diff --git a/apps/app/src/components/dialogs/ThreadArchiveDialog.tsx b/apps/app/src/components/dialogs/ThreadArchiveDialog.tsx
new file mode 100644
index 0000000000..d916da6c26
--- /dev/null
+++ b/apps/app/src/components/dialogs/ThreadArchiveDialog.tsx
@@ -0,0 +1,39 @@
+import type { Thread } from "@bb/domain";
+import {
+  ConfirmDeleteDialog,
+  ConfirmDeleteDialogContent,
+} from "./ConfirmDeleteDialog";
+
+export interface ThreadArchiveDialogTarget {
+  thread: Thread;
+  workingThreadCount: number;
+}
+
+interface ThreadArchiveDialogProps {
+  target: ThreadArchiveDialogTarget | null;
+  pending: boolean;
+  onOpenChange: (open: boolean) => void;
+  onArchive: (target: ThreadArchiveDialogTarget) => void;
+}
+
+export function ThreadArchiveDialog({
+  target,
+  pending,
+  onOpenChange,
+  onArchive,
+}: ThreadArchiveDialogProps) {
+  return (
+    <ConfirmDeleteDialog open={target !== null} onOpenChange={onOpenChange}>
+      {target ? (
+        <ConfirmDeleteDialogContent
+          title="Archive active work?"
+          description={`${target.workingThreadCount === 1 ? "A thread is" : `${target.workingThreadCount} threads are`} currently working in this thread tree. Archiving may interrupt the active work.`}
+          confirmLabel="Archive anyway"
+          pending={pending}
+          onConfirm={() => onArchive(target)}
+          onCancel={() => onOpenChange(false)}
+        />
+      ) : null}
+    </ConfirmDeleteDialog>
+  );
+}
diff --git a/apps/app/src/components/thread/ThreadActionsProvider.test.tsx b/apps/app/src/components/thread/ThreadActionsProvider.test.tsx
index aa821a5a36..1a124e37d9 100644
--- a/apps/app/src/components/thread/ThreadActionsProvider.test.tsx
+++ b/apps/app/src/components/thread/ThreadActionsProvider.test.tsx
@@ -13,12 +13,14 @@ import {
 } from "./ThreadActionsProvider";
 
 const mocks = vi.hoisted(() => ({
+  archiveConfirm: vi.fn(),
   closePanesForThreads: vi.fn(),
   dialogOnClose: vi.fn(),
   dialogOnOpen: vi.fn(),
   dialogOnOpenChange: vi.fn(),
   mutation: vi.fn(),
   navigate: vi.fn(),
+  workingArchiveThreadCount: vi.fn(() => 0),
 }));
 
 vi.mock("react-router-dom", async (importOriginal) => {
@@ -42,6 +44,27 @@ vi.mock("@/components/dialogs/ThreadRenameDialog", () => ({
   ThreadRenameDialog: () => null,
 }));
 
+vi.mock("@/components/dialogs/ThreadArchiveDialog", () => ({
+  ThreadArchiveDialog: ({
+    onArchive,
+  }: {
+    onArchive: (target: unknown) => void;
+  }) => {
+    mocks.archiveConfirm.mockImplementation(onArchive);
+    return null;
+  },
+}));
+
+vi.mock(
+  "@/hooks/cache-owners/thread-archive-cache",
+  async (importOriginal) => ({
+    ...(await importOriginal<
+      typeof import("@/hooks/cache-owners/thread-archive-cache")
+    >()),
+    getCachedWorkingArchiveThreadCount: mocks.workingArchiveThreadCount,
+  }),
+);
+
 vi.mock("@/components/ui/app-toast", () => ({
   appToast: {
     dismiss: vi.fn(),
@@ -153,6 +176,7 @@ beforeEach(() => {
     focusedRoute: null,
     removedAny: false,
   });
+  mocks.workingArchiveThreadCount.mockReturnValue(0);
 });
 
 afterEach(() => {
@@ -161,6 +185,38 @@ afterEach(() => {
 });
 
 describe("ThreadActionsProvider archive feedback", () => {
+  it("asks for confirmation instead of archiving when cached work is active", () => {
+    const thread = makeThread();
+    mocks.workingArchiveThreadCount.mockReturnValue(1);
+    renderProvider(<ArchiveButton thread={thread} />);
+
+    fireEvent.click(screen.getByRole("button", { name: "Archive" }));
+
+    expect(mocks.dialogOnOpen).toHaveBeenCalledWith({
+      thread,
+      workingThreadCount: 1,
+    });
+    expect(sdk.threads.archiveAll).not.toHaveBeenCalled();
+  });
+
+  it("archives exactly once after active-work confirmation", async () => {
+    const thread = makeThread();
+    const target = { thread, workingThreadCount: 2 };
+    mocks.workingArchiveThreadCount.mockReturnValue(2);
+    renderProvider(<ArchiveButton thread={thread} />);
+
+    fireEvent.click(screen.getByRole("button", { name: "Archive" }));
+    mocks.archiveConfirm(target);
+
+    await vi.waitFor(() => {
+      expect(sdk.threads.archiveAll).toHaveBeenCalledTimes(1);
+    });
+    expect(sdk.threads.archiveAll).toHaveBeenCalledWith({
+      threadId: thread.id,
+    });
+    expect(mocks.dialogOnClose).toHaveBeenCalledTimes(1);
+  });
+
   it("shows one archive toast whose Undo restores the parent and children", async () => {
     renderProvider(<ArchiveButton thread={makeThread()} />);
 
diff --git a/apps/app/src/components/thread/ThreadActionsProvider.tsx b/apps/app/src/components/thread/ThreadActionsProvider.tsx
index b5dfdf289d..8737d1619e 100644
--- a/apps/app/src/components/thread/ThreadActionsProvider.tsx
+++ b/apps/app/src/components/thread/ThreadActionsProvider.tsx
@@ -8,6 +8,7 @@ import {
   type ReactNode,
 } from "react";
 import { useSetAtom } from "jotai";
+import { useQueryClient } from "@tanstack/react-query";
 import { appToast } from "@/components/ui/app-toast";
 import {
   closePanesForThreadsAtom,
@@ -41,12 +42,17 @@ import {
   ThreadDeleteDialog,
   type ThreadDeleteDialogTarget,
 } from "@/components/dialogs/ThreadDeleteDialog";
+import {
+  ThreadArchiveDialog,
+  type ThreadArchiveDialogTarget,
+} from "@/components/dialogs/ThreadArchiveDialog";
 import { ArchivedThreadToastTitle } from "@/components/thread/ArchivedThreadToastTitle";
 import { destroyPersistedBrowserViewsForThread } from "@/components/secondary-panel/browserViewVisibilityCoordinator";
 import { getThreadReadToggleAction } from "@bb/client-core";
 import { getRootComposeRoutePath, getThreadRoutePath } from "@/lib/route-paths";
 import { getDesktopBrowserApi } from "@/lib/bb-desktop";
 import { useRouteNavigate } from "@/components/ui/app-route-anchor";
+import { getCachedWorkingArchiveThreadCount } from "@/hooks/cache-owners/thread-archive-cache";
 
 export interface ThreadActionsContextValue {
   archiveThreadAndChildren: (thread: Thread) => void;
@@ -99,6 +105,7 @@ export function ThreadActionsProvider({
   // Stable across navigations: the context value below must not change per
   // pathname, or every mounted sidebar ThreadRow re-renders on each route change.
   const navigate = useRouteNavigate();
+  const queryClient = useQueryClient();
   const { threadId: viewedThreadId } = useRouteState();
   // Read the currently-viewed thread live inside async mutation callbacks: a
   // pane's stale-prune (deleted/archived thread) can move the URL between a
@@ -138,9 +145,12 @@ export function ThreadActionsProvider({
 
   const renameDialog = useDialogState<ThreadRenameDialogTarget>();
   const deleteDialog = useDialogState<ThreadDeleteDialogTarget>();
+  const archiveDialog = useDialogState<ThreadArchiveDialogTarget>();
 
   const { onClose: closeRenameDialog, onOpen: openRenameDialog } = renameDialog;
   const { onClose: closeDeleteDialog, onOpen: openDeleteDialog } = deleteDialog;
+  const { onClose: closeArchiveDialog, onOpen: openArchiveDialog } =
+    archiveDialog;
 
   useEffect(() => {
     return () => {
@@ -330,7 +340,7 @@ export function ThreadActionsProvider({
     [unarchiveMutate],
   );
 
-  const archiveThreadAndChildrenAction = useCallback(
+  const performArchiveThreadAndChildren = useCallback(
     (thread: Thread) => {
       archiveThreadAndChildrenMutateAsync({ id: thread.id }).then(
         (response) => {
@@ -396,6 +406,29 @@ export function ThreadActionsProvider({
     ],
   );
 
+  const archiveThreadAndChildrenAction = useCallback(
+    (thread: Thread) => {
+      const workingThreadCount = getCachedWorkingArchiveThreadCount({
+        queryClient,
+        threadId: thread.id,
+      });
+      if (workingThreadCount > 0) {
+        openArchiveDialog({ thread, workingThreadCount });
+        return;
+      }
+      performArchiveThreadAndChildren(thread);
+    },
+    [openArchiveDialog, performArchiveThreadAndChildren, queryClient],
+  );
+
+  const confirmArchive = useCallback(
+    (target: ThreadArchiveDialogTarget) => {
+      closeArchiveDialog();
+      performArchiveThreadAndChildren(target.thread);
+    },
+    [closeArchiveDialog, performArchiveThreadAndChildren],
+  );
+
   const toggleRead = useCallback(
     (thread: Thread) => {
       if (getThreadReadToggleAction(thread) === "mark_unread") {
@@ -466,6 +499,12 @@ export function ThreadActionsProvider({
         onOpenChange={renameDialog.onOpenChange}
         onRename={submitRename}
       />
+      <ThreadArchiveDialog
+        target={archiveDialog.target}
+        pending={archiveThreadAndChildrenMutation.isPending}
+        onOpenChange={archiveDialog.onOpenChange}
+        onArchive={confirmArchive}
+      />
       <ThreadDeleteDialog
         target={deleteDialog.target}
         pending={deleteThread.isPending}
diff --git a/apps/app/src/hooks/cache-owners/thread-archive-cache.test.ts b/apps/app/src/hooks/cache-owners/thread-archive-cache.test.ts
new file mode 100644
index 0000000000..1bb158762d
--- /dev/null
+++ b/apps/app/src/hooks/cache-owners/thread-archive-cache.test.ts
@@ -0,0 +1,138 @@
+import type { ThreadListEntry } from "@bb/domain";
+import type { SidebarBootstrapResponse } from "@bb/server-contract";
+import { describe, expect, it } from "vitest";
+import { createQueryClientTestHarness } from "@/test/queryClientTestHarness";
+import { sidebarNavigationQueryKey } from "../queries/query-keys";
+import { getCachedWorkingArchiveThreadCount } from "./thread-archive-cache";
+
+function makeThread(thread: Partial<ThreadListEntry> = {}): ThreadListEntry {
+  return {
+    id: "thread-parent",
+    projectId: "project-1",
+    environmentId: "env-1",
+    providerId: "codex",
+    title: null,
+    titleFallback: null,
+    sectionId: null,
+    status: "idle",
+    parentThreadId: null,
+    sourceThreadId: null,
+    originKind: null,
+    originPluginId: null,
+    visibility: "visible",
+    archivedAt: null,
+    pinnedAt: null,
+    deletedAt: null,
+    lastReadAt: null,
+    latestAttentionAt: 1,
+    createdAt: 1,
+    updatedAt: 1,
+    runtime: {
+      displayStatus: "idle",
+      hostReconnectGraceExpiresAt: null,
+    },
+    activity: {
+      activeWorkflowCount: 0,
+      activeBackgroundAgentCount: 0,
+      activeBackgroundCommandCount: 0,
+      activePlanModeCount: 0,
+      activeGoalCount: 0,
+    },
+    pinSortKey: null,
+    hasPendingInteraction: false,
+    environmentHostId: "host-1",
+    environmentName: "Environment",
+    environmentBranchName: "main",
+    environmentWorkspaceDisplayKind: "managed-worktree",
+    ...thread,
+  };
+}
+
+function makeSidebarNavigation(
+  threads: ThreadListEntry[],
+): SidebarBootstrapResponse {
+  const project = {
+    id: "project-1",
+    kind: "standard" as const,
+    name: "Project",
+    gitRemoteUrl: null,
+    createdAt: 1,
+    updatedAt: 1,
+    sources: [],
+    threads,
+    defaultExecutionOptions: null,
+  };
+  return {
+    sections: [],
+    projects: [project],
+    personalProject: {
+      ...project,
+      id: "proj_personal",
+      kind: "personal",
+      threads: [],
+    },
+  };
+}
+
+describe("getCachedWorkingArchiveThreadCount", () => {
+  it("does not guard an idle thread tree", () => {
+    const { queryClient } = createQueryClientTestHarness();
+    queryClient.setQueryData(
+      sidebarNavigationQueryKey(),
+      makeSidebarNavigation([
+        makeThread(),
+        makeThread({ id: "thread-child", parentThreadId: "thread-parent" }),
+      ]),
+    );
+
+    expect(
+      getCachedWorkingArchiveThreadCount({
+        queryClient,
+        threadId: "thread-parent",
+      }),
+    ).toBe(0);
+  });
+
+  it("counts working activity on the thread and its children", () => {
+    const { queryClient } = createQueryClientTestHarness();
+    queryClient.setQueryData(
+      sidebarNavigationQueryKey(),
+      makeSidebarNavigation([
+        makeThread({
+          runtime: {
+            displayStatus: "active",
+            hostReconnectGraceExpiresAt: null,
+          },
+        }),
+        makeThread({
+          id: "thread-child",
+          parentThreadId: "thread-parent",
+          activity: {
+            activeWorkflowCount: 0,
+            activeBackgroundAgentCount: 1,
+            activeBackgroundCommandCount: 0,
+            activePlanModeCount: 0,
+            activeGoalCount: 0,
+          },
+        }),
+        makeThread({
+          id: "unrelated-thread",
+          activity: {
+            activeWorkflowCount: 1,
+            activeBackgroundAgentCount: 0,
+            activeBackgroundCommandCount: 0,
+            activePlanModeCount: 0,
+            activeGoalCount: 0,
+          },
+        }),
+      ]),
+    );
+
+    expect(
+      getCachedWorkingArchiveThreadCount({
+        queryClient,
+        threadId: "thread-parent",
+      }),
+    ).toBe(2);
+  });
+});
diff --git a/apps/app/src/hooks/cache-owners/thread-archive-cache.ts b/apps/app/src/hooks/cache-owners/thread-archive-cache.ts
index 85855cfbd1..af9419335f 100644
--- a/apps/app/src/hooks/cache-owners/thread-archive-cache.ts
+++ b/apps/app/src/hooks/cache-owners/thread-archive-cache.ts
@@ -1,5 +1,13 @@
 import type { QueryClient } from "@tanstack/react-query";
 import type { ThreadListEntry, ThreadWithRuntime } from "@bb/domain";
+import {
+  hasActiveBackgroundAgentActivity,
+  hasActiveBackgroundCommandActivity,
+  hasActiveGoalActivity,
+  hasActivePlanModeActivity,
+  hasActiveWorkflowActivity,
+  isRuntimeBusyThread,
+} from "@bb/client-core";
 import { threadQueryKey, threadsQueryKey } from "../queries/query-keys";
 import {
   applyToCachedSidebarNavigationThreads,
@@ -60,6 +68,51 @@ export function getCachedLiveThreadIdsMatching({
   return Array.from(threadIds);
 }
 
+/**
+ * Counts cached live threads in an archive cascade that currently have working
+ * activity. This stays synchronous so idle archives never wait on preflight.
+ */
+export function getCachedWorkingArchiveThreadCount({
+  queryClient,
+  threadId,
+}: {
+  queryClient: QueryClient;
+  threadId: string;
+}): number {
+  const workingThreadIds = new Set<string>();
+  const collectMatchingThread = (thread: ThreadListEntry) => {
+    if (
+      thread.archivedAt !== null ||
+      (thread.id !== threadId && thread.parentThreadId !== threadId)
+    ) {
+      return;
+    }
+    if (
+      isRuntimeBusyThread(thread) ||
+      hasActiveWorkflowActivity(thread) ||
+      hasActiveBackgroundAgentActivity(thread) ||
+      hasActiveBackgroundCommandActivity(thread) ||
+      hasActivePlanModeActivity(thread) ||
+      hasActiveGoalActivity(thread)
+    ) {
+      workingThreadIds.add(thread.id);
+    }
+  };
+
+  for (const { data } of getCachedThreadLists(queryClient, {
+    queryKey: threadsQueryKey(),
+  })) {
+    for (const thread of iterateThreadListCacheEntries(data)) {
+      collectMatchingThread(thread);
+    }
+  }
+  for (const thread of getCachedSidebarNavigationThreads(queryClient)) {
+    collectMatchingThread(thread);
+  }
+
+  return workingThreadIds.size;
+}
+
 export function getCachedThreadSnapshots({
   queryClient,
   threadIds,