diff --git a/src/web-ui/src/flow_chat/services/FlowChatManager.test.ts b/src/web-ui/src/flow_chat/services/FlowChatManager.test.ts
index 554ee37c6d..2c49ae98d4 100644
--- a/src/web-ui/src/flow_chat/services/FlowChatManager.test.ts
+++ b/src/web-ui/src/flow_chat/services/FlowChatManager.test.ts
@@ -333,6 +333,10 @@ describe('FlowChatManager initialization', () => {
       projectWorkspaceId: workspaceFixture('/project', 'ssh-user@server', 'server').id,
       workspaceHostname: 'server',
     });
+    // Legacy record: it predates execution-workspace stamping, so it has no
+    // session workspace ID and is attributed through its project scope only.
+    session.workspaceId = undefined;
+    delete (session as { config?: Record<string, unknown> }).config.workspaceId;
     storeMocks.store = {
       registerPersistUnreadCompletionCallback: vi.fn(),
       getSurfaceGeneration: vi.fn(() => 0),
diff --git a/src/web-ui/src/flow_chat/utils/sessionOrdering.test.ts b/src/web-ui/src/flow_chat/utils/sessionOrdering.test.ts
index 9a2c2987b4..08935646ed 100644
--- a/src/web-ui/src/flow_chat/utils/sessionOrdering.test.ts
+++ b/src/web-ui/src/flow_chat/utils/sessionOrdering.test.ts
@@ -104,6 +104,22 @@ describe('sessionOrdering', () => {
     expect(sessionBelongsToWorkspaceNavRow({ workspaceId: 'host-b-project' }, 'host-a-project')).toBe(false);
   });
 
+  it('does not show a worktree session under its main project group', () => {
+    const worktreeSession = {
+      workspaceId: 'worktree-cli',
+      projectWorkspaceId: 'main-project',
+    };
+    expect(sessionBelongsToWorkspaceNavRow(worktreeSession, 'worktree-cli')).toBe(true);
+    expect(sessionBelongsToWorkspaceNavRow(worktreeSession, 'main-project')).toBe(false);
+    expect(sessionBelongsToWorkspaceNavRow(worktreeSession, 'sibling-worktree')).toBe(false);
+  });
+
+  it('still attributes a legacy record that only carries the project ID', () => {
+    const legacySession = { projectWorkspaceId: 'main-project' };
+    expect(sessionBelongsToWorkspaceNavRow(legacySession, 'main-project')).toBe(true);
+    expect(sessionBelongsToWorkspaceNavRow(legacySession, 'other-project')).toBe(false);
+  });
+
   it('does not guess membership from a missing or stale ID', () => {
     expect(sessionBelongsToWorkspaceNavRow({}, 'known')).toBe(false);
     expect(sessionBelongsToWorkspaceNavRow({ workspaceId: 'stale' }, 'known')).toBe(false);
diff --git a/src/web-ui/src/flow_chat/utils/sessionOrdering.ts b/src/web-ui/src/flow_chat/utils/sessionOrdering.ts
index 1451ced407..be667adc2f 100644
--- a/src/web-ui/src/flow_chat/utils/sessionOrdering.ts
+++ b/src/web-ui/src/flow_chat/utils/sessionOrdering.ts
@@ -1,11 +1,18 @@
 import type { Session } from '../types/flow-chat';
 import type { SessionMetadata } from '@/shared/types/session-history';
-/** Session list membership is the owning host's workspace ID, never a path. */
+/**
+ * Session list membership is the owning execution workspace ID (`workspaceId`),
+ * never a path. `projectWorkspaceId` is only the persistence owner (the main
+ * project for linked worktrees) and a legacy fallback for records created
+ * before execution-workspace stamping; it must never widen membership to
+ * sibling worktrees of the same project.
+ */
 export function sessionBelongsToWorkspaceNavRow(
   session: Pick<Session, 'workspaceId' | 'projectWorkspaceId'>,
   workspaceId?: string,
 ): boolean {
-  return Boolean(workspaceId && (session.workspaceId === workspaceId || session.projectWorkspaceId === workspaceId));
+  if (!workspaceId) return false;
+  return (session.workspaceId ?? session.projectWorkspaceId) === workspaceId;
 }
 
 export function getSessionSortTimestamp(session: Pick<Session, 'createdAt' | 'lastFinishedAt'>): number {