diff --git a/ui/src/components/AnnouncementWell.tsx b/ui/src/components/AnnouncementWell.tsx
index b71ce16f095..48612619d8f 100644
--- a/ui/src/components/AnnouncementWell.tsx
+++ b/ui/src/components/AnnouncementWell.tsx
@@ -41,7 +41,10 @@ export function AnnouncementWell({ health }: { health?: HealthStatus }) {
   const modalOpen = useModalOpen(Boolean(announcement));
   if (!announcement || modalOpen || (toasts?.length ?? 0) > 0) return null;
   return (
-    <aside aria-label="Paperclip announcements" className="announcement-well fixed left-3 bottom-(--announcement-mobile-bottom) z-40 w-(--announcement-available-width) max-w-(--announcement-width) max-h-(--announcement-mobile-max-height) overflow-y-auto md:bottom-3 md:max-h-(--announcement-max-height)">
+    // `md:left` clears the desktop sidebar, whose footer holds the account menu
+    // and the Share feedback button. Mobile keeps the viewport edge: its sidebar
+    // is a drawer that reserves no width.
+    <aside aria-label="Paperclip announcements" className="announcement-well fixed left-3 bottom-(--announcement-mobile-bottom) z-40 w-(--announcement-available-width) max-w-(--announcement-width) max-h-(--announcement-mobile-max-height) overflow-y-auto md:left-(--overlay-gutter-left) md:bottom-3 md:max-h-(--announcement-max-height)">
       <AnnouncementCard key={announcement.id} announcement={announcement} onDismiss={() => dismiss(announcement.id)} />
     </aside>
   );
diff --git a/ui/src/components/SidebarShell.production.tsx b/ui/src/components/SidebarShell.production.tsx
index 5d706761606..2500bf6d40f 100644
--- a/ui/src/components/SidebarShell.production.tsx
+++ b/ui/src/components/SidebarShell.production.tsx
@@ -122,6 +122,19 @@ export function SidebarShell({
   // collapsed (the rail width is a fixed constant, not user-resizable).
   const canResize = resizable && open && !collapsed;
 
+  // The toast viewport is anchored to the viewport's bottom-left, which is this
+  // panel's footer — the account menu and the Share feedback button. It is a
+  // sibling of the shell rather than a descendant, so the width it has to clear
+  // goes on the root. Reserved width, not panel width: a peeking panel overlays
+  // content deliberately and is transient.
+  useEffect(() => {
+    const root = document.documentElement;
+    root.style.setProperty("--sidebar-reserved-width", `${reservedWidth}px`);
+    return () => {
+      root.style.removeProperty("--sidebar-reserved-width");
+    };
+  }, [reservedWidth]);
+
   const reservedStyle = useMemo(
     () => ({ width: `${reservedWidth}px` }),
     [reservedWidth],
diff --git a/ui/src/components/SidebarShell.test.tsx b/ui/src/components/SidebarShell.test.tsx
index b4571b81952..f106d504b84 100644
--- a/ui/src/components/SidebarShell.test.tsx
+++ b/ui/src/components/SidebarShell.test.tsx
@@ -65,6 +65,73 @@ describe("SidebarShell", () => {
     expect(handle()?.getAttribute("aria-valuenow")).toBe("320");
   });
 
+  // The bottom-left overlays — the announcement well and the toast viewport —
+  // are fixed to the viewport, so without this width they land on the sidebar
+  // footer, on top of the account menu and the Share feedback button.
+  function reservedWidthVariable() {
+    return document.documentElement.style.getPropertyValue("--sidebar-reserved-width");
+  }
+
+  it("publishes the width the bottom-left overlays have to clear", () => {
+    window.localStorage.setItem("test.sidebar.width", "320");
+
+    act(() => {
+      root.render(
+        <SidebarShell open resizable storageKey="test.sidebar.width">
+          <div>Sidebar</div>
+        </SidebarShell>,
+      );
+    });
+
+    expect(reservedWidthVariable()).toBe("320px");
+
+    const separator = handle()!;
+    separator.setPointerCapture = vi.fn();
+    act(() => {
+      separator.dispatchEvent(pointerEvent("pointerdown", 320));
+      separator.dispatchEvent(pointerEvent("pointermove", 260));
+      separator.dispatchEvent(pointerEvent("pointerup", 260));
+    });
+
+    // It tracks a drag, so the overlays do not hang over a narrowed sidebar.
+    expect(reservedWidthVariable()).toBe("260px");
+  });
+
+  it("publishes only the rail width when collapsed, and none when closed", () => {
+    act(() => {
+      root.render(
+        <SidebarShell open collapsed storageKey="test.sidebar.width">
+          <div>Sidebar</div>
+        </SidebarShell>,
+      );
+    });
+    expect(reservedWidthVariable()).toBe(`${SIDEBAR_RAIL_WIDTH}px`);
+
+    act(() => {
+      root.render(
+        <SidebarShell open={false} storageKey="test.sidebar.width">
+          <div>Sidebar</div>
+        </SidebarShell>,
+      );
+    });
+    // Nothing reserved means the overlays return to the viewport edge.
+    expect(reservedWidthVariable()).toBe("0px");
+  });
+
+  it("reserves nothing to clear while the panel is only peeking", () => {
+    act(() => {
+      root.render(
+        <SidebarShell open collapsed peeking storageKey="test.sidebar.width">
+          <div>Sidebar</div>
+        </SidebarShell>,
+      );
+    });
+
+    // A peeking panel overlays content on purpose and is transient, so the
+    // overlays must not jump sideways as the pointer crosses the rail.
+    expect(reservedWidthVariable()).toBe(`${SIDEBAR_RAIL_WIDTH}px`);
+  });
+
   it("resizes by dragging and persists the new width", () => {
     act(() => {
       root.render(
diff --git a/ui/src/components/SidebarShell.tsx b/ui/src/components/SidebarShell.tsx
index 9834434c0bf..9cb50805fea 100644
--- a/ui/src/components/SidebarShell.tsx
+++ b/ui/src/components/SidebarShell.tsx
@@ -122,6 +122,19 @@ export function SidebarShell({
   // collapsed (the rail width is a fixed constant, not user-resizable).
   const canResize = resizable && open && !collapsed;
 
+  // The announcement well and the toast viewport are anchored to the viewport's
+  // bottom-left, which is this panel's footer — the account menu and the Share
+  // feedback button. They are siblings of the shell rather than descendants, so
+  // the width they have to clear goes on the root. Reserved width, not panel
+  // width: a peeking panel overlays content deliberately and is transient.
+  useEffect(() => {
+    const root = document.documentElement;
+    root.style.setProperty("--sidebar-reserved-width", `${reservedWidth}px`);
+    return () => {
+      root.style.removeProperty("--sidebar-reserved-width");
+    };
+  }, [reservedWidth]);
+
   const reservedStyle = useMemo(
     () => ({ width: `${reservedWidth}px` }),
     [reservedWidth],
diff --git a/ui/src/components/ToastViewport.tsx b/ui/src/components/ToastViewport.tsx
index 23a53bda246..f9ba1bf72d0 100644
--- a/ui/src/components/ToastViewport.tsx
+++ b/ui/src/components/ToastViewport.tsx
@@ -101,7 +101,9 @@ export function ToastViewport() {
     <aside
       aria-live="polite"
       aria-atomic="false"
-      className="pointer-events-none fixed bottom-3 left-3 z-(--z-120) w-full max-w-sm px-1"
+      // Same bottom-left slot as the announcement well, so it clears the
+      // desktop sidebar the same way. Mobile keeps the viewport edge.
+      className="pointer-events-none fixed bottom-3 left-3 z-(--z-120) w-full max-w-sm px-1 md:left-(--overlay-gutter-left) md:w-(--overlay-available-width)"
     >
       <ol className="flex w-full flex-col-reverse gap-2">
         {toasts.map((toast) => (
diff --git a/ui/src/index.css b/ui/src/index.css
index 951556f36f6..86ee94ccd8e 100644
--- a/ui/src/index.css
+++ b/ui/src/index.css
@@ -3029,8 +3029,28 @@ span.paperclip-mention-chip[data-mention-kind="external-object"] {
   }
 }
 :root {
+  /*
+   * Width the desktop sidebar currently reserves. SidebarShell publishes the
+   * live value, including while the panel is being dragged; the fallback of 0
+   * covers every surface without one — the mobile drawer, and any host that
+   * renders the overlays outside the app shell.
+   */
+  --sidebar-reserved-width: 0px;
+  /*
+   * Left edge for the viewport-anchored overlays at the bottom of the screen:
+   * the announcement well and the toast viewport. They are fixed, so without
+   * this they land on top of the sidebar footer, which is where the account
+   * menu and the Share feedback button live.
+   */
+  --overlay-gutter-left: calc(var(--sidebar-reserved-width) + var(--spacing) * 3);
+  /*
+   * Width still on screen after the gutter, so a widened sidebar on a narrow
+   * window shortens the overlay instead of pushing it off the right edge.
+   * With no sidebar this is the full-bleed width these overlays always had.
+   */
+  --overlay-available-width: calc(100vw - var(--overlay-gutter-left) - var(--spacing) * 3);
   --announcement-width: 22rem;
-  --announcement-available-width: calc(100vw - var(--spacing) * 6);
+  --announcement-available-width: var(--overlay-available-width);
   --announcement-image-height: 8.5rem;
   --announcement-image-mobile-height: 6rem;
   --announcement-mobile-bottom: calc(var(--spacing) * 19 + env(safe-area-inset-bottom, 0px));