diff --git a/apps/backend/internal/office/dashboard/service_tasks.go b/apps/backend/internal/office/dashboard/service_tasks.go
index 6073951839..6dc275092d 100644
--- a/apps/backend/internal/office/dashboard/service_tasks.go
+++ b/apps/backend/internal/office/dashboard/service_tasks.go
@@ -111,6 +111,7 @@ func (s *DashboardService) UpdateTaskProjectID(ctx context.Context, taskID, proj
 		}
 	}
 	s.publishTaskUpdated(ctx, taskID, []string{"project_id"})
+	s.publishCanonicalTaskUpdated(ctx, taskID)
 	return nil
 }
 
diff --git a/apps/backend/internal/office/dashboard/task_updated_events_test.go b/apps/backend/internal/office/dashboard/task_updated_events_test.go
index dff1605a73..0292887759 100644
--- a/apps/backend/internal/office/dashboard/task_updated_events_test.go
+++ b/apps/backend/internal/office/dashboard/task_updated_events_test.go
@@ -93,3 +93,23 @@ func TestUpdateTaskStatus_NoPublisherWiredIsSafe(t *testing.T) {
 		t.Fatalf("update task status: %v", err)
 	}
 }
+
+func TestUpdateTaskProjectIDPublishesCanonicalTaskUpdatedForAssignAndClear(t *testing.T) {
+	deps := newTestDeps(t)
+	insertTestTask(t, deps.db, "project-transition", "ws-project", "Project transition", "todo", 0)
+	insertTestProject(t, deps, "project-1", "ws-project")
+	pub := &recordingTaskLifecyclePublisher{}
+	deps.svc.SetTaskLifecyclePublisher(pub)
+
+	if err := deps.svc.UpdateTaskProjectID(context.Background(), "project-transition", "project-1"); err != nil {
+		t.Fatalf("assign project: %v", err)
+	}
+	if err := deps.svc.UpdateTaskProjectID(context.Background(), "project-transition", ""); err != nil {
+		t.Fatalf("clear project: %v", err)
+	}
+
+	want := []string{"project-transition", "project-transition"}
+	if len(pub.published) != len(want) || pub.published[0] != want[0] || pub.published[1] != want[1] {
+		t.Fatalf("published = %v, want %v", pub.published, want)
+	}
+}
diff --git a/apps/backend/internal/task/service/service_events.go b/apps/backend/internal/task/service/service_events.go
index 6d3dff3804..7631ed3dd0 100644
--- a/apps/backend/internal/task/service/service_events.go
+++ b/apps/backend/internal/task/service/service_events.go
@@ -426,6 +426,9 @@ func (s *Service) publishTaskEventNow(ctx context.Context, eventType string, tas
 		"updated_at":         task.UpdatedAt.Format(time.RFC3339Nano),
 		"is_ephemeral":       task.IsEphemeral,
 		"autopilot":          task.Autopilot,
+		// Always explicit because a project reassignment can change Office
+		// ownership in either direction while a client has the task cached.
+		"is_from_office": task.IsFromOffice,
 		// Consumers that restore quick-chat tabs filter on origin, so it has to
 		// travel with the event and not just the HTTP DTO.
 		"origin": task.Origin,
diff --git a/apps/backend/internal/task/service/service_events_test.go b/apps/backend/internal/task/service/service_events_test.go
index 2ffaf04615..61ff5c5860 100644
--- a/apps/backend/internal/task/service/service_events_test.go
+++ b/apps/backend/internal/task/service/service_events_test.go
@@ -717,6 +717,30 @@ func TestPublishTaskUpdated_EmitsAutopilot(t *testing.T) {
 	}
 }
 
+func TestPublishTaskUpdatedEmitsOfficeIdentityExplicitly(t *testing.T) {
+	for _, tc := range []struct {
+		name         string
+		isFromOffice bool
+	}{
+		{name: "office", isFromOffice: true},
+		{name: "kanban", isFromOffice: false},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			svc, eventBus, _ := createTestService(t)
+			svc.PublishTaskUpdated(context.Background(), &models.Task{
+				ID: "task-office-identity", WorkspaceID: "ws-1", WorkflowID: "wf-1", WorkflowStepID: "step-1",
+				IsFromOffice: tc.isFromOffice,
+			})
+
+			data := singlePublishedEventData(t, eventBus)
+			got, ok := data["is_from_office"].(bool)
+			if !ok || got != tc.isFromOffice {
+				t.Fatalf("is_from_office payload = %#v, want %t", data["is_from_office"], tc.isFromOffice)
+			}
+		})
+	}
+}
+
 func TestPublishTaskUpdatedRedactsDeferredLaunchAttribution(t *testing.T) {
 	svc, eventBus, _ := createTestService(t)
 	task := &models.Task{
diff --git a/apps/web/components/task/mobile/session-task-switcher-sheet-item.test.ts b/apps/web/components/task/mobile/session-task-switcher-sheet-item.test.ts
index 8398285455..5f99aa594d 100644
--- a/apps/web/components/task/mobile/session-task-switcher-sheet-item.test.ts
+++ b/apps/web/components/task/mobile/session-task-switcher-sheet-item.test.ts
@@ -75,3 +75,12 @@ describe("toSheetItem remote executor projection", () => {
     expect(item.primarySessionId).toBe("session-1");
   });
 });
+
+describe("toSheetItem Office identity projection", () => {
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.4
+  it("carries Office identity into the phone task drawer row", () => {
+    const item = toSheetItem(task({ isFromOffice: true }), emptyCtx());
+
+    expect(item.isFromOffice).toBe(true);
+  });
+});
diff --git a/apps/web/components/task/mobile/session-task-switcher-sheet-item.ts b/apps/web/components/task/mobile/session-task-switcher-sheet-item.ts
index 01c85260af..936aa8a642 100644
--- a/apps/web/components/task/mobile/session-task-switcher-sheet-item.ts
+++ b/apps/web/components/task/mobile/session-task-switcher-sheet-item.ts
@@ -114,6 +114,7 @@ export function toSheetItem(
     primaryExecutorProfileId: task.primaryExecutorProfileId ?? undefined,
     workflowStepColor: facts.workflowStepColor,
     isArchived: task.isArchived === true,
+    isFromOffice: task.isFromOffice,
     isRemoteExecutor: task.isRemoteExecutor,
     remoteExecutorId: task.primaryExecutorId ?? undefined,
     remoteExecutorType: task.primaryExecutorType ?? undefined,
diff --git a/apps/web/components/task/mobile/session-task-switcher-sheet-props.ts b/apps/web/components/task/mobile/session-task-switcher-sheet-props.ts
index 961679b36a..589e122f40 100644
--- a/apps/web/components/task/mobile/session-task-switcher-sheet-props.ts
+++ b/apps/web/components/task/mobile/session-task-switcher-sheet-props.ts
@@ -20,6 +20,7 @@ export function buildMobileTaskSwitcherProps(
 ): TaskSwitcherProps {
   return {
     grouped: helpers.grouped,
+    nestHierarchyTasks: props.tasks,
     workflows: props.workflows,
     stepsByWorkflowId: props.stepsByWorkflowId,
     activeTaskId: props.activeTaskId,
diff --git a/apps/web/components/task/task-nest-context-menu.tsx b/apps/web/components/task/task-nest-context-menu.tsx
index cfdfd0dd52..14c8e66ddd 100644
--- a/apps/web/components/task/task-nest-context-menu.tsx
+++ b/apps/web/components/task/task-nest-context-menu.tsx
@@ -8,7 +8,6 @@ import {
   ContextMenuSubContent,
   ContextMenuSubTrigger,
 } from "@kandev/ui/context-menu";
-import { useAppStore } from "@/components/state-provider";
 import { useNestTask } from "@/hooks/use-nest-task";
 import { computeNestCandidates } from "@/lib/sidebar/nest-candidates";
 import type { TaskSwitcherItem } from "./task-switcher";
@@ -16,6 +15,8 @@ import { useTranslation } from "react-i18next";
 
 type TaskNestContextMenuItemsProps = {
   task: TaskSwitcherItem;
+  nestCandidateTasks?: TaskSwitcherItem[];
+  nestHierarchyTasks?: TaskSwitcherItem[];
   disabled?: boolean;
 };
 
@@ -25,25 +26,23 @@ type TaskNestContextMenuItemsProps = {
  * other tasks in the same workflow, excluding the task's own descendants
  * (which would create a cycle) and its current parent.
  */
-export function TaskNestContextMenuItems({ task, disabled }: TaskNestContextMenuItemsProps) {
+export function TaskNestContextMenuItems({
+  task,
+  nestCandidateTasks = [],
+  nestHierarchyTasks,
+  disabled,
+}: TaskNestContextMenuItemsProps) {
   const { t } = useTranslation();
   const workflowId = task.workflowId;
-  // Prefer the all-workflows snapshot; fall back to the active kanban tasks,
-  // which the sidebar also renders from before the multi-snapshot fetch
-  // resolves (e.g. initial /t/:id load). Without the fallback the menu would
-  // show "No other tasks" even though rows are visible.
-  const tasks = useAppStore((s) => {
-    if (!workflowId) return undefined;
-    return (
-      s.kanbanMulti?.snapshots?.[workflowId]?.tasks ??
-      (s.kanban?.workflowId === workflowId ? s.kanban?.tasks : undefined)
-    );
-  });
   const nestTask = useNestTask();
 
   if (!workflowId || task.isArchived) return null;
 
-  const candidates = computeNestCandidates(tasks ?? [], task.id);
+  const candidates = computeNestCandidates(
+    nestCandidateTasks.filter((candidate) => candidate.workflowId === workflowId),
+    task.id,
+    nestHierarchyTasks ?? nestCandidateTasks,
+  );
   const hasParent = Boolean(task.parentTaskId);
 
   return (
diff --git a/apps/web/components/task/task-session-sidebar-aggregate.test.ts b/apps/web/components/task/task-session-sidebar-aggregate.test.ts
index 86b003c3d6..526e42571e 100644
--- a/apps/web/components/task/task-session-sidebar-aggregate.test.ts
+++ b/apps/web/components/task/task-session-sidebar-aggregate.test.ts
@@ -233,6 +233,26 @@ describe("aggregateSidebarTasks active precedence", () => {
 
     expect(result.allTasks[0].autopilot).toBe(true);
   });
+
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.4
+  it("preserves projected Office identity through a newer partial active task", () => {
+    const projected = makeTask("t1", "s1", {
+      isFromOffice: true,
+      updatedAt: "2026-08-03T00:00:00Z",
+    });
+    const active = makeTask("t1", "s1", {
+      updatedAt: "2026-08-03T00:00:01Z",
+    });
+
+    const result = aggregateSidebarTasks(
+      { "wf-1": makeSnapshot([makeStep("s1", 0)], [projected]) },
+      "wf-1",
+      [active],
+      [makeStep("s1", 0)],
+    );
+
+    expect(result.allTasks[0].isFromOffice).toBe(true);
+  });
 });
 
 describe("aggregateSidebarTasks task lifecycle freshness", () => {
diff --git a/apps/web/components/task/task-session-sidebar-aggregate.ts b/apps/web/components/task/task-session-sidebar-aggregate.ts
index 6c7790d0bc..5bb097a272 100644
--- a/apps/web/components/task/task-session-sidebar-aggregate.ts
+++ b/apps/web/components/task/task-session-sidebar-aggregate.ts
@@ -114,6 +114,7 @@ function reconcileTaskProjection(
     // a partial WS event, so do not let it erase the value from the full
     // workflow projection.
     autopilot: active.autopilot ?? projected.autopilot,
+    isFromOffice: active.isFromOffice ?? projected.isFromOffice,
     _workflowId: activeWorkflowId,
   };
   const task = activeTaskIsNewer(active, projected) ? activeTask : projected;
diff --git a/apps/web/components/task/task-session-sidebar-item.test.ts b/apps/web/components/task/task-session-sidebar-item.test.ts
index 564915a8ad..eb5090da3a 100644
--- a/apps/web/components/task/task-session-sidebar-item.test.ts
+++ b/apps/web/components/task/task-session-sidebar-item.test.ts
@@ -168,6 +168,15 @@ describe("buildSidebarItem", () => {
   });
 });
 
+describe("buildSidebarItem Office identity", () => {
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.4
+  it("preserves isFromOffice for candidate filtering", () => {
+    const item = buildSidebarItem(task({ isFromOffice: true }), emptyContext());
+
+    expect(item.isFromOffice).toBe(true);
+  });
+});
+
 describe("buildSidebarItem — parked on background work", () => {
   it("carries the parked-on-background-work projection through to the sidebar row", () => {
     const item = buildSidebarItem(task({ parkedOnBackgroundWork: true }), emptyContext());
diff --git a/apps/web/components/task/task-session-sidebar-item.ts b/apps/web/components/task/task-session-sidebar-item.ts
index 3ece9e0d3f..52dd0ea00c 100644
--- a/apps/web/components/task/task-session-sidebar-item.ts
+++ b/apps/web/components/task/task-session-sidebar-item.ts
@@ -159,6 +159,7 @@ export function buildSidebarItem(
     remoteExecutorName: task.primaryExecutorName ?? undefined,
     createdAt: task.createdAt,
     isArchived: task.isArchived === true,
+    isFromOffice: task.isFromOffice,
     parentTaskTitle: task.parentTaskId ? context.titleById.get(task.parentTaskId) : undefined,
     parentTaskId: task.parentTaskId ?? undefined,
     workspaceMode: task.workspaceMode,
diff --git a/apps/web/components/task/task-session-sidebar-switcher-props.ts b/apps/web/components/task/task-session-sidebar-switcher-props.ts
index e3afc193ff..00aabac358 100644
--- a/apps/web/components/task/task-session-sidebar-switcher-props.ts
+++ b/apps/web/components/task/task-session-sidebar-switcher-props.ts
@@ -14,6 +14,7 @@ type TaskSwitcherComponentProps = ComponentProps<typeof TaskSwitcher>;
  */
 export function buildTaskSwitcherProps(args: {
   grouped: TaskSwitcherComponentProps["grouped"];
+  nestHierarchyTasks: TaskSwitcherComponentProps["nestHierarchyTasks"];
   workflows: TaskSwitcherComponentProps["workflows"];
   stepsByWorkflowId: TaskSwitcherComponentProps["stepsByWorkflowId"];
   highlightedTaskId: string | null;
@@ -49,6 +50,7 @@ export function buildTaskSwitcherProps(args: {
 }): TaskSwitcherComponentProps {
   return {
     grouped: args.grouped,
+    nestHierarchyTasks: args.nestHierarchyTasks,
     workflows: args.workflows,
     stepsByWorkflowId: args.stepsByWorkflowId,
     activeTaskId: args.highlightedTaskId,
diff --git a/apps/web/components/task/task-session-sidebar.tsx b/apps/web/components/task/task-session-sidebar.tsx
index 79e9c2c64f..1fa8a27926 100644
--- a/apps/web/components/task/task-session-sidebar.tsx
+++ b/apps/web/components/task/task-session-sidebar.tsx
@@ -501,6 +501,7 @@ export const TaskSessionSidebar = memo(function TaskSessionSidebar({
   );
   const switcherProps = buildTaskSwitcherProps({
     grouped,
+    nestHierarchyTasks: displayTasks,
     workflows,
     stepsByWorkflowId,
     highlightedTaskId,
diff --git a/apps/web/components/task/task-switcher-context-menu-items.tsx b/apps/web/components/task/task-switcher-context-menu-items.tsx
index ac1068094f..d182f0de25 100644
--- a/apps/web/components/task/task-switcher-context-menu-items.tsx
+++ b/apps/web/components/task/task-switcher-context-menu-items.tsx
@@ -47,6 +47,8 @@ type LinkActions = ReturnType<typeof selectTaskLinkActions>;
 type SingleGroupProps = Pick<
   SingleSelectionMenuProps,
   | "task"
+  | "nestCandidateTasks"
+  | "nestHierarchyTasks"
   | "isDeleting"
   | "isArchiving"
   | "isPinned"
@@ -282,6 +284,8 @@ function SingleEditGroup({
 
 function SingleRelationshipsGroup({
   task,
+  nestCandidateTasks,
+  nestHierarchyTasks,
   isDeleting,
   onCreateSubtask,
   onDetachTask,
@@ -292,6 +296,8 @@ function SingleRelationshipsGroup({
 }: Pick<
   SingleGroupProps,
   | "task"
+  | "nestCandidateTasks"
+  | "nestHierarchyTasks"
   | "isDeleting"
   | "onCreateSubtask"
   | "onDetachTask"
@@ -303,7 +309,14 @@ function SingleRelationshipsGroup({
   return (
     <>
       <TaskCreateSubtaskItem task={task} disabled={isDeleting} onCreateSubtask={onCreateSubtask} />
-      {!task.isArchived && <TaskNestContextMenuItems task={task} disabled={isDeleting} />}
+      {!task.isArchived && (
+        <TaskNestContextMenuItems
+          task={task}
+          nestCandidateTasks={nestCandidateTasks}
+          nestHierarchyTasks={nestHierarchyTasks}
+          disabled={isDeleting}
+        />
+      )}
       {hasLinkGroup && linkActions && pluginLinkActions && (
         <TaskPluginLinkMenu
           disabled={isDeleting}
diff --git a/apps/web/components/task/task-switcher-context-menu.test.tsx b/apps/web/components/task/task-switcher-context-menu.test.tsx
index 08087231e3..57eafe1c12 100644
--- a/apps/web/components/task/task-switcher-context-menu.test.tsx
+++ b/apps/web/components/task/task-switcher-context-menu.test.tsx
@@ -12,6 +12,7 @@ import type { TaskSwitcherItem } from "./task-switcher-types";
 const PLUGIN_ID = "example-task-actions";
 const PLUGIN_ACTION_LABEL = "Inspect task";
 const WORKFLOW_1_ID = "workflow-1";
+const WORKFLOW_1_NAME = "Workflow 1";
 const WORKFLOW_ID = WORKFLOW_1_ID;
 const STEP_ID = "step-1";
 const SEPARATOR_SLOT = "context-menu-separator";
@@ -171,7 +172,7 @@ describe("TaskItemWithContextMenu — grouped single-task actions", () => {
               parentTaskId: "parent-1",
             })}
             workflows={[
-              { id: WORKFLOW_ID, name: "Workflow 1" },
+              { id: WORKFLOW_ID, name: WORKFLOW_1_NAME },
               { id: "workflow-2", name: "Workflow 2" },
             ]}
             stepsByWorkflowId={{
@@ -246,7 +247,7 @@ describe("TaskItemWithContextMenu — grouped bulk actions", () => {
             task={task({ workflowId: WORKFLOW_ID, workflowStepId: STEP_ID })}
             selectedTaskIds={new Set(["task-1", "task-2"])}
             workflows={[
-              { id: WORKFLOW_ID, name: "Workflow 1" },
+              { id: WORKFLOW_ID, name: WORKFLOW_1_NAME },
               { id: "workflow-2", name: "Workflow 2" },
             ]}
             stepsByWorkflowId={{
@@ -309,7 +310,7 @@ function renderWorkflowMoveMenu({
               workflowId: WORKFLOW_1_ID,
               workflowStepId: "step-1",
             })}
-            workflows={[{ id: WORKFLOW_1_ID, name: "Workflow 1" }]}
+            workflows={[{ id: WORKFLOW_1_ID, name: WORKFLOW_1_NAME }]}
             stepsByWorkflowId={{
               [WORKFLOW_1_ID]: [
                 { id: "step-1", title: "Todo" },
diff --git a/apps/web/components/task/task-switcher-context-menu.tsx b/apps/web/components/task/task-switcher-context-menu.tsx
index d940ba032b..4a13017f4c 100644
--- a/apps/web/components/task/task-switcher-context-menu.tsx
+++ b/apps/web/components/task/task-switcher-context-menu.tsx
@@ -26,6 +26,10 @@ export { createTaskLinkSelectAction } from "./task-switcher-link-menu";
 
 type ContextMenuProps = TaskLinkHandlers & {
   task: TaskSwitcherItem;
+  nestCandidateTasks?: TaskSwitcherItem[];
+  nestHierarchyTasks?: TaskSwitcherItem[];
+  getNestCandidateTasks?: () => TaskSwitcherItem[];
+  getNestHierarchyTasks?: () => TaskSwitcherItem[] | undefined;
   workflows?: TaskMoveWorkflow[];
   stepsByWorkflowId?: Record<string, StepDef[]>;
   steps?: StepDef[];
@@ -55,19 +59,40 @@ type ContextMenuProps = TaskLinkHandlers & {
   isMixedWorkflowSelection?: boolean;
 };
 
+type OpenedNestSources = {
+  candidates?: TaskSwitcherItem[];
+  hierarchy?: TaskSwitcherItem[];
+};
+
+function useContextMenuOpenState(props: ContextMenuProps) {
+  const [contextOpen, setContextOpen] = useState(false);
+  const [openedNestSources, setOpenedNestSources] = useState<OpenedNestSources>({});
+  const handleContextOpenChange = (open: boolean) => {
+    if (open) {
+      setOpenedNestSources({
+        candidates: props.getNestCandidateTasks?.() ?? props.nestCandidateTasks,
+        hierarchy: props.getNestHierarchyTasks?.() ?? props.nestHierarchyTasks,
+      });
+    }
+    setContextOpen(open);
+  };
+  return { contextOpen, setContextOpen, openedNestSources, handleContextOpenChange };
+}
+
 // This component coordinates the context menu and drag cancellation. Archive
 // state lives in its focused adapter so unavailable actions stay unavailable.
 export function TaskItemWithContextMenu(props: ContextMenuProps) {
   const { children, ...menuProps } = props;
   const { task, stepsByWorkflowId, steps, onRequestMoveOptions, onBeforeMoveOptionsOpen } = props;
-  const [contextOpen, setContextOpen] = useState(false);
+  const { contextOpen, setContextOpen, openedNestSources, handleContextOpenChange } =
+    useContextMenuOpenState(props);
   const [menuKey, setMenuKey] = useState(0);
   const moveTasks = useTaskWorkflowMove();
   const closeMenu = () => {
     setContextOpen(false);
     setMenuKey((k) => k + 1);
   };
-  const { handleOpenChange, triggerProps } = useMenuTouchDragCancel(setContextOpen);
+  const { handleOpenChange, triggerProps } = useMenuTouchDragCancel(handleContextOpenChange);
   const { isFinePointer, isMobile } = useResponsiveBreakpoint();
   const usesTouchDrawer = useTouchDrawer();
   const archive = useTaskSwitcherArchiveConfirmation({
@@ -107,11 +132,17 @@ export function TaskItemWithContextMenu(props: ContextMenuProps) {
   // owns its own drawer, so the inline path stays off there too.
   const inlineSubmitWithOptions =
     !usesTouchDrawer && !onRequestMoveOptions ? submitMoveOptionsForStep : undefined;
-  const contextMenuProps = buildTaskContextMenuItemsProps(props, closeMenu, moveTasks, {
-    onMoveToStepWithOptions: handleMoveToStepWithOptions,
-    onSubmitWithOptions: inlineSubmitWithOptions,
-    isMoving,
-  });
+  const contextMenuProps = buildTaskContextMenuItemsProps(
+    props,
+    openedNestSources,
+    closeMenu,
+    moveTasks,
+    {
+      onMoveToStepWithOptions: handleMoveToStepWithOptions,
+      onSubmitWithOptions: inlineSubmitWithOptions,
+      isMoving,
+    },
+  );
 
   return (
     <>
@@ -157,7 +188,11 @@ export function TaskItemWithContextMenu(props: ContextMenuProps) {
 
 export type TaskContextMenuItemsProps = Omit<
   ContextMenuProps,
-  "children" | "onBeforeMoveOptionsOpen" | "onRequestMoveOptions"
+  | "children"
+  | "onBeforeMoveOptionsOpen"
+  | "onRequestMoveOptions"
+  | "getNestCandidateTasks"
+  | "getNestHierarchyTasks"
 > & {
   closeMenu: () => void;
   moveTasks: ReturnType<typeof useTaskWorkflowMove>;
@@ -171,6 +206,7 @@ export type TaskContextMenuItemsProps = Omit<
 
 function buildTaskContextMenuItemsProps(
   props: ContextMenuProps,
+  nestSources: OpenedNestSources,
   closeMenu: () => void,
   moveTasks: ReturnType<typeof useTaskWorkflowMove>,
   moveOptions: {
@@ -186,10 +222,14 @@ function buildTaskContextMenuItemsProps(
     children: _children,
     onRequestMoveOptions: _onRequestMoveOptions,
     onBeforeMoveOptionsOpen: _onBeforeMoveOptionsOpen,
+    getNestCandidateTasks: _getNestCandidateTasks,
+    getNestHierarchyTasks: _getNestHierarchyTasks,
     ...rest
   } = props;
   return {
     ...rest,
+    nestCandidateTasks: nestSources.candidates ?? rest.nestCandidateTasks,
+    nestHierarchyTasks: nestSources.hierarchy ?? rest.nestHierarchyTasks,
     onMoveToStepWithOptions: moveOptions.onMoveToStepWithOptions,
     onSubmitWithOptions: moveOptions.onSubmitWithOptions,
     moveOptionsBusy: moveOptions.isMoving,
diff --git a/apps/web/components/task/task-switcher-nest-context-menu.test.tsx b/apps/web/components/task/task-switcher-nest-context-menu.test.tsx
new file mode 100644
index 0000000000..a753682484
--- /dev/null
+++ b/apps/web/components/task/task-switcher-nest-context-menu.test.tsx
@@ -0,0 +1,112 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { cleanup, fireEvent, render, screen } from "@testing-library/react";
+import { StateProvider } from "@/components/state-provider";
+import { ToastProvider } from "@/components/toast-provider";
+import { TaskItemWithContextMenu } from "./task-switcher-context-menu";
+import type { TaskSwitcherItem } from "./task-switcher-types";
+
+const WORKFLOW_ID = "workflow-1";
+const STEP_ID = "step-1";
+
+function task(overrides: Partial<TaskSwitcherItem> = {}): TaskSwitcherItem {
+  return { id: "task-1", title: "Task 1", state: "IN_PROGRESS", ...overrides };
+}
+
+function storeTask(item: TaskSwitcherItem) {
+  return {
+    id: item.id,
+    title: item.title,
+    workflowId: item.workflowId,
+    workflowStepId: item.workflowStepId,
+    position: 0,
+  };
+}
+
+afterEach(cleanup);
+
+describe("TaskItemWithContextMenu Nest under candidates", () => {
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.1
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.2
+  it("uses rendered candidates when the workflow snapshot is partial", async () => {
+    const current = task({ workflowId: WORKFLOW_ID, workflowStepId: STEP_ID });
+    const target = task({
+      id: "visible-target",
+      title: "Visible target",
+      workflowId: WORKFLOW_ID,
+      workflowStepId: STEP_ID,
+    });
+
+    render(
+      <StateProvider
+        initialState={
+          {
+            kanban: {
+              workflowId: WORKFLOW_ID,
+              steps: [],
+              tasks: [storeTask(current), storeTask(target)],
+            },
+            kanbanMulti: {
+              isLoading: false,
+              snapshots: {
+                [WORKFLOW_ID]: {
+                  workflowId: WORKFLOW_ID,
+                  workflowName: "Workflow 1",
+                  steps: [],
+                  tasks: [storeTask(current)],
+                  isPlaceholder: true,
+                },
+              },
+            },
+          } as never
+        }
+      >
+        <ToastProvider>
+          <TaskItemWithContextMenu task={current} nestCandidateTasks={[current, target]}>
+            <div data-testid="task-row">Task 1</div>
+          </TaskItemWithContextMenu>
+        </ToastProvider>
+      </StateProvider>,
+    );
+
+    fireEvent.contextMenu(screen.getByTestId("task-row"));
+    const nestUnder = await screen.findByRole("menuitem", { name: "Nest under" });
+    fireEvent.pointerMove(nestUnder, { pointerType: "mouse" });
+
+    expect(await screen.findByRole("menuitem", { name: "Visible target" })).not.toBeNull();
+  });
+
+  it("captures the latest candidates when the menu opens", async () => {
+    const current = task({ workflowId: WORKFLOW_ID, workflowStepId: STEP_ID });
+    const staleTarget = task({
+      id: "stale-target",
+      title: "Stale target",
+      workflowId: WORKFLOW_ID,
+      workflowStepId: STEP_ID,
+    });
+    const latestTarget = task({
+      id: "latest-target",
+      title: "Latest target",
+      workflowId: WORKFLOW_ID,
+      workflowStepId: STEP_ID,
+    });
+    let candidates = [current, staleTarget];
+
+    render(
+      <StateProvider>
+        <ToastProvider>
+          <TaskItemWithContextMenu task={current} getNestCandidateTasks={() => candidates}>
+            <div data-testid="fresh-task-row">Task 1</div>
+          </TaskItemWithContextMenu>
+        </ToastProvider>
+      </StateProvider>,
+    );
+    candidates = [current, latestTarget];
+
+    fireEvent.contextMenu(screen.getByTestId("fresh-task-row"));
+    const nestUnder = await screen.findByRole("menuitem", { name: "Nest under" });
+    fireEvent.pointerMove(nestUnder, { pointerType: "mouse" });
+
+    expect(await screen.findByRole("menuitem", { name: "Latest target" })).not.toBeNull();
+    expect(screen.queryByRole("menuitem", { name: "Stale target" })).toBeNull();
+  });
+});
diff --git a/apps/web/components/task/task-switcher-render-stability.test.tsx b/apps/web/components/task/task-switcher-render-stability.test.tsx
index f31590f286..72ac56efa5 100644
--- a/apps/web/components/task/task-switcher-render-stability.test.tsx
+++ b/apps/web/components/task/task-switcher-render-stability.test.tsx
@@ -8,6 +8,7 @@ const renderCounts = vi.hoisted(() => ({
   groups: new Map<string, number>(),
   nodes: new Map<string, number>(),
   rows: new Map<string, number>(),
+  nestCandidateGetters: new Map<string, (() => TaskSwitcherItem[]) | undefined>(),
 }));
 
 function increment(counts: Map<string, number>, key: string) {
@@ -22,7 +23,18 @@ vi.mock("./task-item", () => ({
 }));
 
 vi.mock("./task-switcher-context-menu", () => ({
-  TaskItemWithContextMenu: ({ children }: { children: ReactElement }) => children,
+  TaskItemWithContextMenu: ({
+    task,
+    children,
+    getNestCandidateTasks,
+  }: {
+    task: TaskSwitcherItem;
+    children: ReactElement;
+    getNestCandidateTasks?: () => TaskSwitcherItem[];
+  }) => {
+    renderCounts.nestCandidateGetters.set(task.id, getNestCandidateTasks);
+    return children;
+  },
 }));
 
 vi.mock("./task-switcher-subtask-dnd", () => ({
@@ -131,6 +143,7 @@ beforeEach(() => {
   renderCounts.groups.clear();
   renderCounts.nodes.clear();
   renderCounts.rows.clear();
+  renderCounts.nestCandidateGetters.clear();
 });
 
 afterEach(() => cleanup());
@@ -189,6 +202,20 @@ describe("TaskSwitcher render stability", () => {
 
     expect(latestHandler).toHaveBeenCalledWith(TASK_B);
   });
+
+  it("gives a skipped row the latest nest candidates", () => {
+    const taskA = task(TASK_A, WORKFLOW_A);
+    const taskB = task(TASK_B, WORKFLOW_A);
+    const view = render(switcher(groupedTogether([taskA, taskB])));
+
+    view.rerender(switcher(groupedTogether([{ ...taskA, title: TASK_A_UPDATED }, { ...taskB }])));
+
+    expect(
+      renderCounts.nestCandidateGetters
+        .get(TASK_B)?.()
+        .map((item) => item.title),
+    ).toEqual([TASK_A_UPDATED, TASK_B]);
+  });
 });
 
 describe("TaskSwitcher nested render stability", () => {
diff --git a/apps/web/components/task/task-switcher-row.tsx b/apps/web/components/task/task-switcher-row.tsx
index 9693185d0f..4ad84f70c2 100644
--- a/apps/web/components/task/task-switcher-row.tsx
+++ b/apps/web/components/task/task-switcher-row.tsx
@@ -16,6 +16,9 @@ export type SubtaskToggleInfo = {
 
 export type TaskRowProps = {
   task: TaskSwitcherItem;
+  nestCandidateTasks?: TaskSwitcherItem[];
+  getNestCandidateTasks?: () => TaskSwitcherItem[];
+  getNestHierarchyTasks?: () => TaskSwitcherItem[] | undefined;
   isSubTask?: boolean;
   depth?: number;
   subtaskToggle?: SubtaskToggleInfo;
@@ -66,6 +69,9 @@ function archiveAware<T>(value: T | undefined, isArchived: boolean): T | undefin
 
 function getContextMenuProps(props: TaskRowProps, isArchived: boolean) {
   return {
+    nestCandidateTasks: props.nestCandidateTasks,
+    getNestCandidateTasks: props.getNestCandidateTasks,
+    getNestHierarchyTasks: props.getNestHierarchyTasks,
     onEditTask: archiveAware(props.onEditTask, isArchived),
     onRenameTask: archiveAware(props.onRenameTask, isArchived),
     onArchiveTask: archiveAware(props.onArchiveTask, isArchived),
diff --git a/apps/web/components/task/task-switcher-subtask-dnd.test.ts b/apps/web/components/task/task-switcher-subtask-dnd.test.ts
index 2755231701..212995b5dd 100644
--- a/apps/web/components/task/task-switcher-subtask-dnd.test.ts
+++ b/apps/web/components/task/task-switcher-subtask-dnd.test.ts
@@ -17,13 +17,24 @@ const WORKFLOW = "wf-1";
 const OTHER_WORKFLOW = "wf-2";
 const PWC = "parent-with-child"; // root that already has a child
 const PWC_CHILD = "child-of-pwc";
+const OFFICE_SUBJECT = "office-subject";
+const OFFICE_TARGET_ROOT = "office-target-root";
+const HIDDEN_CHILD = "hidden-child";
 
-function task(id: string, overrides: { workflowId?: string; parentTaskId?: string | null } = {}) {
+function task(
+  id: string,
+  overrides: {
+    workflowId?: string;
+    parentTaskId?: string | null;
+    isFromOffice?: boolean;
+  } = {},
+) {
   return {
     id,
     title: id,
     workflowId: overrides.workflowId ?? WORKFLOW,
     parentTaskId: overrides.parentTaskId ?? undefined,
+    isFromOffice: overrides.isFromOffice,
   };
 }
 
@@ -154,4 +165,39 @@ describe("computeNestTargets", () => {
     expect(targets.has("other-wf")).toBe(false);
     expect(targets.has(SUB_C)).toBe(false);
   });
+
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.1
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.4
+  it("matches the shared Office candidates for a subject with children", () => {
+    const officeTasks = [
+      task(OFFICE_SUBJECT, { isFromOffice: true }),
+      task("office-descendant", {
+        parentTaskId: OFFICE_SUBJECT,
+        isFromOffice: true,
+      }),
+      task(OFFICE_TARGET_ROOT, { isFromOffice: true }),
+      task("office-target-child", {
+        parentTaskId: OFFICE_TARGET_ROOT,
+        isFromOffice: true,
+      }),
+    ];
+
+    expect(computeNestTargets(officeTasks[0], officeTasks)).toEqual(
+      new Set([OFFICE_TARGET_ROOT, "office-target-child"]),
+    );
+  });
+
+  it("excludes a descendant whose intermediate ancestor is hidden", () => {
+    const hierarchy = [
+      task(OFFICE_SUBJECT, { isFromOffice: true }),
+      task(HIDDEN_CHILD, { parentTaskId: OFFICE_SUBJECT, isFromOffice: true }),
+      task("visible-grandchild", { parentTaskId: HIDDEN_CHILD, isFromOffice: true }),
+      task("valid-target", { isFromOffice: true }),
+    ];
+    const visibleTasks = hierarchy.filter((candidate) => candidate.id !== HIDDEN_CHILD);
+
+    expect(computeNestTargets(hierarchy[0], visibleTasks, hierarchy)).toEqual(
+      new Set(["valid-target"]),
+    );
+  });
 });
diff --git a/apps/web/components/task/task-switcher-subtask-dnd.tsx b/apps/web/components/task/task-switcher-subtask-dnd.tsx
index edcdec6785..f1c5819062 100644
--- a/apps/web/components/task/task-switcher-subtask-dnd.tsx
+++ b/apps/web/components/task/task-switcher-subtask-dnd.tsx
@@ -122,16 +122,20 @@ const GROUP_ROOT_LEVEL = "__group_root__";
 /**
  * Computes the rows a drag may nest the active task under, mirroring the
  * context menu's candidate rules (computeNestCandidates) scoped to the
- * active task's workflow: same-workflow roots, excluding the task itself,
- * its current parent, and any subtask (which also excludes its descendants).
+ * active task's workflow. Every mode excludes the task, its current parent,
+ * and descendants; Kanban keeps the root-only depth limit while Office may
+ * target tasks at any depth.
  */
 export function computeNestTargets(
   activeTask: Pick<TaskSwitcherItem, "id" | "workflowId" | "parentTaskId"> | undefined,
   groupTasks: TaskSwitcherItem[],
+  hierarchyTasks: readonly TaskSwitcherItem[] = groupTasks,
 ): Set<string> {
   if (!activeTask) return new Set();
   const sameWorkflow = groupTasks.filter((t) => t.workflowId === activeTask.workflowId);
-  return new Set(computeNestCandidates(sameWorkflow, activeTask.id).map((t) => t.id));
+  return new Set(
+    computeNestCandidates(sameWorkflow, activeTask.id, hierarchyTasks).map((task) => task.id),
+  );
 }
 
 /**
@@ -328,6 +332,47 @@ export function SortableTaskLevel({
   );
 }
 
+function useActiveNestDrag(
+  groupTasks: TaskSwitcherItem[],
+  tasksById: ReadonlyMap<string, TaskSwitcherItem>,
+  getNestHierarchyTasks?: () => TaskSwitcherItem[] | undefined,
+) {
+  const [activeDrag, setActiveDrag] = useState<{
+    id: string;
+    hierarchyTasks?: TaskSwitcherItem[];
+  } | null>(null);
+  const activeDragId = activeDrag?.id ?? null;
+  const nestTargetIds = useMemo(
+    () =>
+      computeNestTargets(
+        activeDragId ? tasksById.get(activeDragId) : undefined,
+        groupTasks,
+        activeDrag?.hierarchyTasks ?? groupTasks,
+      ),
+    [activeDrag, activeDragId, tasksById, groupTasks],
+  );
+  const handleDragStart = useCallback(
+    (event: DragStartEvent) => {
+      setActiveDrag({
+        id: String(event.active.id),
+        hierarchyTasks: getNestHierarchyTasks?.(),
+      });
+    },
+    [getNestHierarchyTasks],
+  );
+  const clearActiveDrag = useCallback(() => setActiveDrag(null), []);
+  return { activeDragId, nestTargetIds, handleDragStart, clearActiveDrag };
+}
+
+type TaskTreeDndGroupProps = {
+  groupTasks: TaskSwitcherItem[];
+  getNestHierarchyTasks?: () => TaskSwitcherItem[] | undefined;
+  onReorderGroup?: (groupTaskIds: string[]) => void;
+  onReorderSubtasks?: (parentTaskId: string, orderedSubtaskIds: string[]) => void;
+  onNestTask?: (activeTaskId: string, parentTaskId: string) => void;
+  children: (nestTargetIds: Set<string>, activeDragId: string | null) => React.ReactNode;
+};
+
 /**
  * One group-spanning DndContext for the sidebar task tree. Replaces the
  * per-level contexts so a drag can target rows outside its own sibling level:
@@ -339,18 +384,12 @@ export function SortableTaskLevel({
  */
 export function TaskTreeDndGroup({
   groupTasks,
+  getNestHierarchyTasks,
   onReorderGroup,
   onReorderSubtasks,
   onNestTask,
   children,
-}: {
-  groupTasks: TaskSwitcherItem[];
-  onReorderGroup?: (groupTaskIds: string[]) => void;
-  onReorderSubtasks?: (parentTaskId: string, orderedSubtaskIds: string[]) => void;
-  onNestTask?: (activeTaskId: string, parentTaskId: string) => void;
-  children: (nestTargetIds: Set<string>, activeDragId: string | null) => React.ReactNode;
-}) {
-  const [activeDragId, setActiveDragId] = useState<string | null>(null);
+}: TaskTreeDndGroupProps) {
   const sensors = useSensors(
     useSensor(MouseSensor, {
       activationConstraint: TASK_SWITCHER_DRAG_ACTIVATION_CONSTRAINTS.pointer,
@@ -378,18 +417,14 @@ export function TaskTreeDndGroup({
       childrenByParent: byParent,
     };
   }, [groupTasks]);
-  const nestTargetIds = useMemo(
-    () => computeNestTargets(activeDragId ? tasksById.get(activeDragId) : undefined, groupTasks),
-    [activeDragId, tasksById, groupTasks],
+  const { activeDragId, nestTargetIds, handleDragStart, clearActiveDrag } = useActiveNestDrag(
+    groupTasks,
+    tasksById,
+    getNestHierarchyTasks,
   );
-
-  const handleDragStart = useCallback((event: DragStartEvent) => {
-    setActiveDragId(String(event.active.id));
-  }, []);
-  const handleDragCancel = useCallback(() => setActiveDragId(null), []);
   const handleDragEnd = useCallback(
     (event: DragEndEvent) => {
-      setActiveDragId(null);
+      clearActiveDrag();
       const { active, over } = event;
       if (!over) return;
       const decision = resolveSidebarDrop({
@@ -411,7 +446,14 @@ export function TaskTreeDndGroup({
           break;
       }
     },
-    [groupRootIds, childrenByParent, onReorderGroup, onReorderSubtasks, onNestTask],
+    [
+      clearActiveDrag,
+      groupRootIds,
+      childrenByParent,
+      onReorderGroup,
+      onReorderSubtasks,
+      onNestTask,
+    ],
   );
 
   const collisionDetection = useCallback<CollisionDetection>((args) => {
@@ -432,7 +474,7 @@ export function TaskTreeDndGroup({
       collisionDetection={collisionDetection}
       onDragStart={handleDragStart}
       onDragEnd={handleDragEnd}
-      onDragCancel={handleDragCancel}
+      onDragCancel={clearActiveDrag}
     >
       {children(nestTargetIds, activeDragId)}
     </DndContext>
diff --git a/apps/web/components/task/task-switcher-tree.tsx b/apps/web/components/task/task-switcher-tree.tsx
index abc6eb521e..dc197fe0e7 100644
--- a/apps/web/components/task/task-switcher-tree.tsx
+++ b/apps/web/components/task/task-switcher-tree.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { memo, useCallback, useMemo } from "react";
+import { memo, useCallback, useLayoutEffect, useMemo, useRef } from "react";
 import { countGroupTasks, type SidebarGroup } from "@/lib/sidebar/apply-view";
 import {
   SortableTaskLevel,
@@ -14,7 +14,14 @@ import type { TaskSwitcherItem } from "./task-switcher-types";
 
 export type TaskRowBaseProps = Omit<
   TaskRowProps,
-  "task" | "subtaskToggle" | "isPinned" | "isSubTask" | "depth"
+  | "task"
+  | "nestCandidateTasks"
+  | "getNestCandidateTasks"
+  | "getNestHierarchyTasks"
+  | "subtaskToggle"
+  | "isPinned"
+  | "isSubTask"
+  | "depth"
 >;
 
 type TaskTreeContext = {
@@ -23,6 +30,8 @@ type TaskTreeContext = {
   onToggleSubtasks?: (parentTaskId: string) => void;
   pinnedSet: Set<string>;
   rowProps: TaskRowBaseProps;
+  getNestCandidateTasks: () => TaskSwitcherItem[];
+  getNestHierarchyTasks?: () => TaskSwitcherItem[] | undefined;
   onReorderGroup?: (groupTaskIds: string[]) => void;
   onReorderSubtasks?: (parentTaskId: string, orderedSubtaskIds: string[]) => void;
   onNestTask?: (taskId: string, parentTaskId: string) => void;
@@ -74,6 +83,8 @@ function taskTreeNodeEqual(previous: TaskTreeNodeProps, next: TaskTreeNodeProps)
     previous.depth !== next.depth ||
     previous.isDraggable !== next.isDraggable ||
     previous.ctx.rowProps !== next.ctx.rowProps ||
+    previous.ctx.getNestCandidateTasks !== next.ctx.getNestCandidateTasks ||
+    previous.ctx.getNestHierarchyTasks !== next.ctx.getNestHierarchyTasks ||
     previous.ctx.onToggleSubtasks !== next.ctx.onToggleSubtasks ||
     previous.ctx.onReorderGroup !== next.ctx.onReorderGroup ||
     previous.ctx.onReorderSubtasks !== next.ctx.onReorderSubtasks ||
@@ -125,6 +136,8 @@ const TaskTreeNode = memo(function TaskTreeNode({
         subtaskToggle={toggleInfo}
         isPinned={isRoot && ctx.pinnedSet.has(task.id)}
         {...ctx.rowProps}
+        getNestCandidateTasks={ctx.getNestCandidateTasks}
+        getNestHierarchyTasks={ctx.getNestHierarchyTasks}
         onTogglePin={isRoot ? ctx.rowProps.onTogglePin : undefined}
       />
       {isNestTarget && <NestDropZone taskId={task.id} title={task.title} />}
@@ -184,6 +197,7 @@ function getReorderHandler(parentTaskId: string | null, ctx: TaskTreeContext) {
 export type GroupSectionProps = {
   group: SidebarGroup;
   subTasksByParentId: Map<string, TaskSwitcherItem[]>;
+  getNestHierarchyTasks?: () => TaskSwitcherItem[] | undefined;
   rowProps: TaskRowBaseProps;
   pinnedSet: Set<string>;
   isCollapsed: boolean;
@@ -230,6 +244,7 @@ function groupSectionEqual(previous: GroupSectionProps, next: GroupSectionProps)
   if (
     previous.group !== next.group ||
     previous.rowProps !== next.rowProps ||
+    previous.getNestHierarchyTasks !== next.getNestHierarchyTasks ||
     previous.pinnedSet !== next.pinnedSet ||
     previous.isCollapsed !== next.isCollapsed ||
     previous.onToggleGroup !== next.onToggleGroup ||
@@ -286,6 +301,7 @@ function flattenGroupTasks(
 export const GroupSection = memo(function GroupSection({
   group,
   subTasksByParentId,
+  getNestHierarchyTasks,
   rowProps,
   pinnedSet,
   isCollapsed,
@@ -302,6 +318,11 @@ export const GroupSection = memo(function GroupSection({
     () => flattenGroupTasks(group.tasks, subTasksByParentId),
     [group.tasks, subTasksByParentId],
   );
+  const groupTasksRef = useRef(groupTasks);
+  useLayoutEffect(() => {
+    groupTasksRef.current = groupTasks;
+  }, [groupTasks]);
+  const getNestCandidateTasks = useCallback(() => groupTasksRef.current, []);
 
   const renderTree = (nestTargetIds: Set<string>) => {
     if (isCollapsed) return null;
@@ -311,6 +332,8 @@ export const GroupSection = memo(function GroupSection({
       onToggleSubtasks,
       pinnedSet,
       rowProps,
+      getNestCandidateTasks,
+      getNestHierarchyTasks,
       onReorderGroup,
       onReorderSubtasks,
       onNestTask,
@@ -333,6 +356,7 @@ export const GroupSection = memo(function GroupSection({
       )}
       <TaskTreeDndGroup
         groupTasks={groupTasks}
+        getNestHierarchyTasks={getNestHierarchyTasks}
         onReorderGroup={onReorderGroup}
         onReorderSubtasks={onReorderSubtasks}
         onNestTask={onNestTask}
diff --git a/apps/web/components/task/task-switcher-types.ts b/apps/web/components/task/task-switcher-types.ts
index d20b28b3f4..466f42aaa8 100644
--- a/apps/web/components/task/task-switcher-types.ts
+++ b/apps/web/components/task/task-switcher-types.ts
@@ -62,6 +62,7 @@ export type TaskSwitcherItem = {
   lastActivityAt?: string;
   createdAt?: string;
   isArchived?: boolean;
+  isFromOffice?: boolean;
   primarySessionId?: string | null;
   hasPendingClarification?: boolean;
   hasPendingPermission?: boolean;
@@ -81,6 +82,8 @@ export type TaskSwitcherItem = {
 
 export type TaskSwitcherProps = {
   grouped: GroupedSidebarList;
+  /** Complete unfiltered task set used only to validate hierarchy constraints. */
+  nestHierarchyTasks?: TaskSwitcherItem[];
   workflows?: TaskMoveWorkflow[];
   stepsByWorkflowId?: Record<string, StepDef[]>;
   activeTaskId: string | null;
diff --git a/apps/web/components/task/task-switcher.tsx b/apps/web/components/task/task-switcher.tsx
index 793d33e447..e16537ebbb 100644
--- a/apps/web/components/task/task-switcher.tsx
+++ b/apps/web/components/task/task-switcher.tsx
@@ -158,12 +158,14 @@ function buildGroupSectionProps(
     pinnedSet: Set<string>;
     collapsedSet: Set<string>;
     showHeader: boolean;
+    getNestHierarchyTasks: () => TaskSwitcherProps["nestHierarchyTasks"];
   },
 ): GroupSectionProps {
-  const { group, rowProps, pinnedSet, collapsedSet, showHeader } = options;
+  const { group, rowProps, pinnedSet, collapsedSet, showHeader, getNestHierarchyTasks } = options;
   return {
     group,
     subTasksByParentId: grouped.subTasksByParentId,
+    getNestHierarchyTasks,
     rowProps,
     pinnedSet,
     isCollapsed: collapsedSet.has(group.key),
@@ -218,6 +220,11 @@ export const TaskSwitcher = memo(function TaskSwitcher(props: TaskSwitcherProps)
     () => new Set(props.collapsedGroupKeys ?? []),
     [props.collapsedGroupKeys],
   );
+  const nestHierarchyTasksRef = useRef(props.nestHierarchyTasks);
+  useLayoutEffect(() => {
+    nestHierarchyTasksRef.current = props.nestHierarchyTasks;
+  }, [props.nestHierarchyTasks]);
+  const getNestHierarchyTasks = useMemo(() => () => nestHierarchyTasksRef.current, []);
 
   if (isLoading) return <TaskSwitcherSkeleton />;
 
@@ -258,6 +265,7 @@ export const TaskSwitcher = memo(function TaskSwitcher(props: TaskSwitcherProps)
             pinnedSet,
             collapsedSet,
             showHeader: showHeaders,
+            getNestHierarchyTasks,
           })}
         />
       ))}
diff --git a/apps/web/e2e/tests/task/mobile-office-sidebar-nest-task.spec.ts b/apps/web/e2e/tests/task/mobile-office-sidebar-nest-task.spec.ts
new file mode 100644
index 0000000000..ff4c508ee6
--- /dev/null
+++ b/apps/web/e2e/tests/task/mobile-office-sidebar-nest-task.spec.ts
@@ -0,0 +1,89 @@
+import { test, expect } from "../../fixtures/office-fixture";
+import { SessionPage } from "../../pages/session-page";
+
+function taskId(task: Record<string, unknown>): string {
+  return task.id as string;
+}
+
+test.describe("Mobile Office sidebar Nest under", () => {
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.1
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.4
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.5
+  test("nests an Office parent under another Office task", async ({
+    testPage,
+    apiClient,
+    officeApi,
+    seedData,
+  }) => {
+    const project = await officeApi.createProject(
+      seedData.workspaceId,
+      "Mobile Office nesting project",
+    );
+    const projectId = taskId(project);
+    const subject = await apiClient.createTask(
+      seedData.workspaceId,
+      "Mobile Office nesting subject",
+      {
+        workflow_id: seedData.workflowId,
+        workflow_step_id: seedData.startStepId,
+        repository_ids: [seedData.repositoryId],
+      },
+    );
+    const descendant = await apiClient.createTask(
+      seedData.workspaceId,
+      "Mobile Office nesting descendant",
+      {
+        workflow_id: seedData.workflowId,
+        workflow_step_id: seedData.startStepId,
+        repository_ids: [seedData.repositoryId],
+        parent_id: subject.id,
+      },
+    );
+    const target = await apiClient.createTask(
+      seedData.workspaceId,
+      "Mobile Office nesting target",
+      {
+        workflow_id: seedData.workflowId,
+        workflow_step_id: seedData.startStepId,
+        repository_ids: [seedData.repositoryId],
+      },
+    );
+
+    await Promise.all([
+      officeApi.updateTaskProjectId(subject.id, projectId),
+      officeApi.updateTaskProjectId(descendant.id, projectId),
+      officeApi.updateTaskProjectId(target.id, projectId),
+    ]);
+    await apiClient.saveUserSettings({
+      workspace_id: seedData.workspaceId,
+      workflow_filter_id: seedData.workflowId,
+      keyboard_shortcuts: {},
+      enable_preview_on_click: false,
+      sidebar_views: [],
+    });
+
+    await testPage.goto(`/t/${subject.id}`);
+    await new SessionPage(testPage).waitForLoad();
+    await testPage.getByTestId("mobile-session-menu").tap();
+
+    const taskSheet = testPage.getByRole("dialog", { name: "Tasks" });
+    const subjectRow = taskSheet
+      .getByTestId("sidebar-task-item")
+      .filter({ hasText: "Mobile Office nesting subject" });
+    await expect(subjectRow).toBeVisible();
+    await subjectRow.getByRole("button", { name: "Task actions" }).tap();
+    await testPage.getByRole("menuitem", { name: "Nest under" }).tap();
+    await expect(
+      testPage.getByRole("menuitem", { name: "Mobile Office nesting descendant" }),
+    ).toHaveCount(0);
+    await testPage.getByRole("menuitem", { name: "Mobile Office nesting target" }).tap();
+
+    const taskBlock = (id: string) =>
+      taskSheet.locator(`[data-testid="sortable-task-block"][data-task-id="${id}"]`);
+    await expect(taskBlock(subject.id)).toHaveAttribute("data-depth", "1", { timeout: 10_000 });
+    await expect(taskBlock(descendant.id)).toHaveAttribute("data-depth", "2");
+    await expect
+      .poll(async () => (await apiClient.getTask(subject.id)).parent_id ?? "")
+      .toBe(target.id);
+  });
+});
diff --git a/apps/web/e2e/tests/task/office-sidebar-nest-task.spec.ts b/apps/web/e2e/tests/task/office-sidebar-nest-task.spec.ts
new file mode 100644
index 0000000000..829523d004
--- /dev/null
+++ b/apps/web/e2e/tests/task/office-sidebar-nest-task.spec.ts
@@ -0,0 +1,96 @@
+import { test, expect } from "../../fixtures/office-fixture";
+import { SessionPage } from "../../pages/session-page";
+
+function taskId(task: Record<string, unknown>): string {
+  return task.id as string;
+}
+
+test.describe("Office sidebar Nest under", () => {
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.1
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.4
+  test("nests an Office parent under another Office task", async ({
+    testPage,
+    apiClient,
+    officeApi,
+    seedData,
+  }) => {
+    const project = await officeApi.createProject(seedData.workspaceId, "Office nesting project");
+    const projectId = taskId(project);
+    const subject = await apiClient.createTask(seedData.workspaceId, "Office nesting subject", {
+      workflow_id: seedData.workflowId,
+      workflow_step_id: seedData.startStepId,
+      repository_ids: [seedData.repositoryId],
+    });
+    const subjectId = subject.id;
+    const descendant = await apiClient.createTask(
+      seedData.workspaceId,
+      "Office nesting descendant",
+      {
+        workflow_id: seedData.workflowId,
+        workflow_step_id: seedData.startStepId,
+        repository_ids: [seedData.repositoryId],
+        parent_id: subjectId,
+      },
+    );
+    const descendantId = descendant.id;
+    const target = await apiClient.createTask(seedData.workspaceId, "Office nesting target", {
+      workflow_id: seedData.workflowId,
+      workflow_step_id: seedData.startStepId,
+      repository_ids: [seedData.repositoryId],
+    });
+    const targetId = target.id;
+
+    await Promise.all([
+      officeApi.updateTaskProjectId(subjectId, projectId),
+      officeApi.updateTaskProjectId(descendantId, projectId),
+      officeApi.updateTaskProjectId(targetId, projectId),
+    ]);
+    await apiClient.saveUserSettings({
+      workspace_id: seedData.workspaceId,
+      workflow_filter_id: seedData.workflowId,
+      keyboard_shortcuts: {},
+      enable_preview_on_click: false,
+      sidebar_views: [],
+    });
+
+    await testPage.goto(`/t/${subjectId}`);
+    const session = new SessionPage(testPage);
+    await session.waitForLoad();
+    await expect(session.sidebar).toBeVisible({ timeout: 10_000 });
+
+    const taskBlock = (id: string) =>
+      session.sidebar.locator(`[data-testid="sortable-task-block"][data-task-id="${id}"]`);
+    await expect(taskBlock(subjectId)).toHaveAttribute("data-depth", "0");
+    await expect(taskBlock(descendantId)).toHaveAttribute("data-depth", "1");
+    await expect(taskBlock(targetId)).toHaveAttribute("data-depth", "0");
+
+    const subjectRow = session.sidebar
+      .locator("[data-testid='sidebar-task-item']")
+      .filter({ hasText: "Office nesting subject" });
+    await subjectRow.hover();
+    await subjectRow.getByRole("button", { name: "Task actions" }).click();
+    await testPage.getByRole("menuitem", { name: "Nest under" }).hover();
+
+    await expect(testPage.getByRole("menuitem", { name: "Office nesting descendant" })).toHaveCount(
+      0,
+    );
+    await testPage.getByRole("menuitem", { name: "Office nesting target" }).click();
+
+    await expect(taskBlock(subjectId)).toHaveAttribute("data-depth", "1", { timeout: 10_000 });
+    await expect(taskBlock(descendantId)).toHaveAttribute("data-depth", "2");
+    await expect(
+      taskBlock(targetId).locator(
+        `[data-testid="sortable-task-block"][data-task-id="${subjectId}"]`,
+      ),
+    ).toHaveCount(1);
+
+    await expect
+      .poll(async () => (await apiClient.getTask(subjectId)).parent_id ?? "")
+      .toBe(targetId);
+
+    await testPage.reload();
+    await session.waitForLoad();
+    await expect(taskBlock(subjectId)).toHaveAttribute("data-depth", "1", { timeout: 10_000 });
+    await expect(taskBlock(descendantId)).toHaveAttribute("data-depth", "2");
+  });
+});
diff --git a/apps/web/e2e/tests/workflow/workflow-step-move-overrides.spec.ts b/apps/web/e2e/tests/workflow/workflow-step-move-overrides.spec.ts
index 226eb66428..84baee9700 100644
--- a/apps/web/e2e/tests/workflow/workflow-step-move-overrides.spec.ts
+++ b/apps/web/e2e/tests/workflow/workflow-step-move-overrides.spec.ts
@@ -172,6 +172,7 @@ test("keeps the desktop next-step form open when clicking a checkbox label", asy
   const form = testPage.getByTestId("proceed-next-step-options");
   await expect(form).toBeVisible();
 
+  await form.hover();
   await form.getByText("Skip the step prompt", { exact: true }).click();
 
   await expect(form).toBeVisible();
diff --git a/apps/web/lib/kanban/map-task.test.ts b/apps/web/lib/kanban/map-task.test.ts
index 58973ce421..dc2809d12d 100644
--- a/apps/web/lib/kanban/map-task.test.ts
+++ b/apps/web/lib/kanban/map-task.test.ts
@@ -67,6 +67,16 @@ function wsPayload(overrides: Partial<TaskLike> = {}): TaskLike {
 }
 
 describe("toKanbanTask — HTTP DTO / WS payload parity", () => {
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.4
+  it("maps is_from_office to isFromOffice", () => {
+    const officeProjection: Partial<TaskLike> = { is_from_office: true };
+    const http = toKanbanTask(httpDTO(officeProjection));
+    const ws = toKanbanTask(wsPayload(officeProjection));
+
+    expect(http.isFromOffice).toBe(true);
+    expect(ws.isFromOffice).toBe(true);
+  });
+
   it("carries workspace identity and archived state through both task shapes", () => {
     const archivedAt = "2026-08-04T10:00:00Z";
     const http = toKanbanTask(httpDTO({ archived_at: archivedAt } as Partial<TaskLike>));
diff --git a/apps/web/lib/kanban/map-task.ts b/apps/web/lib/kanban/map-task.ts
index c71d6a2d4f..b0847ec97b 100644
--- a/apps/web/lib/kanban/map-task.ts
+++ b/apps/web/lib/kanban/map-task.ts
@@ -84,6 +84,7 @@ export type TaskLike = {
   primary_agent_profile_id?: string | null;
   labels?: string | string[] | null;
   is_remote_executor?: boolean;
+  is_from_office?: boolean;
   parent_id?: string | null;
   assignee_user_id?: string;
   updated_at?: string;
@@ -305,6 +306,7 @@ export function toKanbanTask(source: TaskLike): KanbanTask {
     primaryAgentName: source.primary_agent_name ?? undefined,
     labels: pickLabels(source),
     assigneeUserId: pickAssignee(source.assignee_user_id),
+    isFromOffice: source.is_from_office,
     parentTaskId: source.parent_id ?? undefined,
     workspaceMode: workspaceModeFromMetadata(source.metadata),
     updatedAt: source.updated_at,
diff --git a/apps/web/lib/sidebar/nest-candidates.test.ts b/apps/web/lib/sidebar/nest-candidates.test.ts
index 6bb3e43ec7..c5638e7610 100644
--- a/apps/web/lib/sidebar/nest-candidates.test.ts
+++ b/apps/web/lib/sidebar/nest-candidates.test.ts
@@ -1,7 +1,12 @@
 import { describe, expect, it } from "vitest";
 import { computeNestCandidates } from "./nest-candidates";
 
-type T = { id: string; title: string; parentTaskId?: string | null };
+type T = {
+  id: string;
+  title: string;
+  parentTaskId?: string | null;
+  isFromOffice?: boolean;
+};
 
 const tasks: T[] = [
   { id: "a", title: "A" }, // root with a child
@@ -9,6 +14,8 @@ const tasks: T[] = [
   { id: "d", title: "D" }, // root, leaf
   { id: "e", title: "E" }, // root, leaf
 ];
+const OFFICE_ROOT = "office-root";
+const OFFICE_CHILD = "office-child";
 
 describe("computeNestCandidates", () => {
   it("excludes the task itself", () => {
@@ -52,4 +59,94 @@ describe("computeNestCandidates", () => {
   it("returns empty when the task is the only one", () => {
     expect(computeNestCandidates([{ id: "solo", title: "Solo" }], "solo")).toEqual([]);
   });
+
+  it("returns empty when the subject is missing", () => {
+    expect(computeNestCandidates(tasks, "missing")).toEqual([]);
+  });
+});
+
+describe("computeNestCandidates for Office hierarchies", () => {
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.4
+  it("allows deep Office targets and excludes descendants", () => {
+    const officeTasks: T[] = [
+      { id: OFFICE_ROOT, title: "Office root", isFromOffice: true },
+      {
+        id: OFFICE_CHILD,
+        title: "Office child",
+        parentTaskId: OFFICE_ROOT,
+        isFromOffice: true,
+      },
+      {
+        id: "office-grandchild",
+        title: "Office grandchild",
+        parentTaskId: OFFICE_CHILD,
+        isFromOffice: true,
+      },
+      { id: "office-target-root", title: "Target root", isFromOffice: true },
+      {
+        id: "office-target-child",
+        title: "Target child",
+        parentTaskId: "office-target-root",
+        isFromOffice: true,
+      },
+    ];
+
+    expect(computeNestCandidates(officeTasks, "office-root").map((task) => task.id)).toEqual([
+      "office-target-root",
+      "office-target-child",
+    ]);
+  });
+
+  it("uses the complete hierarchy to exclude descendants hidden by the current view", () => {
+    const hierarchy: T[] = [
+      { id: "subject", title: "Subject", isFromOffice: true },
+      {
+        id: "hidden-child",
+        title: "Hidden child",
+        parentTaskId: "subject",
+        isFromOffice: true,
+      },
+      {
+        id: "visible-grandchild",
+        title: "Visible grandchild",
+        parentTaskId: "hidden-child",
+        isFromOffice: true,
+      },
+      { id: "valid-target", title: "Valid target", isFromOffice: true },
+    ];
+    const visibleTasks = hierarchy.filter((task) => task.id !== "hidden-child");
+
+    expect(
+      computeNestCandidates(visibleTasks, "subject", hierarchy).map((task) => task.id),
+    ).toEqual(["valid-target"]);
+  });
+
+  it("allows an Office subject to nest under a non-Office subtask", () => {
+    const mixedTasks: T[] = [
+      { id: "office-subject", title: "Office subject", isFromOffice: true },
+      { id: "kanban-root", title: "Kanban root" },
+      { id: "kanban-child", title: "Kanban child", parentTaskId: "kanban-root" },
+    ];
+
+    expect(computeNestCandidates(mixedTasks, "office-subject").map((task) => task.id)).toContain(
+      "kanban-child",
+    );
+  });
+
+  it("allows a non-Office subject to nest under an Office subtask", () => {
+    const mixedTasks: T[] = [
+      { id: "kanban-subject", title: "Kanban subject" },
+      { id: OFFICE_ROOT, title: "Office root", isFromOffice: true },
+      {
+        id: OFFICE_CHILD,
+        title: "Office child",
+        parentTaskId: OFFICE_ROOT,
+        isFromOffice: true,
+      },
+    ];
+
+    expect(computeNestCandidates(mixedTasks, "kanban-subject").map((task) => task.id)).toContain(
+      OFFICE_CHILD,
+    );
+  });
 });
diff --git a/apps/web/lib/sidebar/nest-candidates.ts b/apps/web/lib/sidebar/nest-candidates.ts
index d470d1ddec..2a640397ba 100644
--- a/apps/web/lib/sidebar/nest-candidates.ts
+++ b/apps/web/lib/sidebar/nest-candidates.ts
@@ -6,29 +6,54 @@ export type NestCandidate = {
   id: string;
   title: string;
   parentTaskId?: string | null;
+  isFromOffice?: boolean;
 };
 
 /**
  * computeNestCandidates returns the tasks that `taskId` may be nested under,
- * enforcing the one-level kanban subtask limit.
+ * enforcing the one-level Kanban limit while allowing arbitrary-depth Office
+ * hierarchies.
  *
- * When the task itself already has children, nesting it under any parent would
- * push those children to depth 2, so no candidate is offered. Otherwise a
- * candidate is any task that is NOT:
- *  - the task itself,
- *  - the task's current parent (selecting it would be a no-op), or
- *  - already a subtask — nesting under a subtask would create a grandchild
- *    (this also structurally excludes the task's own descendants, so no cycle
- *    can be introduced).
+ * Every hierarchy excludes the task, its current parent, and descendants.
+ * Kanban additionally requires a childless subject and root candidate. Office
+ * uses the backend's endpoint rule: either endpoint being Office lifts those
+ * two depth restrictions.
  *
  * Order is preserved from the input list.
  */
-export function computeNestCandidates<T extends NestCandidate>(tasks: T[], taskId: string): T[] {
-  const task = tasks.find((t) => t.id === taskId);
+export function computeNestCandidates<T extends NestCandidate>(
+  tasks: T[],
+  taskId: string,
+  hierarchyTasks: readonly NestCandidate[] = tasks,
+): T[] {
+  const tasksById = new Map(hierarchyTasks.map((candidate) => [candidate.id, candidate]));
+  const task = tasksById.get(taskId) ?? tasks.find((candidate) => candidate.id === taskId);
+  if (!task) return [];
 
-  // A task with children cannot be nested without exceeding the one-level limit.
-  if (tasks.some((t) => t.parentTaskId === taskId)) return [];
+  const taskHasChildren = hierarchyTasks.some((candidate) => candidate.parentTaskId === taskId);
+  const currentParent = task.parentTaskId ?? undefined;
 
-  const currentParent = task?.parentTaskId ?? undefined;
-  return tasks.filter((t) => t.id !== taskId && t.id !== currentParent && !t.parentTaskId);
+  return tasks.filter((candidate) => {
+    const hierarchyCandidate = tasksById.get(candidate.id) ?? candidate;
+    if (hierarchyCandidate.id === taskId || hierarchyCandidate.id === currentParent) return false;
+    if (hasAncestor(hierarchyCandidate, taskId, tasksById)) return false;
+    if (task.isFromOffice || hierarchyCandidate.isFromOffice) return true;
+    return !taskHasChildren && !hierarchyCandidate.parentTaskId;
+  });
+}
+
+function hasAncestor(
+  candidate: NestCandidate,
+  ancestorId: string,
+  tasksById: ReadonlyMap<string, NestCandidate>,
+): boolean {
+  const visited = new Set<string>();
+  let parentId = candidate.parentTaskId;
+  while (parentId) {
+    if (parentId === ancestorId) return true;
+    if (visited.has(parentId)) return true;
+    visited.add(parentId);
+    parentId = tasksById.get(parentId)?.parentTaskId;
+  }
+  return false;
 }
diff --git a/apps/web/lib/state/slices/kanban/types.ts b/apps/web/lib/state/slices/kanban/types.ts
index 4156e71b99..b0c5beb88a 100644
--- a/apps/web/lib/state/slices/kanban/types.ts
+++ b/apps/web/lib/state/slices/kanban/types.ts
@@ -164,6 +164,8 @@ export type KanbanState = {
     primaryAgentName?: string | null;
     labels?: string[];
     isRemoteExecutor?: boolean;
+    /** Backend-owned discriminator for task hierarchy rules. Absent means non-Office. */
+    isFromOffice?: boolean;
     /** Human assignee (user id). Independent of any agent assignment. */
     assigneeUserId?: string;
     parentTaskId?: string | null;
diff --git a/apps/web/lib/ws/handlers/task-merge.ts b/apps/web/lib/ws/handlers/task-merge.ts
index 155a80e98e..3a8b85ce05 100644
--- a/apps/web/lib/ws/handlers/task-merge.ts
+++ b/apps/web/lib/ws/handlers/task-merge.ts
@@ -134,6 +134,10 @@ export function mergeTaskUpdate(
   if (!hasPayloadField(payload, "metadata")) merged.metadata = existing.metadata;
   if (!hasPayloadField(payload, "labels")) merged.labels = existing.labels;
   if (!hasPayloadField(payload, "origin")) merged.origin = existing.origin;
+  preserveOmittedField(existing, merged, payload, nextTask, {
+    payloadKey: "is_from_office",
+    taskField: "isFromOffice",
+  });
   preserveOmittedField(existing, merged, payload, nextTask, {
     payloadKey: "task_pending_action",
     taskField: "taskPendingAction",
diff --git a/apps/web/lib/ws/handlers/tasks-office-identity.test.ts b/apps/web/lib/ws/handlers/tasks-office-identity.test.ts
new file mode 100644
index 0000000000..afdca34607
--- /dev/null
+++ b/apps/web/lib/ws/handlers/tasks-office-identity.test.ts
@@ -0,0 +1,77 @@
+import { describe, expect, it } from "vitest";
+import { registerTasksHandlers } from "./tasks";
+import { makeMessage, makeStore } from "./tasks.test-helpers";
+
+const TASK_ID = "office-task";
+const WORKFLOW_ID = "office-workflow";
+const STEP_ID = "office-step";
+
+function cachedOfficeTask() {
+  return {
+    id: TASK_ID,
+    title: "Office task",
+    workflowId: WORKFLOW_ID,
+    workflowStepId: STEP_ID,
+    position: 0,
+    isFromOffice: true,
+  };
+}
+
+function storeWithCachedOfficeTask() {
+  return makeStore({
+    kanban: {
+      workflowId: WORKFLOW_ID,
+      steps: [],
+      tasks: [cachedOfficeTask()],
+    },
+    kanbanMulti: {
+      isLoading: false,
+      snapshots: {
+        [WORKFLOW_ID]: {
+          workflowId: WORKFLOW_ID,
+          workflowName: "Office",
+          steps: [],
+          tasks: [cachedOfficeTask()],
+        },
+      },
+    },
+  } as never);
+}
+
+describe("task.updated Office identity preservation", () => {
+  // @covers AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.4
+  it("preserves Office identity when a lightweight update omits it", () => {
+    const store = storeWithCachedOfficeTask();
+
+    registerTasksHandlers(store)["task.updated"]!(
+      makeMessage({
+        task_id: TASK_ID,
+        workflow_id: WORKFLOW_ID,
+        workflow_step_id: STEP_ID,
+        title: "Renamed Office task",
+        state: "IN_PROGRESS",
+      }),
+    );
+
+    expect(store.getState().kanban.tasks[0]?.isFromOffice).toBe(true);
+    expect(store.getState().kanbanMulti.snapshots[WORKFLOW_ID]?.tasks[0]?.isFromOffice).toBe(true);
+  });
+
+  it("clears Office identity when task.updated explicitly sets it to false", () => {
+    const store = storeWithCachedOfficeTask();
+
+    registerTasksHandlers(store)["task.updated"]!(
+      makeMessage({
+        task_id: TASK_ID,
+        workflow_id: WORKFLOW_ID,
+        workflow_step_id: STEP_ID,
+        title: "Project removed",
+        state: "IN_PROGRESS",
+        is_from_office: false,
+      }),
+    );
+
+    expect(store.getState().kanban.tasks[0]?.isFromOffice).toBe(false);
+    expect(store.getState().kanbanMulti.snapshots[WORKFLOW_ID]?.tasks[0]?.isFromOffice).toBe(false);
+  });
+});
diff --git a/docs/plans/fix-nest-under-candidates/plan.md b/docs/plans/fix-nest-under-candidates/plan.md
new file mode 100644
index 0000000000..aa4f98f6f2
--- /dev/null
+++ b/docs/plans/fix-nest-under-candidates/plan.md
@@ -0,0 +1,164 @@
+---
+created: 2026-09-16
+status: implemented
+requirements:
+  - REQ-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001
+system_design:
+  - ../../specs/tasks/system-design/subtask-reparenting-drag-drop.md
+legacy_specs: []
+---
+
+# Implementation Plan: Fix Nest under candidates
+
+## Overview
+
+The sidebar can incorrectly disable `Nest under` for two independent reasons. Its context menu
+prefers any present multi-workflow snapshot even when that snapshot is a partial placeholder, and
+the shared eligibility helper applies the one-level Kanban rule to Office tasks despite the
+backend's explicit Office depth exemption. The correction preserves Office identity through the
+frontend projection, uses the rendered task group for menu and drag candidates, and makes their
+shared pure filter match the backend depth contract. Review validation also requires the complete
+unfiltered hierarchy for cycle checks, stable candidate access that preserves row memoization, and
+an explicit Office discriminator on canonical task updates after project reassignment.
+
+No endpoint, persistence schema, localization, or layout change is required.
+
+The smallest reproductions are focused frontend tests. An Office subject with a child and an
+eligible Office root produces an empty candidate array, while a context menu rendered beside an
+eligible task cannot find that task when the raw multi-workflow snapshot contains only the subject.
+Before the correction, the focused command reports two failures and 28 passes:
+
+```bash
+(cd apps/web && pnpm test -- lib/sidebar/nest-candidates.test.ts components/task/task-switcher-context-menu.test.tsx)
+```
+
+The permanent regressions are `allows deep Office targets and excludes descendants` in
+`lib/sidebar/nest-candidates.test.ts` and `uses rendered candidates when the workflow snapshot is
+partial` in `components/task/task-switcher-nest-context-menu.test.tsx`.
+
+## Scope
+
+### In scope
+
+- Preserve the existing backend Office discriminator through the sidebar task projection.
+- Publish the canonical task update after Office project reassignment and serialize its Office
+  discriminator explicitly in both directions.
+- Make menu and drag candidates share the rendered task group and eligibility helper.
+- Validate descendants and child depth against the complete pre-filter hierarchy without adding
+  hidden tasks to the candidate list.
+- Preserve memoized row stability while menus and drags read the latest committed candidate data.
+- Match the backend's existing Office depth exemption and retain Kanban's one-level rule.
+- Prove the correction with focused tests and an Office sidebar browser flow.
+
+### Out of scope
+
+- Backend validation, API shape, persistence, and workspace materialization.
+- Kanban card drag, sibling reorder, and the separate Office list parent picker.
+- New controls, menu copy, layout, touch gestures, or navigation.
+
+## Technical approach
+
+1. Preserve the DTO's optional `is_from_office` value through `toKanbanTask`, the Kanban snapshot
+   type, desktop and phone sidebar projections, and `TaskSwitcherItem` as `isFromOffice`. Preserve
+   the cached value when a lightweight `task.updated` event omits the field and when a partial
+   active-board task is reconciled with a full multi-workflow projection.
+2. Extend `computeNestCandidates` with explicit descendant detection against a separate complete
+   hierarchy. Preserve the current Kanban root-only and childless-subject rules, while allowing
+   arbitrary-depth Office candidates when either endpoint is Office.
+3. Pass the flattened rendered `groupTasks` and complete pre-filter hierarchy through stable getters
+   in the task-tree composition. Remove the menu's direct raw-snapshot selection. Keep drag target
+   discovery on the same collections and helper without rerendering unaffected rows.
+4. Add `is_from_office` explicitly to canonical task events and publish one after Office project
+   assignment or removal so live caches observe both transitions.
+5. Add focused projection, event, helper, menu, drag, and render-stability tests. Add an Office E2E
+   that reproduces the disabled submenu with a parent task and proves successful deeper nesting.
+
+## ASCII UI preview
+
+### UI-01: `Nest under` submenu with a visible eligible Office target
+
+Current behavior established by the focused component reproduction:
+
+```text
+Nest under  >  +--------------------+
+                 | No other tasks     |  disabled
+                 +--------------------+
+```
+
+Required behavior:
+
+```text
+Nest under  >  +--------------------+
+                 | Office target      |  selectable
+                 +--------------------+
+```
+
+For a Kanban subject that already has a child, the disabled `No other tasks` state remains correct.
+Desktop retains the anchored submenu shown above. On phone, the existing inset menu sheet renders
+the same candidate row and retains its existing touch target, scroll, dismissal, and safe-area
+behavior. Spacing is illustrative; candidate presence and eligibility are required by
+AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.1 through .5.
+
+## Tests
+
+- AC `AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.1` and
+  `AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.2`:
+  `task-switcher-nest-context-menu.test.tsx`, `uses rendered candidates when the workflow snapshot
+is partial`; `task-switcher-subtask-dnd.test.ts`, `matches menu candidates for Office and Kanban
+tasks`.
+- AC `AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.3`: `nest-candidates.test.ts`, `keeps the
+one-level Kanban limit`.
+- AC `AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.4`: `map-task.test.ts`, `maps is_from_office to
+isFromOffice`;
+  `tasks-office-identity.test.ts`, `preserves Office identity when task.updated omits it`;
+  `task-session-sidebar-aggregate.test.ts`, `preserves projected Office identity through a newer
+partial active task`;
+  `task-session-sidebar-item.test.ts`, `preserves isFromOffice`;
+  `session-task-switcher-sheet-item.test.ts`, `carries Office identity into the phone task drawer
+row`;
+  `nest-candidates.test.ts`, `allows deep Office targets and excludes descendants`.
+- Filtered hierarchy and stable composition: `nest-candidates.test.ts`, `uses the complete hierarchy
+to exclude descendants hidden by the current view`; `task-switcher-render-stability.test.tsx`,
+  `gives a skipped row the latest nest candidates`.
+- Live Office identity: backend dashboard and task-service event tests cover canonical publication
+  for project assignment/removal and explicit true/false serialization; the frontend handler test
+  covers clearing the cached identity.
+- AC `AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.5`: the shared component tests render the
+  existing menu and drag paths without separate desktop and phone candidate logic.
+
+## E2E tests
+
+- AC `AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.1` through
+  `AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.4`: add `office-sidebar-nest-task.spec.ts`, Chromium
+  scenario `nests an Office parent under another Office task`, asserting the target row, persisted
+  parent, and deeper rendered hierarchy.
+- AC `AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.3`: rerun `sidebar-nest-task.spec.ts` in
+  Chromium as the shallow Kanban regression.
+- AC `AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.5`: rerun
+  `mobile-subtask-reparent-drag-drop.spec.ts` and add `mobile-office-sidebar-nest-task.spec.ts` in
+  `mobile-chrome` to preserve the shared touch path and prove the Office candidate reaches the
+  phone task sheet.
+
+## Work orders
+
+- [x] [Task 01: Correct Nest under candidates](task-01-correct-nest-under-candidates.md)
+
+One sequential work order owns the projection, shared algorithm, both interaction paths, and browser
+proof because each layer is required for one observable result. A wave does not authorize
+delegation.
+
+## Verification results
+
+Implemented in one work order. The focused Vitest suite passes 107 tests across nine files, and the
+full frontend suite passes 18,454 tests with four skipped. Typecheck, full frontend lint, the i18n
+ratchet, and the complete Office dashboard and task service Go package tests pass. Chromium passes
+the existing Kanban nesting flow and the new Office depth-two nesting and reload flow. The
+`mobile-chrome` project passes the existing touch re-parenting cases and the new Office phone-sheet
+flow. Specification validation and the scoped diff check also pass.
+
+## Risks
+
+- The backend remains the final validator for races and hierarchy data absent from the complete
+  client projection; rejected mutations must retain the existing rollback and error path.
+- Reading a raw snapshot in either consumer would recreate the menu/drag divergence.
+- Missing Office metadata must default to non-Office so the correction cannot relax Kanban rules.
diff --git a/docs/plans/fix-nest-under-candidates/task-01-correct-nest-under-candidates.md b/docs/plans/fix-nest-under-candidates/task-01-correct-nest-under-candidates.md
new file mode 100644
index 0000000000..d3cdf85028
--- /dev/null
+++ b/docs/plans/fix-nest-under-candidates/task-01-correct-nest-under-candidates.md
@@ -0,0 +1,171 @@
+---
+id: "01-correct-nest-under-candidates"
+title: "Correct Nest under candidates"
+status: done
+wave: 1
+depends_on: []
+plan: "plan.md"
+requirements:
+  - REQ-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001
+acceptance_criteria:
+  - AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.1
+  - AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.2
+  - AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.3
+  - AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.4
+  - AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.5
+system_design:
+  - ../../specs/tasks/system-design/subtask-reparenting-drag-drop.md
+---
+
+# Task 01: Correct Nest under candidates
+
+## Summary
+
+The sidebar context menu and drag interaction offer the same eligible tasks from the rendered group,
+including arbitrary-depth Office targets, while Kanban keeps its one-level hierarchy.
+
+## In scope
+
+- Preserve the backend Office discriminator through HTTP mapping, lightweight WebSocket merges,
+  active/snapshot reconciliation, and the sidebar projection.
+- Make canonical task updates serialize Office identity explicitly and publish after project
+  assignment or removal.
+- Make `computeNestCandidates` distinguish Office from Kanban and reject descendants through the
+  complete pre-filter hierarchy.
+- Supply the rendered group and complete hierarchy through stable getters, preserving row
+  memoization, and remove the context menu's independent raw-snapshot lookup.
+- Keep drag target discovery on the same group and helper.
+- Add focused unit, component, and Office E2E regression coverage.
+
+## Out of scope
+
+- Backend validation, task PATCH semantics, and persistence.
+- Kanban card drag, sibling reorder, Office list parent picker, menu copy, and menu geometry.
+- Lifting the one-level Kanban hierarchy limit.
+
+## Acceptance
+
+- A visible eligible task remains selectable from `Nest under` when the corresponding raw
+  multi-workflow snapshot is partial, and drag exposes exactly the same candidate identifiers.
+- Office subjects with children and Office targets at any depth are eligible except for self,
+  current parent, and descendants, including a descendant behind a filtered intermediate ancestor;
+  the existing Kanban root-only and childless-subject behavior remains unchanged.
+- Unaffected rows retain their memoization while a subsequently opened menu sees current candidates,
+  and project assignment/removal updates cached Office identity without a reload.
+- The focused frontend suites, Office browser scenario, typecheck, lint, i18n ratchet, and spec
+  validation pass.
+
+## ASCII UI preview
+
+### UI-01: `Nest under` submenu with a visible eligible Office target
+
+See the combined preview in
+[plan.md](plan.md#ui-01-nest-under-submenu-with-a-visible-eligible-office-target).
+
+```text
+Before                         After
++--------------------+        +--------------------+
+| No other tasks     |        | Office target      |
+| disabled           |        | selectable         |
++--------------------+        +--------------------+
+```
+
+Desktop keeps its anchored submenu. Phone uses the same candidate row in the existing inset menu
+sheet. No spacing, control, or navigation change is part of this work order.
+
+## Verification
+
+```bash
+(cd apps && pnpm install --frozen-lockfile)
+(cd apps/web && pnpm test -- lib/kanban/map-task.test.ts lib/ws/handlers/tasks-office-identity.test.ts lib/sidebar/nest-candidates.test.ts components/task/task-session-sidebar-aggregate.test.ts components/task/task-session-sidebar-item.test.ts components/task/mobile/session-task-switcher-sheet-item.test.ts components/task/task-switcher-subtask-dnd.test.ts components/task/task-switcher-context-menu.test.tsx components/task/task-switcher-nest-context-menu.test.tsx)
+(cd apps/web && pnpm run typecheck)
+(cd apps/web && pnpm run lint)
+(cd apps/web && pnpm run i18n:ratchet)
+(cd apps/backend && go test ./internal/office/dashboard ./internal/task/service)
+(cd apps/web && pnpm e2e:run tests/task/sidebar-nest-task.spec.ts tests/task/office-sidebar-nest-task.spec.ts)
+(cd apps/web && pnpm e2e:run --project mobile-chrome tests/task/mobile-subtask-reparent-drag-drop.spec.ts tests/task/mobile-office-sidebar-nest-task.spec.ts)
+python3 scripts/list-docs.py validate
+python3 scripts/lint-spec-files.py --all
+git diff --check -- docs/specs/tasks docs/plans/fix-nest-under-candidates apps/web
+```
+
+Run the complete block from the repository root.
+
+## Files likely touched
+
+- `apps/web/lib/kanban/map-task.ts`
+- `apps/web/lib/kanban/map-task.test.ts`
+- `apps/web/lib/state/slices/kanban/types.ts`
+- `apps/web/lib/ws/handlers/task-merge.ts`
+- `apps/web/lib/ws/handlers/tasks-office-identity.test.ts`
+- `apps/web/lib/sidebar/nest-candidates.ts`
+- `apps/web/lib/sidebar/nest-candidates.test.ts`
+- `apps/web/components/task/task-switcher-types.ts`
+- `apps/web/components/task/task-session-sidebar-aggregate.ts`
+- `apps/web/components/task/task-session-sidebar-aggregate.test.ts`
+- `apps/web/components/task/task-session-sidebar-item.ts`
+- `apps/web/components/task/task-session-sidebar-item.test.ts`
+- `apps/web/components/task/mobile/session-task-switcher-sheet-item.ts`
+- `apps/web/components/task/mobile/session-task-switcher-sheet-item.test.ts`
+- `apps/web/components/task/task-switcher-tree.tsx`
+- `apps/web/components/task/task-switcher.tsx`
+- `apps/web/components/task/task-switcher-render-stability.test.tsx`
+- `apps/web/components/task/task-session-sidebar-switcher-props.ts`
+- `apps/web/components/task/mobile/session-task-switcher-sheet-props.ts`
+- `apps/web/components/task/task-switcher-row.tsx`
+- `apps/web/components/task/task-switcher-context-menu.tsx`
+- `apps/web/components/task/task-switcher-context-menu-items.tsx`
+- `apps/web/components/task/task-nest-context-menu.tsx`
+- `apps/web/components/task/task-switcher-context-menu.test.tsx`
+- `apps/web/components/task/task-switcher-nest-context-menu.test.tsx`
+- `apps/web/components/task/task-switcher-subtask-dnd.tsx`
+- `apps/web/components/task/task-switcher-subtask-dnd.test.ts`
+- `apps/web/e2e/tests/task/office-sidebar-nest-task.spec.ts`
+- `apps/web/e2e/tests/task/mobile-office-sidebar-nest-task.spec.ts`
+- `apps/backend/internal/office/dashboard/service_tasks.go`
+- `apps/backend/internal/office/dashboard/task_updated_events_test.go`
+- `apps/backend/internal/task/service/service_events.go`
+- `apps/backend/internal/task/service/service_events_test.go`
+
+Use fewer files when the rendered-group value can be threaded through an existing context or prop.
+Do not introduce a general task registry for this local composition.
+
+## Dependencies
+
+None.
+
+## Parallelism
+
+`sequential`
+
+## Inputs
+
+- Requirement acceptance criteria and scenarios in
+  `docs/specs/tasks/requirements/subtask-reparenting-drag-drop.md`.
+- Candidate-source and eligibility contracts in
+  `docs/specs/tasks/system-design/subtask-reparenting-drag-drop.md`.
+- Existing canonical mutation path in `apps/web/hooks/use-nest-task.ts` and backend validation in
+  `apps/backend/internal/task/service/service_tasks.go`.
+
+## Risks
+
+- A cycle filter based only on visible tasks would expose deeper descendants when a view hides an
+  intermediate ancestor; follow ancestor chains in the complete hierarchy with a visited set.
+- Defaulting a missing Office discriminator to true would relax Kanban rules; absent values remain
+  non-Office.
+- A menu-local store selector would let menu and drag diverge again; both consume the rendered
+  group supplied by their common tree composition.
+- Passing changing candidate arrays directly to every row would defeat memoization; stable getters
+  must resolve the latest committed arrays at interaction time.
+
+## Results
+
+Implemented the shared rendered-group candidate path, Office-aware hierarchy filtering, Office
+identity projection and partial-update preservation, and descendant-cycle prevention. The new
+browser scenario proves an Office parent with a child can be nested under another Office task,
+persists the relationship, renders depth two, and survives reload.
+
+Verification passed: frozen install; 107 focused Vitest tests across nine files; the full frontend
+suite with 18,454 passing tests and four skipped; TypeScript typecheck; full frontend lint; i18n
+ratchet; complete Office dashboard and task service Go package tests; desktop Chromium and
+`mobile-chrome` nesting flows; both specification validators; and the scoped diff check.
diff --git a/docs/specs/tasks/README.md b/docs/specs/tasks/README.md
index a547b05308..3102237fc7 100644
--- a/docs/specs/tasks/README.md
+++ b/docs/specs/tasks/README.md
@@ -171,6 +171,7 @@ signals, and task-scoped scheduling contracts.
 - [Prompt attachments](system-design/prompt-attachments.md)
 - [Task Actions Menu on Preview and Detail Surfaces](system-design/task-actions-menu.md)
 - [Task menu grouping](system-design/task-menu-grouping.md)
+- [Subtask re-parenting by drag and drop](system-design/subtask-reparenting-drag-drop.md)
 - [Task Archive Confirmation](system-design/archive-confirmation.md)
 - [Task removal navigation](system-design/removal-navigation.md)
 - [Task plan content size limit](system-design/plan-content-size-limit.md)
diff --git a/docs/specs/tasks/requirements/subtask-reparenting-drag-drop.md b/docs/specs/tasks/requirements/subtask-reparenting-drag-drop.md
index bf5d950d93..733ee8cde4 100644
--- a/docs/specs/tasks/requirements/subtask-reparenting-drag-drop.md
+++ b/docs/specs/tasks/requirements/subtask-reparenting-drag-drop.md
@@ -5,6 +5,7 @@ created: 2026-08-04
 owners:
   - kandev
 ---
+
 # Subtask re-parenting by drag and drop Requirements
 
 ## Overview
@@ -15,11 +16,30 @@ This document is the migrated task-system source for the capability. The source
 
 ### REQ-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001: Subtask re-parenting by drag and drop
 
-**Intent:** Preserve the observable task or workflow behavior recorded by the legacy specification.
+**Intent:** Let users re-parent tasks from the sidebar menu or drag gesture while presenting every
+eligible target that is already visible in the task group.
 
 #### Acceptance criteria
 
-- **AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.1:** When a consumer uses this capability, the system shall provide the observable behavior and exclusions documented below.
+- **AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.1:** The sidebar `Nest under` submenu and the
+  drag nest zones shall derive their targets from the same rendered task group and the same
+  eligibility rules.
+- **AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.2:** When an eligible same-workflow target is
+  visible in the rendered task group, the target shall remain available while workflow snapshots
+  are loading, refreshing, or temporarily incomplete; the submenu shall show `No other tasks` only
+  when that rendered group has no eligible target.
+- **AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.3:** For non-Office tasks, an eligible target shall
+  be a root task, and a task that already has children shall have no nest target, preserving the
+  one-level Kanban hierarchy.
+- **AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.4:** When the subject or target is an Office task,
+  the UI shall allow arbitrary-depth re-parenting while excluding the subject, its current parent,
+  and its descendants, including descendants whose intermediate ancestors are hidden by the active
+  sidebar view. The backend remains authoritative for self, cycle, archive, workspace, and
+  concurrency validation. Live project assignment and removal shall update the cached Office
+  identity in both directions.
+- **AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.5:** Desktop sidebar and mobile task-switcher
+  presentations shall use the same menu and drag eligibility without changing their existing
+  pointer, touch, focus, dismissal, or scrolling behavior.
 
 ## Migrated source detail
 
@@ -31,7 +51,13 @@ Users can detach a subtask or nest a task under another via context-menu actions
 
 - The sidebar task tree (desktop sidebar and the mobile task switcher sheet) lets a user re-parent a task by dragging its row onto another row's **nest drop zone**.
 - The result is strictly equivalent to choosing `Un-nest (remove parent)` then `Nest under <target>` from the task's context menu: the task's parent becomes the target, and a task whose workspace mode is `inherit_parent` ends with mode `shared_group` — its materialized workspace and workspace-group membership are unchanged.
-- Drop targets are exactly the candidates the context menu offers (the `computeNestCandidates` rules): same-workflow root tasks in the visible group, excluding the dragged task, its current parent, and any subtask (which also excludes the task's own descendants). A task that already has children offers no nest targets.
+- Drop targets are exactly the candidates the context menu offers (the `computeNestCandidates`
+  rules) from the same-workflow tasks in the rendered group. The complete unfiltered task
+  hierarchy is used only to validate ancestry and depth, so filtering an intermediate ancestor
+  cannot expose a descendant as a target. Every mode excludes the subject and its current parent.
+  Kanban candidates must be roots and a Kanban subject with children has no targets. Office
+  candidates may be at any depth, and an Office subject may keep its descendants, but its
+  descendants are excluded as targets so the UI cannot offer a known cycle.
 - While a drag with valid targets is active, candidate rows show a nest drop zone (a left-edge strip) with a `Nest under <title>` affordance. Dropping on a zone re-parents; dropping between rows keeps the existing sibling-reorder behavior; any other drop is a no-op.
 - Re-parenting is a single API call on the existing canonical path (`PATCH /api/v1/tasks/:id` with `parent_id`), which already rejects self-parenting, missing/archived/cross-workspace targets, descendant cycles, and one-level-depth violations for kanban tasks.
 - The sidebar `Nest under` menu, the Office parent picker, the WS task-update path, and the Office dashboard PATCH all share the same composite semantics: any effective parent change normalizes an `inherit_parent` workspace mode to `shared_group`.
@@ -56,11 +82,18 @@ No new endpoint. Reuses and extends existing contracts:
 
 - `PATCH /api/v1/tasks/:id` with `parent_id` (non-empty nests; `""` un-nests) — already validated by `Service.resolveParentID` (self, existence, archived, same workspace, descendant cycle) and `validateReparentDepth` (one-level kanban limit, Office trees exempt). **Behavior addition:** when the effective parent changes, `inherit_parent` workspace mode is normalized to `shared_group` (mirroring the detach operation). Success returns the updated task DTO; invalid targets map to `400`; missing task to `404`.
 - `PATCH /api/v1/office/tasks/:id` with non-empty `parent_id` — same normalization added for parity; empty parent continues to route through the canonical detach operation.
-- WS `task.updated` payload unchanged: `parent_id` is always present (nil when cleared), and `metadata` carries the normalized workspace mode.
+- WS `task.updated` includes an explicit `is_from_office` boolean so either Office-identity
+  transition clears or establishes the cached value. `parent_id` is always present (nil when
+  cleared), and `metadata` carries the normalized workspace mode. Office project reassignment
+  publishes this canonical event after the write.
 
 ## Failure modes
 
-- **Invalid target** (self, descendant, subtask, archived, missing, cross-workspace, or depth violation): the UI filters these targets out before a drop can land, so a drop outside every nest zone is a no-op. If a request is nevertheless rejected by the backend (e.g. a valid-zone target became invalid between render and drop), the UI keeps the task in its original tree position, rolls back the optimistic update, and shows a request-error toast.
+- **Invalid target** (self, descendant, archived, missing, cross-workspace, or a Kanban depth
+  violation): the UI filters known-invalid targets before a drop can land, so a drop outside every
+  nest zone is a no-op. If a request is nevertheless rejected by the backend (for example, a
+  valid-zone target became invalid between render and drop), the UI keeps the task in its original
+  tree position, rolls back the optimistic update, and shows a request-error toast.
 - **Persistence failure**: no successful response is returned; the UI rolls back to the original tree.
 - **Concurrent submissions** are safe: setting the same parent twice is idempotent, and the optimistic update is reconciled by the authoritative `task.updated` event.
 - **No valid targets**: the drag offers no nest zones; only reorder remains possible.
@@ -70,8 +103,22 @@ No new endpoint. Reuses and extends existing contracts:
 - **GIVEN** a subtask `C` under parent `A` and a root task `B` in the same workflow, **WHEN** the user drags `C` onto `B`'s nest drop zone, **THEN** `C`'s parent becomes `B`, the sidebar shows `C` nested under `B`, and no reload occurs.
 - **GIVEN** an `inherit_parent` subtask `C` under `A`, **WHEN** `C` is dragged onto root `B`'s nest zone, **THEN** `C`'s persisted workspace mode is `shared_group` and its workspace-group membership is unchanged.
 - **GIVEN** a root task with no children, **WHEN** it is dragged onto another root's nest zone, **THEN** it becomes that root's subtask.
-- **GIVEN** a task that already has children, **WHEN** it is dragged, **THEN** no row offers a nest drop zone (reorder only).
-- **GIVEN** a drag over a subtask row or over a task in a different workflow, **WHEN** the pointer rests on the row, **THEN** no nest drop zone is offered.
+- **GIVEN** a Kanban task that already has children, **WHEN** it is dragged, **THEN** no row offers a nest drop zone (reorder only).
+- **GIVEN** a Kanban drag over a subtask row or a drag over a task in a different workflow, **WHEN** the pointer rests on the row, **THEN** no nest drop zone is offered.
+- **GIVEN** an Office task that already has a child and another eligible Office task in the same
+  rendered group, **WHEN** the user opens `Nest under` or starts a re-parent drag, **THEN** the
+  other task is offered and the resulting Office tree may be deeper than one level.
+- **GIVEN** an eligible target is visible while the stored multi-workflow snapshot is temporarily
+  incomplete, **WHEN** the user opens `Nest under`, **THEN** the visible target is offered instead
+  of a disabled `No other tasks` row.
+- **GIVEN** an Office task and one of its descendants, **WHEN** the menu or drag targets are shown,
+  **THEN** that descendant is not offered as a parent.
+- **GIVEN** a sidebar filter hides the intermediate task between an Office subject and a visible
+  descendant, **WHEN** the menu or drag targets are shown, **THEN** the visible descendant is still
+  excluded as a parent.
+- **GIVEN** a Kanban task is assigned to or removed from an Office project, **WHEN** the canonical
+  `task.updated` event arrives, **THEN** open sidebar caches immediately adopt the explicit new
+  Office identity without a reload.
 - **GIVEN** a subtask dragged toward its current parent, **WHEN** the pointer rests on that parent row, **THEN** no nest drop zone is offered.
 - **GIVEN** a drag dropped between two sibling rows, **WHEN** the drop lands outside every nest zone, **THEN** the siblings reorder as before.
 - **GIVEN** a drop that lands outside every nest zone, **WHEN** the drop completes, **THEN** the task keeps its original parent and no request is sent (a plain no-op); a request-error toast appears only when a valid-zone drop's request is rejected by the backend.
@@ -91,4 +138,5 @@ No new endpoint. Reuses and extends existing contracts:
 
 ## Implementation plan
 
-See [the implementation plan](../../../plans/subtask-reparenting-drag-drop/plan.md).
\ No newline at end of file
+See the original [drag-and-drop implementation plan](../../../plans/subtask-reparenting-drag-drop/plan.md)
+and the [Nest under candidate correction plan](../../../plans/fix-nest-under-candidates/plan.md).
diff --git a/docs/specs/tasks/system-design/subtask-reparenting-drag-drop.md b/docs/specs/tasks/system-design/subtask-reparenting-drag-drop.md
new file mode 100644
index 0000000000..002cd36743
--- /dev/null
+++ b/docs/specs/tasks/system-design/subtask-reparenting-drag-drop.md
@@ -0,0 +1,115 @@
+---
+status: current
+system: tasks
+requirements:
+  - REQ-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001
+---
+
+# Subtask re-parenting by drag and drop system design
+
+## Purpose and boundaries
+
+This design owns sidebar candidate discovery for `Nest under` and drag-to-re-parent. It makes the
+rendered task group the shared candidate source and carries the existing Office discriminator into
+that view model so the UI can apply the same depth boundary as the backend. The complete unfiltered
+task collection supplies hierarchy metadata for cycle and depth validation without adding hidden
+tasks to the candidate list.
+
+The canonical parent mutation, workspace-mode normalization, optimistic update, rollback, and
+WebSocket reconciliation continues to use the canonical task event. Office project reassignment
+now invokes that publisher, and the event carries the authoritative Office identity explicitly.
+The persistence boundaries are described by [Detached Workspace Continuity](detached-workspace-continuity.md).
+No endpoint, database column, feature flag, or user-facing copy is added.
+
+## Requirement mapping
+
+| Criteria                                            | Design section                          |
+| --------------------------------------------------- | --------------------------------------- |
+| AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.1 and .2 | Candidate source and propagation        |
+| AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.3 and .4 | Eligibility algorithm                   |
+| AC-TASKS-SUBTASK-REPARENTING-DRAG-DROP-001.5        | Responsive interaction and verification |
+
+## Candidate source and propagation
+
+The task HTTP DTO already exposes `is_from_office`. `toKanbanTask` preserves it as
+`isFromOffice`, the Kanban snapshot task type owns that property, and both desktop and phone
+sidebar projections preserve it on `TaskSwitcherItem`. This is a data projection only; Office
+identity continues to come from the backend.
+
+The mapped property remains optional on partial inputs: candidate checks treat an absent value as
+non-Office, and `mergeTaskUpdate` retains the cached value when a lightweight event omits
+`is_from_office`. Canonical `task.updated` events always carry an explicit true or false value, so
+project assignment and removal can establish or clear the cached identity. The Office project
+mutation publishes that canonical event after its task-row write. Active-board and multi-workflow
+reconciliation uses the same absent-value fallback so an unrelated partial event cannot erase
+Office identity from a full snapshot.
+
+`GroupSection` already flattens the roots and descendants that it renders into `groupTasks` for
+drag candidate calculation. That collection is the candidate source for both drag and context-menu
+composition. Desktop and phone also pass their complete pre-view task list as hierarchy metadata.
+The menu must not reread `kanbanMulti.snapshots[workflowId].tasks`: a present placeholder or
+partially refreshed snapshot can contain fewer tasks than the reconciled group visible beside the
+menu.
+
+Rows receive stable getter functions for both collections. The owning switcher and group update
+their refs after each committed render, so opening a menu or starting a drag reads current data
+without making every memoized sibling row rerender when one task changes.
+
+Candidate order follows rendered group order. Candidates remain scoped to the subject's workflow,
+so mixed-workflow sidebar groups cannot create an invalid target.
+
+## Eligibility algorithm
+
+`computeNestCandidates` remains the shared pure function. Its input includes `id`, `parentTaskId`,
+and `isFromOffice`. It applies these rules in order:
+
+1. Resolve the subject from the complete hierarchy, falling back to the rendered group. With no
+   subject, return no candidates.
+2. Exclude the subject and its current parent in every mode.
+3. Exclude every rendered task whose ancestor chain reaches the subject. Follow parent links in the
+   complete hierarchy, including ancestors hidden by filtering, and track visited identifiers so
+   corrupt hierarchy data cannot loop in the browser.
+4. When neither the subject nor candidate is an Office task, require the candidate to be a root and
+   return no candidates if the subject has a child. This preserves the one-level Kanban boundary.
+5. When either endpoint is an Office task, allow a candidate at any depth and allow a subject that
+   has children. This mirrors `validateReparentDepth`, which exempts the mutation when either
+   endpoint is Office.
+
+The browser filter is an affordance, not an authorization boundary. `PATCH /api/v1/tasks/:id`
+continues to reject stale, archived, missing, cross-workspace, self, and cyclic targets.
+
+## Mutation and failure flow
+
+The context-menu selection and the drag nest zone call the existing `useNestTask` path with the
+subject workflow and target identifier. The request uses the canonical task PATCH and the existing
+optimistic snapshot update. A rejected request restores the prior tree and shows the existing
+request error. A successful `task.updated` event reconciles the sidebar, board, and task detail.
+
+A temporary multi-workflow placeholder cannot suppress a target that is already present in the
+rendered group. A filtered-out task never becomes a candidate, but it remains available as ancestry
+metadata. If the rendered group itself contains no eligible task, the menu shows its existing
+disabled `No other tasks` row and drag shows no nest zone.
+
+## Responsive interaction
+
+Desktop and phone continue to use the same task-row context-menu and drag components. Desktop keeps
+the anchored submenu; the phone task switcher keeps its inset sheet, touch drag sensor, menu event
+containment, internal scroll owner, safe-area spacing, and current touch targets. Candidate data
+changes do not add controls or alter geometry.
+
+## Verification
+
+Pure tests cover Office and Kanban eligibility, mixed Office/Kanban endpoints, filtered intermediate
+ancestors, descendant exclusion, input order, and missing subjects. Projection and event tests prove
+`is_from_office` reaches `TaskSwitcherItem`, survives omitted fields, and changes explicitly in both
+directions. Component tests prove the context menu uses current rendered-group candidates without
+rerendering unaffected rows, and that menu and drag return the same candidates.
+
+The existing Kanban sidebar E2E remains a shallow-hierarchy regression. Focused desktop and phone
+Office sidebar E2Es create a parent with a child and another target, select that target from `Nest
+under`, and assert the deeper persisted and rendered hierarchy. The existing mobile re-parenting
+E2E continues to cover the shared touch drag path.
+
+## Implementation Plans
+
+- [Fix Nest under candidates](../../../plans/fix-nest-under-candidates/plan.md)