FN-6910: hide subtask quick-add behind experiment

Hide the AI subtask-breakdown quick-add action unless the experimental flag is explicitly enabled.

- Gate Subtask Breakdown callbacks from App on the default-off subtaskBreakdown experiment.
- Remove Subtask quick-add buttons from inline create and quick entry surfaces when no callback is wired.
- Add settings, quick-add, documentation, and changeset coverage for the new experiment toggle.

Files changed:
 .changeset/fn-6910-subtask-breakdown-flag.md       |  5 ++++
 docs/dashboard-guide.md                            |  2 +-
 packages/dashboard/app/App.tsx                     |  8 ++++---
 .../dashboard/app/components/InlineCreateCard.tsx  | 27 ++++++++++++----------
 .../dashboard/app/components/QuickEntryBox.tsx     | 27 ++++++++++++----------
 .../dashboard/app/components/SettingsModal.tsx     |  2 ++
 .../components/__tests__/InlineCreateCard.test.tsx | 13 +++++++++++
 .../components/__tests__/QuickEntryBox.test.tsx    | 12 ++++++++++
 .../components/__tests__/SettingsModal.test.tsx    | 13 +++++++++++
 9 files changed, 81 insertions(+), 28 deletions(-)

Fusion-Task-Id: FN-6910

Fusion-Task-Lineage: a7738b1f-7b41-438d-bd94-79ef56928262

.changeset/fn-6910-subtask-breakdown-flag.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Hide the dashboard AI subtask-breakdown quick-add button behind the default-off `subtaskBreakdown` experimental feature flag.

docs/dashboard-guide.md

@@ -259,7 +259,7 @@ Rules:
 - `Merge target / base branch` stays optional for all modes and uses the same branch-dropdown + `Custom…` fallback behavior as Planning Mode.
 - In **More options → Model Configuration**, **Auto-merge** is a per-task override with three states: **Default** (follow project setting), **Enabled**, or **Disabled**.
 
-The dialog also exposes the board quick-add AI handoffs: **Plan** opens Planning Mode with the current description, and **Subtask** opens Subtask Breakdown with the current description. Both buttons remain disabled until the description has content, matching the quick-add row behavior.
+The dialog also exposes the board quick-add AI handoffs: **Plan** opens Planning Mode with the current description, and **Subtask** opens Subtask Breakdown with the current description when **Settings → Experimental Features → Subtask Breakdown** is enabled. The Subtask handoff is hidden by default; visible handoff buttons remain disabled until the description has content, matching the quick-add row behavior.
 
 ## Chat View
 

packages/dashboard/app/App.tsx

@@ -1029,6 +1029,8 @@ function AppInner() {
   const nodesEnabled = experimentalFeatures.nodesView === true;
   const researchEnabled = experimentalFeatures.researchView === true;
   const evalsEnabled = experimentalFeatures.evalsView === true;
+  /* FNXC:QuickAddSubtaskFlag 2026-06-21-00:00: Missing or false `subtaskBreakdown` settings must hide the AI Subtask quick-add handoff across List, Board, and New Task Modal surfaces; only an explicit true wires the callback. */
+  const subtaskBreakdownEnabled = experimentalFeatures.subtaskBreakdown === true;
   /*
   FNXC:Navigation 2026-06-19-00:00:
   Experimental left sidebar navigation replaces the Header view shortcuts with a persistent sidebar on non-mobile project screens, while mobile continues to use the bottom navigation bar as the only primary navigation surface.
@@ -1861,7 +1863,7 @@ function AppInner() {
             onQuickCreate={handleBoardQuickCreate}
             onNewTask={openNewTaskWithNav}
             onPlanningMode={openPlanningWithInitialPlanWithNav}
-            onSubtaskBreakdown={openSubtaskBreakdownWithNav}
+            onSubtaskBreakdown={subtaskBreakdownEnabled ? openSubtaskBreakdownWithNav : undefined}
             autoMerge={autoMerge}
             onToggleAutoMerge={toggleAutoMerge}
             globalPaused={globalPaused}
@@ -1915,7 +1917,7 @@ function AppInner() {
           onNewTask={openNewTaskWithNav}
           onQuickCreate={handleBoardQuickCreate}
           onPlanningMode={openPlanningWithInitialPlanWithNav}
-          onSubtaskBreakdown={openSubtaskBreakdownWithNav}
+          onSubtaskBreakdown={subtaskBreakdownEnabled ? openSubtaskBreakdownWithNav : undefined}
           availableModels={availableModels}
           favoriteProviders={favoriteProviders}
           favoriteModels={favoriteModels}
@@ -2273,7 +2275,7 @@ function AppInner() {
           handleGitHubImport,
         }}
         onPlanningMode={openPlanningWithInitialPlanWithNav}
-        onSubtaskBreakdown={openSubtaskBreakdownWithNav}
+        onSubtaskBreakdown={subtaskBreakdownEnabled ? openSubtaskBreakdownWithNav : undefined}
         taskOperations={{ moveTask, deleteTask, mergeTask, archiveTask, retryTask, resetTask, duplicateTask }}
         deepLink={{ handleDetailClose }}
         settings={{ prAuthAvailable, autoMerge, themeMode, colorTheme, dashboardFontScalePct, shadcnCustomColors, resolvedThemeMode, setThemeMode, setColorTheme, setDashboardFontScalePct, setShadcnCustomColors }}

packages/dashboard/app/components/InlineCreateCard.tsx

@@ -904,18 +904,21 @@ export function InlineCreateCard({
               <Lightbulb size={12} style={{ verticalAlign: "middle", marginRight: 4 }} />
               {t("inline.plan", "Plan")}
             </button>
-            <button
-              type="button"
-              className="btn btn-sm"
-              onClick={handleSubtaskClick}
-              onMouseDown={(e) => e.preventDefault()}
-              disabled={!description.trim()}
-              data-testid="subtask-button"
-              title={t("inline.breakDownSubtasks", "Break down into AI-generated subtasks")}
-            >
-              <ListTree size={12} style={{ verticalAlign: "middle", marginRight: 4 }} />
-              {t("inline.subtask", "Subtask")}
-            </button>
+            {/* FNXC:QuickAddSubtaskFlag 2026-06-21-00:00: Render no Subtask button or orphaned inline-create click target unless the default-off `subtaskBreakdown` experiment wires this callback. */}
+            {onSubtaskBreakdown && (
+              <button
+                type="button"
+                className="btn btn-sm"
+                onClick={handleSubtaskClick}
+                onMouseDown={(e) => e.preventDefault()}
+                disabled={!description.trim()}
+                data-testid="subtask-button"
+                title={t("inline.breakDownSubtasks", "Break down into AI-generated subtasks")}
+              >
+                <ListTree size={12} style={{ verticalAlign: "middle", marginRight: 4 }} />
+                {t("inline.subtask", "Subtask")}
+              </button>
+            )}
             <div className="dep-trigger-wrap">
               <button
                 type="button"

packages/dashboard/app/components/QuickEntryBox.tsx

@@ -1656,18 +1656,21 @@ export function QuickEntryBox({ onCreate, addToast, tasks = [], availableModels,
               <Lightbulb size={12} style={{ verticalAlign: "middle", marginRight: 4 }} />
               {t("tasks.plan", "Plan")}
             </button>
-            <button
-              type="button"
-              className="btn btn-sm"
-              onClick={handleSubtaskClick}
-              onMouseDown={(e) => e.preventDefault()}
-              disabled={!description.trim()}
-              data-testid="subtask-button"
-              title={t("tasks.subtaskButtonTitle", "Break down into AI-generated subtasks")}
-            >
-              <ListTree size={12} style={{ verticalAlign: "middle", marginRight: 4 }} />
-              {t("tasks.subtask", "Subtask")}
-            </button>
+            {/* FNXC:QuickAddSubtaskFlag 2026-06-21-00:00: Render no Subtask button or click target unless App wires the default-off `subtaskBreakdown` experiment callback. */}
+            {onSubtaskBreakdown && (
+              <button
+                type="button"
+                className="btn btn-sm"
+                onClick={handleSubtaskClick}
+                onMouseDown={(e) => e.preventDefault()}
+                disabled={!description.trim()}
+                data-testid="subtask-button"
+                title={t("tasks.subtaskButtonTitle", "Break down into AI-generated subtasks")}
+              >
+                <ListTree size={12} style={{ verticalAlign: "middle", marginRight: 4 }} />
+                {t("tasks.subtask", "Subtask")}
+              </button>
+            )}
             <div className="refine-trigger-wrap" ref={refineMenuRef}>
               <button
                 type="button"

packages/dashboard/app/components/SettingsModal.tsx

@@ -276,6 +276,8 @@ const KNOWN_EXPERIMENTAL_FEATURES: Record<string, string> = {
   researchView: "Research View",
   evalsView: "Evals View",
   goalsView: "Goals View",
+  /* FNXC:QuickAddSubtaskFlag 2026-06-21-00:00: The AI subtask-breakdown quick-add affordance is exposed only through this default-off experimental flag so missing settings keep every quick-add Subtask button hidden. */
+  subtaskBreakdown: "Subtask Breakdown",
   leftSidebarNav: "Left Sidebar Navigation",
   rightDock: "Right Dock Panel",
   sandbox: "Sandbox (command isolation)",

packages/dashboard/app/components/__tests__/InlineCreateCard.test.tsx

@@ -197,6 +197,7 @@ function renderCard(
     addToast: vi.fn(),
     availableModels: MOCK_MODELS,
     projectId: TEST_PROJECT_ID,
+    onSubtaskBreakdown: vi.fn(),
     ...overrides,
   };
   const result = render(<InlineCreateCard {...props} />);
@@ -859,6 +860,18 @@ describe("InlineCreateCard Plan and Subtask buttons", () => {
     expect((textarea as HTMLTextAreaElement).value).toBe("");
   });
 
+  it("hides the Subtask quick-add action without leaving an inline controls shell when the callback is omitted", () => {
+    renderCard([], { onSubtaskBreakdown: undefined });
+    expandCard();
+
+    const controlsRow = document.querySelector(".inline-create-controls") as HTMLElement;
+    expect(controlsRow).toBeTruthy();
+    expect(screen.queryByTestId("subtask-button")).not.toBeInTheDocument();
+    expect(screen.queryByTitle("Break down into AI-generated subtasks")).not.toBeInTheDocument();
+    expect(controlsRow.contains(screen.getByTestId("plan-button"))).toBe(true);
+    expect(controlsRow.querySelector(".dep-trigger")).toBeTruthy();
+  });
+
   it("shows toast when Plan clicked with empty description (via direct handler call)", () => {
     const addToast = vi.fn();
     const onPlanningMode = vi.fn();

packages/dashboard/app/components/__tests__/QuickEntryBox.test.tsx

@@ -242,6 +242,7 @@ function renderQuickEntryBox(props = {}, { startExpanded = false } = {}) {
     tasks: mockTasks,
     availableModels: MOCK_MODELS,
     projectId: TEST_PROJECT_ID,
+    onSubtaskBreakdown: vi.fn(),
   };
   const result = render(<QuickEntryBox {...defaultProps} {...props} />);
   return { ...result, props: { ...defaultProps, ...props } };
@@ -3349,6 +3350,17 @@ describe("QuickEntryBox", () => {
       expect(actionsContainer.contains(screen.getByTestId("refine-button"))).toBe(true);
     });
 
+    it("hides the Subtask quick-add action without leaving an action-row shell when the callback is omitted", () => {
+      renderQuickEntryBox({ onSubtaskBreakdown: undefined });
+      expandQuickEntry();
+
+      const actionsContainer = screen.getByTestId("quick-entry-actions");
+      expect(screen.queryByTestId("subtask-button")).not.toBeInTheDocument();
+      expect(screen.queryByTitle("Break down into AI-generated subtasks")).not.toBeInTheDocument();
+      expect(actionsContainer.contains(screen.getByTestId("plan-button"))).toBe(true);
+      expect(actionsContainer.contains(screen.getByTestId("refine-button"))).toBe(true);
+    });
+
     it("does not render actions when not expanded", () => {
       renderQuickEntryBox({});
       toggleQuickEntry();

packages/dashboard/app/components/__tests__/SettingsModal.test.tsx

@@ -3777,6 +3777,7 @@ describe("SettingsModal", () => {
       for (const featureLabel of [
         "Research View",
         "Evals View",
+        "Subtask Breakdown",
         "Chat Rooms",
         "Sandbox (command isolation)",
         "Planning-style Agent Onboarding",
@@ -3788,6 +3789,18 @@ describe("SettingsModal", () => {
       expect(screen.getAllByLabelText("Dev Server")).toHaveLength(1);
     });
 
+    it("shows the Subtask Breakdown toggle as off when the setting is missing", async () => {
+      mockFetchSettings.mockResolvedValue({
+        ...defaultSettings,
+        experimentalFeatures: {},
+      });
+
+      renderModal();
+      await openExperimentalFeaturesSection();
+
+      expect(screen.getByLabelText("Subtask Breakdown")).not.toBeChecked();
+    });
+
     it("does not render duplicate Dev Server rows when legacy and canonical keys are both present", async () => {
       mockFetchSettings.mockResolvedValue({
         ...defaultSettings,