(forkedSession!.title).toBe('Fork exploration')
+  })
 })

desktop/src/stores/sessionStore.ts

@@ -125,6 +125,8 @@ export const useSessionStore = create<SessionStore>((set, get) => ({
       projectRoot: sourceSession?.projectRoot ?? sourceSession?.workDir ?? result.workDir ?? null,
       workDir: result.workDir ?? sourceSession?.workDir ?? null,
       workDirExists: true,
+      sourceSessionId: result.sourceSessionId ?? sourceSessionId,
+      sourceMessageId: result.targetMessageId ?? targetMessageId,
     }
 
     set((state) => ({

desktop/src/types/session.ts

@@ -10,6 +10,8 @@ export type SessionListItem = {
   projectRoot?: string | null
   workDir: string | null
   workDirExists: boolean
+  sourceSessionId?: string
+  sourceMessageId?: string
 }
 
 export type MessageEntry = {

src/server/__tests__/sessions.test.ts

@@ -581,6 +581,39 @@ describe('SessionService', () => {
     expect(resultA.sessions[0]!.id).toBe(id1)
   })
 
+  it('should expose sourceSessionId and sourceMessageId for forked sessions', async () => {
+    const sourceSessionId = 'parent-session-0001-0000-0000-000000000000'
+    const forkSessionId = 'forked-session-0001-0000-0000-000000000000'
+    const forkMessageUuid = crypto.randomUUID()
+
+    await writeSessionFile('-tmp-project', forkSessionId, [
+      makeSnapshotEntry(),
+      {
+        parentUuid: null,
+        isSidechain: false,
+        type: 'user',
+        message: { role: 'user', content: 'Forked from parent' },
+        uuid: crypto.randomUUID(),
+        sessionId: forkSessionId,
+        timestamp: new Date().toISOString(),
+        cwd: '/tmp/project',
+        forkedFrom: {
+          sessionId: sourceSessionId,
+          messageUuid: forkMessageUuid,
+        },
+      },
+      makeAssistantEntry('Response in fork'),
+    ])
+
+    const result = await service.listSessions()
+    expect(result.total).toBe(1)
+
+    const session = result.sessions[0]!
+    expect(session.id).toBe(forkSessionId)
+    expect(session.sourceSessionId).toBe(sourceSessionId)
+    expect(session.sourceMessageId).toBe(forkMessageUuid)
+  })
+
   // --------------------------------------------------------------------------
   // getSession
   // --------------------------------------------------------------------------

src/server/services/sessionService.ts

@@ -42,6 +42,8 @@ export type SessionListItem = {
   projectRoot: string | null
   workDir: string | null
   workDirExists: boolean
+  sourceSessionId?: string
+  sourceMessageId?: string
 }
 
 export type DeleteSessionFailure = {
@@ -1321,6 +1323,20 @@ export class SessionService {
           }
         }
 
+        // Extract fork parent relationship from transcript entries
+        let sourceSessionId: string | undefined
+        let sourceMessageId: string | undefined
+        for (const e of entries) {
+          const fork = (e as Record<string, unknown>).forkedFrom as
+            | { sessionId?: string; messageUuid?: string }
+            | undefined
+          if (fork?.sessionId) {
+            sourceSessionId = fork.sessionId
+            sourceMessageId = fork.messageUuid
+            break
+          }
+        }
+
         return {
           id: sessionId,
           title,
@@ -1331,6 +1347,8 @@ export class SessionService {
           projectRoot,
           workDir,
           workDirExists,
+          sourceSessionId,
+          sourceMessageId,
         }
       } catch {
         // Skip unreadable files