diff --git a/ui/src/components/Layout.tsx b/ui/src/components/Layout.tsx
index a93a686920c..43d2c9bbd05 100644
--- a/ui/src/components/Layout.tsx
+++ b/ui/src/components/Layout.tsx
@@ -48,7 +48,7 @@ function getCompanyRouteSegment(pathname: string, companyPrefix: string | undefi
 }
 
 export function Layout() {
-  const { sidebarOpen, setSidebarOpen, toggleSidebar, isMobile } = useSidebar();
+  const { sidebarOpen, setSidebarOpen, toggleSidebar, toggleCollapsed, isMobile } = useSidebar();
   const { openNewIssue, openOnboarding } = useDialogActions();
   const { togglePanelVisible } = usePanel();
   const {
@@ -178,6 +178,15 @@ export function Layout() {
   ]);
 
   const togglePanel = togglePanelVisible;
+  // Cmd/Ctrl+B: collapse/expand the pinned rail on desktop; on mobile keep
+  // toggling the off-canvas drawer.
+  const toggleCollapse = useCallback(() => {
+    if (isMobile) {
+      toggleSidebar();
+    } else {
+      toggleCollapsed();
+    }
+  }, [isMobile, toggleSidebar, toggleCollapsed]);
   const openSearch = useCallback(() => {
     document.dispatchEvent(new KeyboardEvent("keydown", {
       key: "k",
@@ -194,6 +203,7 @@ export function Layout() {
     onNewIssue: () => openNewIssue(),
     onSearch: openSearch,
     onToggleSidebar: toggleSidebar,
+    onToggleCollapse: toggleCollapse,
     onTogglePanel: togglePanel,
     onShowShortcuts: () => setShortcutsOpen(true),
   });
diff --git a/ui/src/context/SidebarContext.test.tsx b/ui/src/context/SidebarContext.test.tsx
new file mode 100644
index 00000000000..7a12c1e3512
--- /dev/null
+++ b/ui/src/context/SidebarContext.test.tsx
@@ -0,0 +1,215 @@
+// @vitest-environment jsdom
+
+import { act } from "react";
+import { createRoot, type Root } from "react-dom/client";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { SidebarProvider, useSidebar } from "./SidebarContext";
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+(globalThis as any).IS_REACT_ACT_ENVIRONMENT = true;
+
+const COLLAPSED_STORAGE_KEY = "paperclip.sidebar.collapsed";
+
+// Mutable media state driving the matchMedia mock.
+const mediaState = { mobile: false, hoverFine: true };
+
+function setViewport({ mobile, hoverFine }: { mobile?: boolean; hoverFine?: boolean }) {
+  if (typeof mobile === "boolean") mediaState.mobile = mobile;
+  if (typeof hoverFine === "boolean") mediaState.hoverFine = hoverFine;
+  Object.defineProperty(window, "innerWidth", {
+    configurable: true,
+    writable: true,
+    value: mediaState.mobile ? 500 : 1280,
+  });
+}
+
+let capturedValue: ReturnType<typeof useSidebar> | null = null;
+
+function Capture() {
+  capturedValue = useSidebar();
+  return null;
+}
+
+function renderProvider(): { root: Root; host: HTMLDivElement } {
+  const host = document.createElement("div");
+  document.body.appendChild(host);
+  const root = createRoot(host);
+  act(() => {
+    root.render(
+      <SidebarProvider>
+        <Capture />
+      </SidebarProvider>,
+    );
+  });
+  return { root, host };
+}
+
+describe("SidebarContext", () => {
+  let active: { root: Root; host: HTMLDivElement } | null = null;
+
+  beforeEach(() => {
+    localStorage.clear();
+    capturedValue = null;
+    setViewport({ mobile: false, hoverFine: true });
+    Object.defineProperty(window, "matchMedia", {
+      configurable: true,
+      writable: true,
+      value: vi.fn().mockImplementation((query: string) => {
+        const isHoverQuery = query.includes("(hover: hover)");
+        const matches = isHoverQuery ? mediaState.hoverFine : mediaState.mobile;
+        return {
+          matches,
+          media: query,
+          onchange: null,
+          addEventListener: vi.fn(),
+          removeEventListener: vi.fn(),
+          addListener: vi.fn(),
+          removeListener: vi.fn(),
+          dispatchEvent: vi.fn(),
+        };
+      }),
+    });
+  });
+
+  afterEach(() => {
+    if (active) {
+      act(() => active!.root.unmount());
+      active.host.remove();
+      active = null;
+    }
+    localStorage.clear();
+  });
+
+  describe("precedence: user pin > route request > default", () => {
+    it("defaults to expanded (collapsed=false) with no pin and no route request", () => {
+      active = renderProvider();
+      expect(capturedValue?.collapsed).toBe(false);
+    });
+
+    it("uses the route request when there is no user pin", () => {
+      active = renderProvider();
+      act(() => capturedValue?.setRouteRequestsCollapsed(true));
+      expect(capturedValue?.collapsed).toBe(true);
+    });
+
+    it("lets an explicit user pin override the route request", () => {
+      active = renderProvider();
+      act(() => capturedValue?.setRouteRequestsCollapsed(true));
+      expect(capturedValue?.collapsed).toBe(true);
+
+      // User pins expanded — this must win over the route's collapse request.
+      act(() => capturedValue?.setCollapsed(false));
+      expect(capturedValue?.collapsed).toBe(false);
+
+      // And pinning collapsed wins too.
+      act(() => capturedValue?.setCollapsed(true));
+      expect(capturedValue?.collapsed).toBe(true);
+    });
+
+    it("toggleCollapsed flips the effective mode and records a pin", () => {
+      active = renderProvider();
+      expect(capturedValue?.collapsed).toBe(false);
+
+      act(() => capturedValue?.toggleCollapsed());
+      expect(capturedValue?.collapsed).toBe(true);
+      expect(localStorage.getItem(COLLAPSED_STORAGE_KEY)).toBe("1");
+
+      act(() => capturedValue?.toggleCollapsed());
+      expect(capturedValue?.collapsed).toBe(false);
+      expect(localStorage.getItem(COLLAPSED_STORAGE_KEY)).toBe("0");
+    });
+
+    it("toggleCollapsed pins expanded when only a route request is active", () => {
+      active = renderProvider();
+      act(() => capturedValue?.setRouteRequestsCollapsed(true));
+      expect(capturedValue?.collapsed).toBe(true);
+
+      // Effective is collapsed (via route); toggling should flip to expanded.
+      act(() => capturedValue?.toggleCollapsed());
+      expect(capturedValue?.collapsed).toBe(false);
+      expect(localStorage.getItem(COLLAPSED_STORAGE_KEY)).toBe("0");
+    });
+  });
+
+  describe("persistence round-trip", () => {
+    it("writes '1'/'0' to localStorage on setCollapsed", () => {
+      active = renderProvider();
+      act(() => capturedValue?.setCollapsed(true));
+      expect(localStorage.getItem(COLLAPSED_STORAGE_KEY)).toBe("1");
+      act(() => capturedValue?.setCollapsed(false));
+      expect(localStorage.getItem(COLLAPSED_STORAGE_KEY)).toBe("0");
+    });
+
+    it("reads the persisted pin synchronously on first paint (no flash)", () => {
+      localStorage.setItem(COLLAPSED_STORAGE_KEY, "1");
+      active = renderProvider();
+      // Collapsed is already true on the very first captured render.
+      expect(capturedValue?.collapsed).toBe(true);
+    });
+
+    it("treats a missing/garbage value as unpinned", () => {
+      localStorage.setItem(COLLAPSED_STORAGE_KEY, "yes");
+      active = renderProvider();
+      expect(capturedValue?.collapsed).toBe(false);
+      // Unpinned, so a route request still applies.
+      act(() => capturedValue?.setRouteRequestsCollapsed(true));
+      expect(capturedValue?.collapsed).toBe(true);
+    });
+  });
+
+  describe("mobile gating", () => {
+    it("never reports collapsed on mobile even with a collapsed pin", () => {
+      localStorage.setItem(COLLAPSED_STORAGE_KEY, "1");
+      setViewport({ mobile: true });
+      active = renderProvider();
+      expect(capturedValue?.isMobile).toBe(true);
+      expect(capturedValue?.collapsed).toBe(false);
+    });
+
+    it("never reports peeking on mobile", () => {
+      localStorage.setItem(COLLAPSED_STORAGE_KEY, "1");
+      setViewport({ mobile: true });
+      active = renderProvider();
+      act(() => capturedValue?.setPeeking(true));
+      expect(capturedValue?.peeking).toBe(false);
+    });
+  });
+
+  describe("peek gating", () => {
+    it("peeks when collapsed on a hover-capable pointer", () => {
+      localStorage.setItem(COLLAPSED_STORAGE_KEY, "1");
+      setViewport({ mobile: false, hoverFine: true });
+      active = renderProvider();
+      expect(capturedValue?.collapsed).toBe(true);
+      act(() => capturedValue?.setPeeking(true));
+      expect(capturedValue?.peeking).toBe(true);
+    });
+
+    it("does not peek when expanded", () => {
+      setViewport({ mobile: false, hoverFine: true });
+      active = renderProvider();
+      expect(capturedValue?.collapsed).toBe(false);
+      act(() => capturedValue?.setPeeking(true));
+      expect(capturedValue?.peeking).toBe(false);
+    });
+
+    it("does not peek on a coarse/non-hover pointer", () => {
+      localStorage.setItem(COLLAPSED_STORAGE_KEY, "1");
+      setViewport({ mobile: false, hoverFine: false });
+      active = renderProvider();
+      expect(capturedValue?.collapsed).toBe(true);
+      act(() => capturedValue?.setPeeking(true));
+      expect(capturedValue?.peeking).toBe(false);
+    });
+  });
+
+  describe("back-compat", () => {
+    it("retains sidebarOpen/toggleSidebar for the drawer", () => {
+      active = renderProvider();
+      const initial = capturedValue?.sidebarOpen;
+      expect(typeof initial).toBe("boolean");
+      act(() => capturedValue?.toggleSidebar());
+      expect(capturedValue?.sidebarOpen).toBe(!initial);
+    });
+  });
+});
diff --git a/ui/src/context/SidebarContext.tsx b/ui/src/context/SidebarContext.tsx
index c563ae00ef2..917bb8e5c9f 100644
--- a/ui/src/context/SidebarContext.tsx
+++ b/ui/src/context/SidebarContext.tsx
@@ -1,20 +1,92 @@
-import { createContext, useCallback, useContext, useState, useEffect, type ReactNode } from "react";
+import {
+  createContext,
+  useCallback,
+  useContext,
+  useEffect,
+  useMemo,
+  useState,
+  type ReactNode,
+} from "react";
 
 interface SidebarContextValue {
+  // Mobile drawer + back-compat (existing behavior, unchanged).
   sidebarOpen: boolean;
   setSidebarOpen: (open: boolean) => void;
   toggleSidebar: () => void;
   isMobile: boolean;
+  // Pinned desktop mode: expanded | collapsed. Desktop-only.
+  collapsed: boolean;
+  setCollapsed: (next: boolean) => void;
+  toggleCollapsed: () => void;
+  // 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).
+  routeRequestsCollapsed: boolean;
+  setRouteRequestsCollapsed: (next: boolean) => void;
 }
 
 const SidebarContext = createContext<SidebarContextValue | null>(null);
 
 const MOBILE_BREAKPOINT = 768;
+const COLLAPSED_STORAGE_KEY = "paperclip.sidebar.collapsed";
+const PEEK_POINTER_QUERY = "(hover: hover) and (pointer: fine)";
+
+// Tri-state read of the persisted user pin:
+//   true  → pinned collapsed ("1")
+//   false → pinned expanded ("0")
+//   null  → no pin (fall through to route request, then global default)
+// Read synchronously in the state initializer so first paint matches the
+// persisted mode (mirrors the `paperclip.sidebar.width` pattern in
+// ResizableSidebarPane and avoids an expand→collapse flash).
+function readStoredCollapsed(): boolean | null {
+  if (typeof window === "undefined") return null;
+
+  try {
+    const stored = window.localStorage.getItem(COLLAPSED_STORAGE_KEY);
+    if (stored === "1") return true;
+    if (stored === "0") return false;
+    return null;
+  } catch {
+    return null;
+  }
+}
+
+function writeStoredCollapsed(value: boolean) {
+  if (typeof window === "undefined") return;
+
+  try {
+    window.localStorage.setItem(COLLAPSED_STORAGE_KEY, value ? "1" : "0");
+  } catch {
+    // Storage can be unavailable in private contexts; pinning should still
+    // work for the current session.
+  }
+}
+
+function readPointerCanPeek(): boolean {
+  if (typeof window === "undefined" || typeof window.matchMedia !== "function") {
+    return false;
+  }
+
+  try {
+    return window.matchMedia(PEEK_POINTER_QUERY).matches;
+  } catch {
+    return false;
+  }
+}
 
 export function SidebarProvider({ children }: { children: ReactNode }) {
   const [isMobile, setIsMobile] = useState(() => window.innerWidth < MOBILE_BREAKPOINT);
   const [sidebarOpen, setSidebarOpen] = useState(() => window.innerWidth >= MOBILE_BREAKPOINT);
 
+  // `null` = unpinned; an explicit user pin takes precedence over route request.
+  const [userCollapsed, setUserCollapsed] = useState<boolean | null>(() => readStoredCollapsed());
+  const [routeRequestsCollapsed, setRouteRequestsCollapsed] = useState(false);
+  const [rawPeeking, setRawPeeking] = useState(false);
+  const [pointerCanPeek, setPointerCanPeek] = useState(() => readPointerCanPeek());
+
   useEffect(() => {
     const mql = window.matchMedia(`(max-width: ${MOBILE_BREAKPOINT - 1}px)`);
     const onChange = (e: MediaQueryListEvent) => {
@@ -25,13 +97,65 @@ export function SidebarProvider({ children }: { children: ReactNode }) {
     return () => mql.removeEventListener("change", onChange);
   }, []);
 
+  useEffect(() => {
+    if (typeof window.matchMedia !== "function") return;
+    const mql = window.matchMedia(PEEK_POINTER_QUERY);
+    const onChange = (e: MediaQueryListEvent) => setPointerCanPeek(e.matches);
+    mql.addEventListener("change", onChange);
+    return () => mql.removeEventListener("change", onChange);
+  }, []);
+
+  // Precedence (highest wins): explicit user pin > route request > default expanded.
+  const desktopCollapsed = userCollapsed !== null ? userCollapsed : routeRequestsCollapsed;
+  // 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;
+  // Peek only applies when collapsed on a hover-capable pointer.
+  const peeking = rawPeeking && collapsed && pointerCanPeek;
+
+  const setCollapsed = useCallback((next: boolean) => {
+    setUserCollapsed(next);
+    writeStoredCollapsed(next);
+  }, []);
+
+  const toggleCollapsed = useCallback(() => {
+    setCollapsed(!desktopCollapsed);
+  }, [desktopCollapsed, setCollapsed]);
+
+  const setPeeking = useCallback((next: boolean) => {
+    setRawPeeking(next);
+  }, []);
+
   const toggleSidebar = useCallback(() => setSidebarOpen((v) => !v), []);
 
-  return (
-    <SidebarContext.Provider value={{ sidebarOpen, setSidebarOpen, toggleSidebar, isMobile }}>
-      {children}
-    </SidebarContext.Provider>
+  const value = useMemo<SidebarContextValue>(
+    () => ({
+      sidebarOpen,
+      setSidebarOpen,
+      toggleSidebar,
+      isMobile,
+      collapsed,
+      setCollapsed,
+      toggleCollapsed,
+      peeking,
+      setPeeking,
+      routeRequestsCollapsed,
+      setRouteRequestsCollapsed,
+    }),
+    [
+      sidebarOpen,
+      toggleSidebar,
+      isMobile,
+      collapsed,
+      setCollapsed,
+      toggleCollapsed,
+      peeking,
+      setPeeking,
+      routeRequestsCollapsed,
+    ],
   );
+
+  return <SidebarContext.Provider value={value}>{children}</SidebarContext.Provider>;
 }
 
 export function useSidebar() {
diff --git a/ui/src/hooks/useKeyboardShortcuts.test.tsx b/ui/src/hooks/useKeyboardShortcuts.test.tsx
index ae98193d94c..f2279ec24a4 100644
--- a/ui/src/hooks/useKeyboardShortcuts.test.tsx
+++ b/ui/src/hooks/useKeyboardShortcuts.test.tsx
@@ -11,14 +11,17 @@ import { useKeyboardShortcuts } from "./useKeyboardShortcuts";
 function TestHarness({
   onNewIssue,
   onSearch,
+  onToggleCollapse,
 }: {
   onNewIssue: () => void;
   onSearch?: () => void;
+  onToggleCollapse?: () => void;
 }) {
   useKeyboardShortcuts({
     enabled: true,
     onNewIssue,
     onSearch,
+    onToggleCollapse,
   });
 
   return <div>keyboard shortcuts test</div>;
@@ -106,4 +109,53 @@ describe("useKeyboardShortcuts", () => {
       root.unmount();
     });
   });
+
+  it("fires onToggleCollapse on Cmd/Ctrl+B", () => {
+    const root = createRoot(container);
+    const onToggleCollapse = vi.fn();
+
+    act(() => {
+      root.render(<TestHarness onNewIssue={vi.fn()} onToggleCollapse={onToggleCollapse} />);
+    });
+
+    document.dispatchEvent(new KeyboardEvent("keydown", {
+      key: "b",
+      metaKey: true,
+      bubbles: true,
+      cancelable: true,
+    }));
+    expect(onToggleCollapse).toHaveBeenCalledTimes(1);
+
+    document.dispatchEvent(new KeyboardEvent("keydown", {
+      key: "b",
+      ctrlKey: true,
+      bubbles: true,
+      cancelable: true,
+    }));
+    expect(onToggleCollapse).toHaveBeenCalledTimes(2);
+
+    act(() => {
+      root.unmount();
+    });
+  });
+
+  it("does not fire onToggleCollapse for a bare 'b' keypress", () => {
+    const root = createRoot(container);
+    const onToggleCollapse = vi.fn();
+
+    act(() => {
+      root.render(<TestHarness onNewIssue={vi.fn()} onToggleCollapse={onToggleCollapse} />);
+    });
+
+    document.dispatchEvent(new KeyboardEvent("keydown", {
+      key: "b",
+      bubbles: true,
+      cancelable: true,
+    }));
+    expect(onToggleCollapse).not.toHaveBeenCalled();
+
+    act(() => {
+      root.unmount();
+    });
+  });
 });
diff --git a/ui/src/hooks/useKeyboardShortcuts.ts b/ui/src/hooks/useKeyboardShortcuts.ts
index 7788b8c359c..2bc51a2ec75 100644
--- a/ui/src/hooks/useKeyboardShortcuts.ts
+++ b/ui/src/hooks/useKeyboardShortcuts.ts
@@ -10,6 +10,7 @@ interface ShortcutHandlers {
   onNewIssue?: () => void;
   onSearch?: () => void;
   onToggleSidebar?: () => void;
+  onToggleCollapse?: () => void;
   onTogglePanel?: () => void;
   onShowShortcuts?: () => void;
 }
@@ -19,6 +20,7 @@ export function useKeyboardShortcuts({
   onNewIssue,
   onSearch,
   onToggleSidebar,
+  onToggleCollapse,
   onTogglePanel,
   onShowShortcuts,
 }: ShortcutHandlers) {
@@ -67,6 +69,12 @@ export function useKeyboardShortcuts({
         onToggleSidebar?.();
       }
 
+      // Cmd/Ctrl+B → Collapse/expand sidebar (desktop) or toggle drawer (mobile)
+      if ((e.key === "b" || e.key === "B") && (e.metaKey || e.ctrlKey) && !e.altKey) {
+        e.preventDefault();
+        onToggleCollapse?.();
+      }
+
       // ] → Toggle Panel
       if (e.key === "]" && !e.metaKey && !e.ctrlKey) {
         e.preventDefault();
@@ -76,5 +84,5 @@ export function useKeyboardShortcuts({
 
     document.addEventListener("keydown", handleKeyDown);
     return () => document.removeEventListener("keydown", handleKeyDown);
-  }, [enabled, onNewIssue, onSearch, onToggleSidebar, onTogglePanel, onShowShortcuts]);
+  }, [enabled, onNewIssue, onSearch, onToggleSidebar, onToggleCollapse, onTogglePanel, onShowShortcuts]);
 }