diff --git a/doc/plugins/PLUGIN_AUTHORING_GUIDE.md b/doc/plugins/PLUGIN_AUTHORING_GUIDE.md
index 33d4ec2abcf..5f8380f7b19 100644
--- a/doc/plugins/PLUGIN_AUTHORING_GUIDE.md
+++ b/doc/plugins/PLUGIN_AUTHORING_GUIDE.md
@@ -346,9 +346,11 @@ route is active the host collapses the app `<Sidebar/>` to its 64px icon rail
 
 Because the host drives this collapse, a plugin should **not** mount
 `RequestCollapsedSidebar` or otherwise try to collapse the app sidebar itself —
-doing so is redundant and fights the host. The host restores the user's previous
-sidebar state when they navigate away, and an explicit user pin (expanded or
-collapsed) always takes precedence over the route-driven collapse.
+doing so is redundant and fights the host. While your route is active the app
+rail is forced collapsed (its expand toggle is hidden), overriding any user pin
+— a secondary sidebar always collapses the primary. This force never changes the
+user's saved expanded/collapsed preference, so the host restores exactly what
+the user chose as soon as they navigate away.
 
 ## Shared host components
 
diff --git a/doc/plugins/PLUGIN_SPEC.md b/doc/plugins/PLUGIN_SPEC.md
index a247221ac24..ccd72f5e6f0 100644
--- a/doc/plugins/PLUGIN_SPEC.md
+++ b/doc/plugins/PLUGIN_SPEC.md
@@ -1144,9 +1144,13 @@ host's own company-settings sidebar.
 The host owns the collapse. Plugins must not mount `RequestCollapsedSidebar` or
 otherwise attempt to collapse the app sidebar from a `routeSidebar` — the host
 applies the collapse while the route is mounted and restores the previous state
-on navigation away. Collapse precedence is: explicit user pin (expanded or
-collapsed) > route-requested collapse > default expanded, so a user's pin always
-wins over the route-driven collapse.
+on navigation away. The collapse is a **hard invariant**: while a secondary
+sidebar is shown the app rail is forced collapsed and its expand/toggle
+affordance is hidden, *overriding* any user pin. Crucially, this force is
+ephemeral — it never mutates the user's persisted expanded/collapsed preference,
+so navigating back to a normal route restores exactly what the user chose.
+Precedence is therefore: secondary-sidebar force > explicit user pin >
+route-requested collapse (`RequestCollapsedSidebar`) > default expanded.
 
 ## 19.6 Shared Components In `@paperclipai/plugin-sdk/ui`
 
diff --git a/packages/plugins/sdk/README.md b/packages/plugins/sdk/README.md
index ca6e2682ab0..020b0b8dedc 100644
--- a/packages/plugins/sdk/README.md
+++ b/packages/plugins/sdk/README.md
@@ -240,7 +240,7 @@ Adds a navigation-style entry to the main company sidebar navigation area, rende
 
 A contextual sidebar shown while the current route is a plugin page route with the same `routePath`. Use this for full-page plugin workspaces that need their own local navigation. It does **not** replace the app sidebar: the host collapses the main `<Sidebar/>` to its 64px icon rail (still hover/peek-able) and renders your `routeSidebar` in a secondary pane beside it, producing `[ app rail ][ your sidebar ][ content ]`. Receives `PluginRouteSidebarProps` with `context.companyId` and `context.companyPrefix` set to the active company. Requires the `ui.sidebar.register` capability.
 
-Do **not** mount `RequestCollapsedSidebar` (or otherwise try to collapse the app sidebar) from a `routeSidebar` plugin — the host drives the collapse automatically while your route is active and restores the user's preference when they navigate away. A user's explicit pin (expanded or collapsed) always wins over this route-driven collapse.
+Do **not** mount `RequestCollapsedSidebar` (or otherwise try to collapse the app sidebar) from a `routeSidebar` plugin — the host drives the collapse automatically while your route is active and restores the user's preference when they navigate away. The collapse is a hard invariant: a secondary sidebar always forces the app rail collapsed (hiding its expand toggle), overriding any user pin, but it never mutates the user's saved expanded/collapsed preference — that is restored as soon as they leave your route.
 
 #### `sidebarPanel`
 
diff --git a/ui/src/components/Layout.test.tsx b/ui/src/components/Layout.test.tsx
index 57c79b7f12e..0a550ca9869 100644
--- a/ui/src/components/Layout.test.tsx
+++ b/ui/src/components/Layout.test.tsx
@@ -17,7 +17,7 @@ const mockInstanceSettingsApi = vi.hoisted(() => ({
 const mockNavigate = vi.hoisted(() => vi.fn());
 const mockSetSelectedCompanyId = vi.hoisted(() => vi.fn());
 const mockSetSidebarOpen = vi.hoisted(() => vi.fn());
-const mockSetRouteRequestsCollapsed = vi.hoisted(() => vi.fn());
+const mockSetForceCollapsed = vi.hoisted(() => vi.fn());
 const mockCompanyState = vi.hoisted(() => ({
   companies: [{ id: "company-1", issuePrefix: "PAP", name: "Paperclip" }],
   selectedCompany: { id: "company-1", issuePrefix: "PAP", name: "Paperclip" },
@@ -173,11 +173,14 @@ vi.mock("../context/SidebarContext", () => ({
     toggleSidebar: vi.fn(),
     toggleCollapsed: vi.fn(),
     collapsed: mockSidebarState.collapsed,
+    collapseLocked: false,
     peeking: mockSidebarState.peeking,
     setPeeking: mockSetPeeking,
     isMobile: mockSidebarState.isMobile,
+    forceCollapsed: false,
+    setForceCollapsed: mockSetForceCollapsed,
     routeRequestsCollapsed: false,
-    setRouteRequestsCollapsed: mockSetRouteRequestsCollapsed,
+    setRouteRequestsCollapsed: vi.fn(),
   }),
 }));
 
@@ -410,7 +413,7 @@ describe("Layout", () => {
     expect(container.textContent).not.toContain("Instance sidebar");
     expect(container.textContent).not.toContain("Plugin route sidebar");
     // The route asks the host to collapse the app sidebar to its rail.
-    expect(mockSetRouteRequestsCollapsed).toHaveBeenCalledWith(true);
+    expect(mockSetForceCollapsed).toHaveBeenCalledWith(true);
 
     await act(async () => {
       root.unmount();
@@ -474,7 +477,7 @@ describe("Layout", () => {
     expect(container.textContent).toContain("Main company nav");
     expect(container.textContent).not.toContain("Company rail");
     expect(container.textContent).not.toContain("Plugin route sidebar");
-    expect(mockSetRouteRequestsCollapsed).toHaveBeenCalledWith(true);
+    expect(mockSetForceCollapsed).toHaveBeenCalledWith(true);
 
     await act(async () => {
       root.unmount();
@@ -529,7 +532,7 @@ describe("Layout", () => {
     expect(container.textContent).toContain("Main company nav");
     expect(container.textContent).not.toContain("Company settings sidebar");
     expect(container.textContent).not.toContain("Instance sidebar");
-    expect(mockSetRouteRequestsCollapsed).toHaveBeenCalledWith(true);
+    expect(mockSetForceCollapsed).toHaveBeenCalledWith(true);
 
     await act(async () => {
       root.unmount();
@@ -713,8 +716,8 @@ describe("Layout", () => {
     expect(container.textContent).toContain("Main company nav");
     expect(container.textContent).not.toContain("Plugin route sidebar");
     // No secondary pane, so the route must not force the sidebar collapsed.
-    expect(mockSetRouteRequestsCollapsed).not.toHaveBeenCalledWith(true);
-    expect(mockSetRouteRequestsCollapsed).toHaveBeenCalledWith(false);
+    expect(mockSetForceCollapsed).not.toHaveBeenCalledWith(true);
+    expect(mockSetForceCollapsed).toHaveBeenCalledWith(false);
 
     await act(async () => {
       root.unmount();
diff --git a/ui/src/components/Layout.tsx b/ui/src/components/Layout.tsx
index caa6c6bdfa3..c23ac306465 100644
--- a/ui/src/components/Layout.tsx
+++ b/ui/src/components/Layout.tsx
@@ -58,7 +58,7 @@ export function Layout() {
     peeking,
     setPeeking,
     isMobile,
-    setRouteRequestsCollapsed,
+    setForceCollapsed,
   } = useSidebar();
   const { openNewIssue, openOnboarding } = useDialogActions();
   const { togglePanelVisible } = usePanel();
@@ -143,14 +143,15 @@ export function Layout() {
     queryFn: () => instanceSettingsApi.getGeneral(),
   }).data?.keyboardShortcuts === true;
 
-  // While a secondary sidebar is shown, ask the app sidebar to default to its
-  // collapsed rail (still peek-able). An explicit user pin always wins — that
-  // precedence lives in SidebarContext. Clearing on cleanup restores the
-  // default when navigating off the takeover route (PAP-10695).
+  // A secondary sidebar always collapses the app sidebar to its rail (still
+  // peek-able) — a hard invariant that overrides the user pin while the route
+  // is active, but does NOT mutate the persisted preference. Clearing the force
+  // on cleanup restores the user's expanded/collapsed choice when navigating
+  // off the takeover route (PAP-10694).
   useEffect(() => {
-    setRouteRequestsCollapsed(hasSecondarySidebar);
-    return () => setRouteRequestsCollapsed(false);
-  }, [hasSecondarySidebar, setRouteRequestsCollapsed]);
+    setForceCollapsed(hasSecondarySidebar);
+    return () => setForceCollapsed(false);
+  }, [hasSecondarySidebar, setForceCollapsed]);
 
   useEffect(() => {
     if (companiesLoading || onboardingTriggered.current) return;
diff --git a/ui/src/components/Sidebar.test.tsx b/ui/src/components/Sidebar.test.tsx
index 960adfcc31d..c711b02a7cd 100644
--- a/ui/src/components/Sidebar.test.tsx
+++ b/ui/src/components/Sidebar.test.tsx
@@ -52,6 +52,7 @@ const mockSidebar = vi.hoisted(() => ({
   isMobile: false,
   setSidebarOpen: vi.fn(),
   collapsed: false,
+  collapseLocked: false,
   peeking: false,
   toggleCollapsed: vi.fn(),
   setCollapsed: vi.fn(),
@@ -315,6 +316,22 @@ describe("Sidebar", () => {
     });
   });
 
+  it("hides the expand/collapse toggle while a secondary sidebar locks the rail", async () => {
+    // A secondary sidebar forces the rail; the user must not be able to expand
+    // the primary while it is shown (PAP-10694).
+    mockInstanceSettingsApi.getExperimental.mockResolvedValue({ enableIsolatedWorkspaces: false });
+    mockSidebar.collapseLocked = true;
+    const root = await renderSidebar();
+
+    expect(container.querySelector('button[aria-label="Collapse sidebar"]')).toBeNull();
+    expect(container.querySelector('button[aria-label="Expand sidebar"]')).toBeNull();
+
+    mockSidebar.collapseLocked = false;
+    flushSync(() => {
+      root.unmount();
+    });
+  });
+
   it("keeps the collapsed rail top bar to just the company logo (no clipped search/toggle)", async () => {
     // In the narrow rail the search/toggle controls don't fit beside the logo and
     // would overflow/clip, shoving the logo out of the icon column (PAP-10676), so
diff --git a/ui/src/components/Sidebar.tsx b/ui/src/components/Sidebar.tsx
index 73d08e59837..592888ad286 100644
--- a/ui/src/components/Sidebar.tsx
+++ b/ui/src/components/Sidebar.tsx
@@ -41,7 +41,7 @@ import { SidebarCompanyMenu } from "./SidebarCompanyMenu";
 export function Sidebar() {
   const { openNewIssue } = useDialogActions();
   const { selectedCompanyId, selectedCompany } = useCompany();
-  const { isMobile, collapsed, peeking, toggleCollapsed, setCollapsed } = useSidebar();
+  const { isMobile, collapsed, collapseLocked, peeking, toggleCollapsed, setCollapsed } = useSidebar();
   const rail = collapsed && !peeking;
   const inboxBadge = useInboxBadge(selectedCompanyId);
   const { data: experimentalSettings } = useQuery({
@@ -94,8 +94,10 @@ export function Sidebar() {
             {/* Desktop-only collapse/expand affordance. While peeking (hover flyout
                 over the collapsed rail) it becomes a Pin that promotes the peek to a
                 pinned-expanded sidebar; otherwise it toggles the pinned rail. Mobile
-                uses the off-canvas drawer, so this control is hidden there. */}
-            {!isMobile ? (
+                uses the off-canvas drawer, so this control is hidden there. It is
+                also hidden while a secondary sidebar forces the rail (collapseLocked):
+                the user cannot expand the primary while a secondary sidebar is shown. */}
+            {!isMobile && !collapseLocked ? (
               peeking ? (
                 <Button
                   variant="ghost"
diff --git a/ui/src/context/SidebarContext.test.tsx b/ui/src/context/SidebarContext.test.tsx
index 7a12c1e3512..ac0412dbd00 100644
--- a/ui/src/context/SidebarContext.test.tsx
+++ b/ui/src/context/SidebarContext.test.tsx
@@ -131,6 +131,53 @@ describe("SidebarContext", () => {
     });
   });
 
+  describe("forced collapse (secondary sidebar): overrides the pin, preserves preference", () => {
+    it("forces collapsed even when the user pinned expanded, without mutating the pin", () => {
+      active = renderProvider();
+      // User prefers expanded site-wide.
+      act(() => capturedValue?.setCollapsed(false));
+      expect(capturedValue?.collapsed).toBe(false);
+      expect(localStorage.getItem(COLLAPSED_STORAGE_KEY)).toBe("0");
+
+      // Entering a secondary-sidebar route forces the rail and locks it.
+      act(() => capturedValue?.setForceCollapsed(true));
+      expect(capturedValue?.collapsed).toBe(true);
+      expect(capturedValue?.collapseLocked).toBe(true);
+      // The persisted preference is untouched.
+      expect(localStorage.getItem(COLLAPSED_STORAGE_KEY)).toBe("0");
+    });
+
+    it("restores the user's preference when the force is cleared (leaving the route)", () => {
+      active = renderProvider();
+      act(() => capturedValue?.setCollapsed(false));
+      act(() => capturedValue?.setForceCollapsed(true));
+      expect(capturedValue?.collapsed).toBe(true);
+
+      // Navigating away clears the force; the expanded preference returns.
+      act(() => capturedValue?.setForceCollapsed(false));
+      expect(capturedValue?.collapsed).toBe(false);
+      expect(capturedValue?.collapseLocked).toBe(false);
+    });
+
+    it("locks the toggle while forced: toggleCollapsed is a no-op and never writes the pin", () => {
+      active = renderProvider();
+      act(() => capturedValue?.setCollapsed(false));
+      act(() => capturedValue?.setForceCollapsed(true));
+
+      act(() => capturedValue?.toggleCollapsed());
+      expect(capturedValue?.collapsed).toBe(true); // still forced
+      expect(localStorage.getItem(COLLAPSED_STORAGE_KEY)).toBe("0"); // pin unchanged
+    });
+
+    it("never forces or locks on mobile", () => {
+      setViewport({ mobile: true });
+      active = renderProvider();
+      act(() => capturedValue?.setForceCollapsed(true));
+      expect(capturedValue?.collapsed).toBe(false);
+      expect(capturedValue?.collapseLocked).toBe(false);
+    });
+  });
+
   describe("persistence round-trip", () => {
     it("writes '1'/'0' to localStorage on setCollapsed", () => {
       active = renderProvider();
diff --git a/ui/src/context/SidebarContext.tsx b/ui/src/context/SidebarContext.tsx
index 917bb8e5c9f..a69fe239283 100644
--- a/ui/src/context/SidebarContext.tsx
+++ b/ui/src/context/SidebarContext.tsx
@@ -18,12 +18,23 @@ interface SidebarContextValue {
   collapsed: boolean;
   setCollapsed: (next: boolean) => void;
   toggleCollapsed: () => void;
+  // True while a secondary sidebar forces the rail: the collapse is locked, so
+  // the expand/toggle affordance must be hidden/inert. Desktop-only.
+  collapseLocked: boolean;
   // Ephemeral peek (hover flyout). Only meaningful on desktop, collapsed,
   // hover-capable pointer. Never persisted.
   peeking: boolean;
   setPeeking: (next: boolean) => void;
-  // Route-requested collapse: a route may default the app sidebar to collapsed.
-  // Lower precedence than an explicit user pin. Wired by Layout/routes (Task 5).
+  // Hard, ephemeral collapse forced by an active secondary sidebar (settings,
+  // plugin `routeSidebar`, …). HIGHER precedence than the user pin — the rule
+  // is "a secondary sidebar always collapses the primary" — but it never
+  // mutates the persisted pin, so leaving the route restores the preference.
+  // Wired by Layout (PAP-10694).
+  forceCollapsed: boolean;
+  setForceCollapsed: (next: boolean) => void;
+  // Route-requested collapse: a route may *default* the app sidebar to
+  // collapsed. LOWER precedence than an explicit user pin. Wired by routes via
+  // RequestCollapsedSidebar.
   routeRequestsCollapsed: boolean;
   setRouteRequestsCollapsed: (next: boolean) => void;
 }
@@ -84,6 +95,7 @@ export function SidebarProvider({ children }: { children: ReactNode }) {
   // `null` = unpinned; an explicit user pin takes precedence over route request.
   const [userCollapsed, setUserCollapsed] = useState<boolean | null>(() => readStoredCollapsed());
   const [routeRequestsCollapsed, setRouteRequestsCollapsed] = useState(false);
+  const [forceCollapsed, setForceCollapsed] = useState(false);
   const [rawPeeking, setRawPeeking] = useState(false);
   const [pointerCanPeek, setPointerCanPeek] = useState(() => readPointerCanPeek());
 
@@ -105,11 +117,16 @@ export function SidebarProvider({ children }: { children: ReactNode }) {
     return () => mql.removeEventListener("change", onChange);
   }, []);
 
-  // Precedence (highest wins): explicit user pin > route request > default expanded.
-  const desktopCollapsed = userCollapsed !== null ? userCollapsed : routeRequestsCollapsed;
+  // Precedence (highest wins): forced (active secondary sidebar) > explicit user
+  // pin > route request > default expanded. The force is ephemeral and never
+  // touches the persisted pin, so dropping it restores the user's preference.
+  const pinnedOrRequested = userCollapsed !== null ? userCollapsed : routeRequestsCollapsed;
+  const desktopCollapsed = forceCollapsed || pinnedOrRequested;
   // Collapsed/peek are desktop-only; mobile always uses the drawer. The user
   // pin is preserved across the breakpoint and reapplies on the desktop side.
   const collapsed = isMobile ? false : desktopCollapsed;
+  // While forced, the pin is locked: the expand/toggle affordance is inert.
+  const collapseLocked = !isMobile && forceCollapsed;
   // Peek only applies when collapsed on a hover-capable pointer.
   const peeking = rawPeeking && collapsed && pointerCanPeek;
 
@@ -119,8 +136,11 @@ export function SidebarProvider({ children }: { children: ReactNode }) {
   }, []);
 
   const toggleCollapsed = useCallback(() => {
-    setCollapsed(!desktopCollapsed);
-  }, [desktopCollapsed, setCollapsed]);
+    // While a secondary sidebar forces the rail, the toggle is locked: it must
+    // neither expand the rail nor mutate the persisted preference.
+    if (forceCollapsed) return;
+    setCollapsed(!pinnedOrRequested);
+  }, [forceCollapsed, pinnedOrRequested, setCollapsed]);
 
   const setPeeking = useCallback((next: boolean) => {
     setRawPeeking(next);
@@ -137,8 +157,11 @@ export function SidebarProvider({ children }: { children: ReactNode }) {
       collapsed,
       setCollapsed,
       toggleCollapsed,
+      collapseLocked,
       peeking,
       setPeeking,
+      forceCollapsed,
+      setForceCollapsed,
       routeRequestsCollapsed,
       setRouteRequestsCollapsed,
     }),
@@ -149,8 +172,10 @@ export function SidebarProvider({ children }: { children: ReactNode }) {
       collapsed,
       setCollapsed,
       toggleCollapsed,
+      collapseLocked,
       peeking,
       setPeeking,
+      forceCollapsed,
       routeRequestsCollapsed,
     ],
   );