diff --git a/docs/decisions.md b/docs/decisions.md
index 962dbbd9f..559a70a85 100644
--- a/docs/decisions.md
+++ b/docs/decisions.md
@@ -1807,3 +1807,33 @@ page to completeness** — rather than option (b) (a generous cap + truncation s
   instead of the single-shot fetch; the #221 stale-query guard and the single add POST are unchanged.
 - **Out of scope (unchanged):** the add POST itself still accepts the full merged id set in one request body
   — bounding *that* surface is a separate concern (see #308 for the add path); #293 is the GET.
+
+## 2026-07-18 — Collapsible sidebar + nav-group accordions: two `ctv-sidebar-*` localStorage keys, labeled groups default-collapsed (#396)
+
+The shell sidebar (`web/src/app/AppShell.tsx`) gained (a) a header toggle that collapses it to a 60px
+icon rail and (b) collapsible accordions per **labeled** nav group (Media, System); the unlabeled
+**Primary** group is always open. Mirrors the Claude Design prototype's updated `Sidebar`.
+
+- **State lives in a small hook, not App.** `web/src/app/sidebarState.ts` `useSidebarState()` owns both
+  pieces of state + persistence; `AppShell` consumes it (nothing else needs it) and stamps
+  `ctv-app-shell-collapsed` on the shell root so the collapse is CSS-driven from one class.
+- **Persistence keys use the established `ctv-` hyphen convention, NOT the prototype's dotted names.**
+  The issue quoted `ctv.sidebar.collapsed` / `ctv.sidebar.groups`, but every existing client-local pref
+  is hyphenated (`ctv-theme`, `ctv-logs-page-size` — spa-conventions §5d), so we use
+  **`ctv-sidebar-collapsed`** (`"1"`/`"0"`) and **`ctv-sidebar-groups`** (JSON `{groupKey: boolean}`,
+  boolean = *collapsed*). Deliberate deviation from the issue's literal key text in favour of the repo
+  convention the issue itself points to; helpers validate/parse defensively (bad JSON / non-boolean
+  values → default).
+- **Labeled groups default to COLLAPSED** (absent `ctv-sidebar-groups` entry ⇒ collapsed), so a fresh
+  load shows only Primary — matching the prototype ("default-collapsed, leaving only Primary visible").
+  A behavior change for existing users; `App.test.tsx`'s shell/nav suite seeds the two groups open
+  because it clicks Media/System nav links directly (the accordion behavior is covered in its own
+  describe).
+- **Accordions apply only in the expanded sidebar.** In the rail, group-collapse is ignored — every
+  item renders as an icon (label kept in the a11y tree via an sr-only span so the accessible name/tests
+  survive; surfaced as a native `title` tooltip), groups separated by a hairline divider, numeric
+  badges shown as a corner dot. The active-route indicator (left rail bar + active background) works in
+  both states.
+- **Group keys are explicit + stable** (`SidebarNavGroupDefinition.key`: `'media'`, `'system'`) rather
+  than derived from the label, so renaming a label doesn't silently orphan persisted state.
+- No route/screen was added or redirected (shell-chrome only), so no `blazor-route-parity.md` change.
diff --git a/docs/spa-conventions.md b/docs/spa-conventions.md
index 14fce2cb7..9414579b1 100644
--- a/docs/spa-conventions.md
+++ b/docs/spa-conventions.md
@@ -459,3 +459,34 @@ integration.
 This module is intentionally reusable beyond SmartCollections — ChannelBuilder and Auto-Tune's
 inline query editing (#69) are candidate future consumers, tracked as separate follow-up issues
 rather than wired in #176.
+
+## 13. Collapsible sidebar + nav-group accordions (#396)
+
+The shell sidebar (`web/src/app/AppShell.tsx`) supports two independent, persisted collapse states.
+Both are shell chrome — no screen participates.
+
+- **State + persistence** live in `web/src/app/sidebarState.ts` (`useSidebarState()`), consumed by
+  `AppShell` alone. It follows the §5d client-local-prefs pattern (a try/catch `getStorage()`, a
+  validating getter, a write-through setter) over two namespaced keys:
+  - `ctv-sidebar-collapsed` — `"1"`/`"0"`; is the sidebar collapsed to the 60px **icon rail**?
+  - `ctv-sidebar-groups` — JSON `{groupKey: boolean}` where the boolean is **collapsed**. A labeled
+    group with **no stored entry defaults to collapsed**, so a fresh load shows only the always-open
+    **Primary** group. (Keys are the `ctv-` hyphen form, not the prototype's `ctv.sidebar.*` — see
+    `decisions.md` 2026-07-18.)
+- **`AppShell` stamps `ctv-app-shell-collapsed` on the shell root** when collapsed; the rail look is
+  entirely CSS-driven from that one class (`shell.css` narrows the tracked `--sidebar-w` to 60px and
+  transitions `grid-template-columns`; `@media (prefers-reduced-motion: reduce)` drops the transition).
+- **Nav is inventory-driven** from `sidebarNavGroups` (`app/routes.tsx`). Only **labeled** groups are
+  collapsible; each has an explicit stable `key` (`'media'`, `'system'`) used for the persisted map —
+  don't derive the key from the label (a rename would orphan persisted state). The unlabeled Primary
+  group is always rendered.
+- **Accordions apply only in the expanded sidebar.** In the rail, group-collapse is ignored: every
+  item renders (icon-only), groups separated by a `.ctv-nav-divider`. The nav item's **label stays in
+  the a11y tree** (visually hidden via CSS, not `display:none`) so the accessible name — and every
+  `getByRole('link', { name })` test — still resolves; the label is also passed as the native `title`
+  tooltip (`NavItem` gained a `title` prop). Numeric badges collapse to a corner dot. The active-route
+  indicator works in both states.
+- **Testing note**: `App.test.tsx`'s shell/nav suite clicks Media/System nav links directly, so its
+  `beforeEach` **seeds both groups open** (`ctv-sidebar-groups`); the default-collapsed / accordion /
+  rail behavior is covered in its own `describe('collapsible sidebar (#396)')`, and the persistence
+  helpers have a colocated `sidebarState.test.ts`.
diff --git a/web/src/App.test.tsx b/web/src/App.test.tsx
index 9bea879ea..ef94623c4 100644
--- a/web/src/App.test.tsx
+++ b/web/src/App.test.tsx
@@ -16,6 +16,13 @@ describe('ChicoryTV SPA scaffold', () => {
 
   beforeEach(() => {
     window.localStorage.clear();
+    // #396: the sidebar's Media/System nav groups now default to COLLAPSED. The shell/nav tests
+    // below click nav links inside those groups directly, so keep them expanded here; the
+    // collapse/accordion behavior itself is covered in its own describe block at the end.
+    window.localStorage.setItem(
+      'ctv-sidebar-groups',
+      JSON.stringify({ media: false, system: false })
+    );
     document.documentElement.removeAttribute('data-theme');
     window.history.replaceState(null, '', '/app');
     vi.restoreAllMocks();
@@ -632,6 +639,88 @@ describe('ChicoryTV SPA scaffold', () => {
     expect(window.location.pathname).toBe('/app/settings/scanner');
     expect(window.fetch).toHaveBeenCalledWith('/api/v1/settings/scanner', expect.any(Object));
   });
+
+  // #396 — collapsible sidebar + collapsible nav groups. These tests want the real defaults
+  // (Media/System groups collapsed), so undo the outer beforeEach's "keep groups open" seed.
+  describe('collapsible sidebar (#396)', () => {
+    beforeEach(() => {
+      window.localStorage.removeItem('ctv-sidebar-groups');
+    });
+
+    it('collapses Media/System nav groups by default, leaving Primary visible', () => {
+      render(<App />);
+
+      // Primary group items always render.
+      expect(screen.getByRole('link', { name: 'Dashboard' })).toBeInTheDocument();
+      expect(screen.getByRole('link', { name: 'Channels' })).toBeInTheDocument();
+
+      // Labeled groups start collapsed → their items are not rendered, but the accordion headers are.
+      expect(screen.queryByRole('link', { name: 'Logs' })).not.toBeInTheDocument();
+      expect(screen.queryByRole('link', { name: 'Libraries' })).not.toBeInTheDocument();
+      expect(screen.getByRole('button', { name: 'Media', expanded: false })).toBeInTheDocument();
+      expect(screen.getByRole('button', { name: 'System', expanded: false })).toBeInTheDocument();
+    });
+
+    it('expands a nav group when its header is clicked and persists the open state', () => {
+      render(<App />);
+
+      const systemHeader = screen.getByRole('button', { name: 'System', expanded: false });
+      fireEvent.click(systemHeader);
+
+      expect(screen.getByRole('button', { name: 'System', expanded: true })).toBeInTheDocument();
+      expect(screen.getByRole('link', { name: 'Logs' })).toBeInTheDocument();
+
+      const stored = JSON.parse(window.localStorage.getItem('ctv-sidebar-groups') ?? '{}');
+      expect(stored.system).toBe(false);
+    });
+
+    it('restores expanded groups from localStorage on load', () => {
+      window.localStorage.setItem('ctv-sidebar-groups', JSON.stringify({ media: false }));
+
+      render(<App />);
+
+      // Media restored open; System still default-collapsed.
+      expect(screen.getByRole('link', { name: 'Libraries' })).toBeInTheDocument();
+      expect(screen.queryByRole('link', { name: 'Logs' })).not.toBeInTheDocument();
+    });
+
+    it('collapses the sidebar to the icon rail and persists it', () => {
+      const { container } = render(<App />);
+
+      const shell = container.querySelector('.ctv-app-shell');
+      expect(shell).not.toHaveClass('ctv-app-shell-collapsed');
+
+      fireEvent.click(screen.getByRole('button', { name: 'Collapse sidebar' }));
+
+      expect(shell).toHaveClass('ctv-app-shell-collapsed');
+      expect(window.localStorage.getItem('ctv-sidebar-collapsed')).toBe('1');
+      // The toggle now offers to expand again.
+      expect(screen.getByRole('button', { name: 'Expand sidebar' })).toBeInTheDocument();
+    });
+
+    it('shows every group item in the rail regardless of accordion state (labels kept for a11y)', () => {
+      // Sidebar collapsed on load; groups at their default (collapsed) accordion state.
+      window.localStorage.setItem('ctv-sidebar-collapsed', '1');
+
+      const { container } = render(<App />);
+
+      expect(container.querySelector('.ctv-app-shell')).toHaveClass('ctv-app-shell-collapsed');
+      // Accordions do not apply in the rail: Media/System items are all present (label stays in the
+      // a11y tree so the link keeps its accessible name), and no accordion header is rendered.
+      expect(screen.getByRole('link', { name: 'Logs' })).toBeInTheDocument();
+      expect(screen.getByRole('link', { name: 'Libraries' })).toBeInTheDocument();
+      expect(screen.queryByRole('button', { name: 'System' })).not.toBeInTheDocument();
+    });
+
+    it('restores the collapsed sidebar from localStorage on load', () => {
+      window.localStorage.setItem('ctv-sidebar-collapsed', '1');
+
+      const { container } = render(<App />);
+
+      expect(container.querySelector('.ctv-app-shell')).toHaveClass('ctv-app-shell-collapsed');
+      expect(screen.getByRole('button', { name: 'Expand sidebar' })).toBeInTheDocument();
+    });
+  });
 });
 
 function jsonResponse(body: unknown, status = 200): Response {
diff --git a/web/src/app/AppShell.tsx b/web/src/app/AppShell.tsx
index 5663c6c21..64097cf8b 100644
--- a/web/src/app/AppShell.tsx
+++ b/web/src/app/AppShell.tsx
@@ -17,6 +17,8 @@ import {
   ClipboardCopy,
   Info,
   ListVideo,
+  PanelLeftClose,
+  PanelLeftOpen,
   Plus,
   Search
 } from 'lucide-react';
@@ -29,8 +31,7 @@ import {
 import {
   Button,
   IconButton,
-  NavItem,
-  NavSection
+  NavItem
 } from '../components';
 import {
   designSystemThemes,
@@ -38,6 +39,7 @@ import {
 } from '../designSystem';
 import { usePrimaryActionHandler } from '../primaryAction';
 import { navigateToPath } from '../routing';
+import { useSidebarState } from './sidebarState';
 import { DashboardHealthSummary } from '../screens/DashboardScreen';
 import { UnauthorizedBanner } from '../UnauthorizedBanner';
 import { UserMenu } from '../UserMenu';
@@ -53,10 +55,12 @@ type NavigateHandler = (route: ScreenRoute, event: MouseEvent) => void;
 
 function SidebarNavGroup({
   activeRoute,
+  collapsed,
   ids,
   onNavigate
 }: {
   activeRoute: ScreenRoute | null;
+  collapsed: boolean;
   ids: readonly ScreenId[];
   onNavigate: NavigateHandler;
 }) {
@@ -70,6 +74,9 @@ function SidebarNavGroup({
             key={route.id}
             icon={route.icon}
             label={route.label}
+            // In the collapsed rail, the label is visually hidden (kept for a11y) so surface it
+            // as a native tooltip; expanded shows the label inline so a tooltip would be redundant.
+            title={collapsed ? route.label : undefined}
             active={route.id === activeRoute?.id}
             badge={route.badge}
             badgeTone="warn"
@@ -111,12 +118,20 @@ function ThemeSwitcher({
 
 function Sidebar({
   activeRoute,
+  collapsed,
   healthState,
-  onNavigate
+  isGroupCollapsed,
+  onNavigate,
+  onToggleCollapsed,
+  onToggleGroup
 }: {
   activeRoute: ScreenRoute | null;
+  collapsed: boolean;
   healthState: DashboardHealthQueryState;
+  isGroupCollapsed: (groupKey: string) => boolean;
   onNavigate: NavigateHandler;
+  onToggleCollapsed: () => void;
+  onToggleGroup: (groupKey: string) => void;
 }) {
   return (
     <aside className="ctv-sidebar">
@@ -125,15 +140,71 @@ function Sidebar({
         <span className="ctv-brand-wordmark">
           Chicory<span>TV</span>
         </span>
+        <button
+          type="button"
+          className="ctv-sidebar-toggle"
+          aria-label={collapsed ? 'Expand sidebar' : 'Collapse sidebar'}
+          aria-pressed={collapsed}
+          title={collapsed ? 'Expand sidebar' : 'Collapse sidebar'}
+          onClick={onToggleCollapsed}
+        >
+          {collapsed ? (
+            <PanelLeftOpen aria-hidden="true" size={17} />
+          ) : (
+            <PanelLeftClose aria-hidden="true" size={17} />
+          )}
+        </button>
       </div>
 
       <nav aria-label="Primary" className="ctv-nav">
-        {sidebarNavGroups.map(({ ids, label }) => (
-          <Fragment key={label ?? 'Primary'}>
-            {label ? <NavSection>{label}</NavSection> : null}
-            <SidebarNavGroup activeRoute={activeRoute} ids={ids} onNavigate={onNavigate} />
-          </Fragment>
-        ))}
+        {sidebarNavGroups.map((group) => {
+          const { ids, key, label } = group;
+
+          // Primary (unlabeled) is always open, in both the rail and the expanded sidebar.
+          if (!label) {
+            return (
+              <SidebarNavGroup
+                key="Primary"
+                activeRoute={activeRoute}
+                collapsed={collapsed}
+                ids={ids}
+                onNavigate={onNavigate}
+              />
+            );
+          }
+
+          const groupKey = key ?? label;
+          // Accordions only apply in the expanded sidebar; the rail always shows every item
+          // (separated by a hairline divider), so group-collapse is ignored there.
+          const groupCollapsed = isGroupCollapsed(groupKey);
+          const showItems = collapsed || !groupCollapsed;
+
+          return (
+            <Fragment key={groupKey}>
+              {collapsed ? (
+                <div className="ctv-nav-divider" role="presentation" />
+              ) : (
+                <button
+                  type="button"
+                  className="ctv-nav-group-header"
+                  aria-expanded={!groupCollapsed}
+                  onClick={() => onToggleGroup(groupKey)}
+                >
+                  <span>{label}</span>
+                  <ChevronDown className="ctv-nav-group-chevron" aria-hidden="true" size={14} />
+                </button>
+              )}
+              {showItems ? (
+                <SidebarNavGroup
+                  activeRoute={activeRoute}
+                  collapsed={collapsed}
+                  ids={ids}
+                  onNavigate={onNavigate}
+                />
+              ) : null}
+            </Fragment>
+          );
+        })}
       </nav>
 
       <div className="ctv-sidebar-health">
@@ -348,9 +419,19 @@ export function AppShell({
   route: ScreenRoute | null;
   theme: DesignSystemThemeId;
 }) {
+  const { collapsed, isGroupCollapsed, toggleCollapsed, toggleGroup } = useSidebarState();
+
   return (
-    <div className="ctv-app-shell">
-      <Sidebar activeRoute={route} healthState={healthState} onNavigate={onNavigate} />
+    <div className={`ctv-app-shell${collapsed ? ' ctv-app-shell-collapsed' : ''}`}>
+      <Sidebar
+        activeRoute={route}
+        collapsed={collapsed}
+        healthState={healthState}
+        isGroupCollapsed={isGroupCollapsed}
+        onNavigate={onNavigate}
+        onToggleCollapsed={toggleCollapsed}
+        onToggleGroup={toggleGroup}
+      />
       <div className="ctv-shell-body">
         <TopBar route={route} />
         <UnauthorizedBanner />
diff --git a/web/src/app/routes.tsx b/web/src/app/routes.tsx
index c0d56a49c..b81fb9ff1 100644
--- a/web/src/app/routes.tsx
+++ b/web/src/app/routes.tsx
@@ -464,6 +464,9 @@ export const routeById = new Map(routes.map((route) => [route.id, route]));
 
 export interface SidebarNavGroupDefinition {
   label?: string;
+  // Stable key for persisted collapse state (ctv-sidebar-groups). Only labeled (collapsible)
+  // groups need one; the unlabeled Primary group is always open (#396).
+  key?: string;
   ids: readonly ScreenId[];
 }
 
@@ -485,6 +488,7 @@ export const sidebarNavGroups: SidebarNavGroupDefinition[] = [
   },
   {
     label: 'Media',
+    key: 'media',
     ids: [
       'media',
       'search',
@@ -500,6 +504,7 @@ export const sidebarNavGroups: SidebarNavGroupDefinition[] = [
   },
   {
     label: 'System',
+    key: 'system',
     ids: [
       'settings',
       'apiKey',
diff --git a/web/src/app/sidebarState.test.ts b/web/src/app/sidebarState.test.ts
new file mode 100644
index 000000000..ec29b579a
--- /dev/null
+++ b/web/src/app/sidebarState.test.ts
@@ -0,0 +1,62 @@
+import { act, renderHook } from '@testing-library/react';
+import { beforeEach, describe, expect, it } from 'vitest';
+import {
+  getStoredSidebarCollapsed,
+  getStoredSidebarGroups,
+  useSidebarState
+} from './sidebarState';
+
+describe('sidebarState persistence (#396)', () => {
+  beforeEach(() => {
+    window.localStorage.clear();
+  });
+
+  it('defaults to expanded when nothing is stored', () => {
+    expect(getStoredSidebarCollapsed()).toBe(false);
+    expect(getStoredSidebarGroups()).toEqual({});
+  });
+
+  it('reads the collapsed flag only from the exact "1" sentinel', () => {
+    window.localStorage.setItem('ctv-sidebar-collapsed', '1');
+    expect(getStoredSidebarCollapsed()).toBe(true);
+
+    window.localStorage.setItem('ctv-sidebar-collapsed', '0');
+    expect(getStoredSidebarCollapsed()).toBe(false);
+  });
+
+  it('ignores malformed or non-boolean group JSON', () => {
+    window.localStorage.setItem('ctv-sidebar-groups', 'not json');
+    expect(getStoredSidebarGroups()).toEqual({});
+
+    window.localStorage.setItem('ctv-sidebar-groups', JSON.stringify(['media']));
+    expect(getStoredSidebarGroups()).toEqual({});
+
+    window.localStorage.setItem(
+      'ctv-sidebar-groups',
+      JSON.stringify({ media: true, system: 'nope' })
+    );
+    expect(getStoredSidebarGroups()).toEqual({ media: true });
+  });
+
+  it('treats an unknown labeled group as collapsed by default, and toggles + persists it', () => {
+    const { result } = renderHook(() => useSidebarState());
+
+    expect(result.current.isGroupCollapsed('media')).toBe(true);
+
+    act(() => result.current.toggleGroup('media'));
+
+    expect(result.current.isGroupCollapsed('media')).toBe(false);
+    expect(getStoredSidebarGroups()).toEqual({ media: false });
+  });
+
+  it('toggles + persists the sidebar collapsed flag', () => {
+    const { result } = renderHook(() => useSidebarState());
+
+    expect(result.current.collapsed).toBe(false);
+
+    act(() => result.current.toggleCollapsed());
+
+    expect(result.current.collapsed).toBe(true);
+    expect(getStoredSidebarCollapsed()).toBe(true);
+  });
+});
diff --git a/web/src/app/sidebarState.ts b/web/src/app/sidebarState.ts
new file mode 100644
index 000000000..f530bb6d8
--- /dev/null
+++ b/web/src/app/sidebarState.ts
@@ -0,0 +1,104 @@
+import { useCallback, useState } from 'react';
+
+// Persisted UI state for the shell sidebar (issue #396). Two client-local preferences,
+// namespaced `ctv-*` per the persisted-UI-state convention (spa-conventions.md §5d):
+//   - ctv-sidebar-collapsed: "1" | "0" — is the sidebar collapsed to the icon rail?
+//   - ctv-sidebar-groups:   JSON {groupKey: boolean} — is a labeled nav group collapsed?
+// A labeled group with no stored entry defaults to COLLAPSED, so a fresh load shows only the
+// always-open Primary group. (The prototype named these keys `ctv.sidebar.*`; we use the
+// established `ctv-` hyphen form — see docs/decisions.md 2026-07-18.)
+
+const COLLAPSED_KEY = 'ctv-sidebar-collapsed';
+const GROUPS_KEY = 'ctv-sidebar-groups';
+
+function getStorage(): Storage | null {
+  try {
+    return window.localStorage;
+  } catch {
+    // Storage can be disabled/unavailable (privacy mode, sandboxed iframe) — degrade to defaults.
+    return null;
+  }
+}
+
+export function getStoredSidebarCollapsed(): boolean {
+  return getStorage()?.getItem(COLLAPSED_KEY) === '1';
+}
+
+function setStoredSidebarCollapsed(collapsed: boolean): void {
+  try {
+    getStorage()?.setItem(COLLAPSED_KEY, collapsed ? '1' : '0');
+  } catch {
+    // ignore write failures (quota / unavailable) — in-memory state still updates
+  }
+}
+
+export function getStoredSidebarGroups(): Record<string, boolean> {
+  const raw = getStorage()?.getItem(GROUPS_KEY);
+  if (!raw) {
+    return {};
+  }
+
+  try {
+    const parsed: unknown = JSON.parse(raw);
+    if (parsed && typeof parsed === 'object' && !Array.isArray(parsed)) {
+      const out: Record<string, boolean> = {};
+      for (const [key, value] of Object.entries(parsed as Record<string, unknown>)) {
+        if (typeof value === 'boolean') {
+          out[key] = value;
+        }
+      }
+      return out;
+    }
+  } catch {
+    // malformed JSON — fall through to defaults
+  }
+
+  return {};
+}
+
+function setStoredSidebarGroups(groups: Record<string, boolean>): void {
+  try {
+    getStorage()?.setItem(GROUPS_KEY, JSON.stringify(groups));
+  } catch {
+    // ignore write failures — in-memory state still updates
+  }
+}
+
+export interface SidebarState {
+  /** Is the sidebar collapsed to the icon rail? */
+  collapsed: boolean;
+  toggleCollapsed: () => void;
+  /** Is a labeled nav group collapsed? Unknown groups default to collapsed. */
+  isGroupCollapsed: (groupKey: string) => boolean;
+  toggleGroup: (groupKey: string) => void;
+}
+
+export function useSidebarState(): SidebarState {
+  const [collapsed, setCollapsed] = useState<boolean>(getStoredSidebarCollapsed);
+  const [groups, setGroups] = useState<Record<string, boolean>>(getStoredSidebarGroups);
+
+  const toggleCollapsed = useCallback(() => {
+    setCollapsed((previous) => {
+      const next = !previous;
+      setStoredSidebarCollapsed(next);
+      return next;
+    });
+  }, []);
+
+  const isGroupCollapsed = useCallback(
+    // Absent entry ⇒ default collapsed for labeled groups.
+    (groupKey: string) => groups[groupKey] ?? true,
+    [groups]
+  );
+
+  const toggleGroup = useCallback((groupKey: string) => {
+    setGroups((previous) => {
+      const currentlyCollapsed = previous[groupKey] ?? true;
+      const next = { ...previous, [groupKey]: !currentlyCollapsed };
+      setStoredSidebarGroups(next);
+      return next;
+    });
+  }, []);
+
+  return { collapsed, toggleCollapsed, isGroupCollapsed, toggleGroup };
+}
diff --git a/web/src/components/components.css b/web/src/components/components.css
index bd9ab3a60..958b76644 100644
--- a/web/src/components/components.css
+++ b/web/src/components/components.css
@@ -861,6 +861,88 @@
   text-transform: uppercase;
 }
 
+/* Collapsible nav-group header (accordion toggle) — the interactive replacement for the static
+   NavSection label in the expanded sidebar (#396). */
+.ctv-nav-group-header {
+  box-sizing: border-box;
+  width: 100%;
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  margin: 8px 0 2px;
+  padding: 6px 10px;
+  border: 0;
+  border-radius: var(--radius-sm);
+  background: transparent;
+  color: var(--text-disabled);
+  cursor: pointer;
+  font-size: var(--text-2xs);
+  font-weight: var(--weight-semibold);
+  letter-spacing: var(--tracking-caps);
+  line-height: 1;
+  text-align: left;
+  text-transform: uppercase;
+  transition: color var(--dur-fast) var(--ease-standard);
+}
+
+.ctv-nav-group-header:hover {
+  color: var(--text-secondary);
+}
+
+.ctv-nav-group-header:focus-visible {
+  outline: none;
+  box-shadow: var(--ring-focus);
+}
+
+.ctv-nav-group-header > span {
+  flex: 1;
+  min-width: 0;
+}
+
+.ctv-nav-group-chevron {
+  flex: 0 0 auto;
+  transition: transform var(--dur-fast) var(--ease-standard);
+}
+
+/* Collapsed group ⇒ chevron points right (closed). */
+.ctv-nav-group-header[aria-expanded='false'] .ctv-nav-group-chevron {
+  transform: rotate(-90deg);
+}
+
+/* ---- Collapsed (icon-rail) nav items (#396) ---- */
+.ctv-app-shell-collapsed .ctv-nav-item {
+  justify-content: center;
+  gap: 0;
+  padding: 0;
+}
+
+/* Keep the label in the a11y tree (accessible name) but visually hidden in the rail; the native
+   title tooltip surfaces it for sighted mouse users. */
+.ctv-app-shell-collapsed .ctv-nav-label {
+  position: absolute;
+  width: 1px;
+  height: 1px;
+  padding: 0;
+  margin: -1px;
+  overflow: hidden;
+  clip-path: inset(50%);
+  white-space: nowrap;
+  border: 0;
+}
+
+/* A numeric badge collapses to a small corner dot in the rail. */
+.ctv-app-shell-collapsed .ctv-nav-badge {
+  position: absolute;
+  top: 5px;
+  right: 9px;
+  min-width: 0;
+  width: 7px;
+  height: 7px;
+  padding: 0;
+  font-size: 0;
+  line-height: 0;
+}
+
 .ctv-tabs {
   display: flex;
   gap: 2px;
diff --git a/web/src/components/navigation.tsx b/web/src/components/navigation.tsx
index bfaec8744..46b99f40a 100644
--- a/web/src/components/navigation.tsx
+++ b/web/src/components/navigation.tsx
@@ -13,6 +13,9 @@ export interface NavItemProps {
   href?: string;
   onClick?: (e: MouseEvent) => void;
   style?: CSSProperties;
+  // Native tooltip; used by the collapsed sidebar rail to surface the label that is visually
+  // hidden there (the label stays in the a11y tree for the accessible name — #396).
+  title?: string;
 }
 
 export function NavItem({
@@ -23,7 +26,8 @@ export function NavItem({
   badgeTone = 'warn',
   href,
   onClick,
-  style
+  style,
+  title
 }: NavItemProps) {
   const content = (
     <>
@@ -44,6 +48,7 @@ export function NavItem({
         onClick={onClick}
         className={classNames('ctv-nav-item', active && 'ctv-nav-item-active')}
         style={style}
+        title={title}
       >
         {content}
       </a>
@@ -57,6 +62,7 @@ export function NavItem({
       onClick={onClick}
       className={classNames('ctv-nav-item', active && 'ctv-nav-item-active')}
       style={style}
+      title={title}
     >
       {content}
     </button>
diff --git a/web/src/shell.css b/web/src/shell.css
index 3d52bfdbf..b6552e8f9 100644
--- a/web/src/shell.css
+++ b/web/src/shell.css
@@ -26,6 +26,21 @@ body {
   color: var(--text-primary);
   font-family: var(--font-sans, system-ui, sans-serif);
   font-size: var(--text-sm, 13px);
+  /* Animate the sidebar collapse/expand (#396). Only the grid track width transitions so the
+     content reflows smoothly without a layout jump. */
+  transition: grid-template-columns var(--dur-base, 140ms) var(--ease-standard);
+}
+
+/* Collapsed sidebar = the 60px icon rail (#396). Narrowing the tracked custom property drives the
+   grid-template-columns transition above. */
+.ctv-app-shell-collapsed {
+  --sidebar-w: 60px;
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .ctv-app-shell {
+    transition: none;
+  }
 }
 
 .ctv-sidebar {
@@ -97,6 +112,72 @@ body {
   font-size: var(--text-2xs, 11px);
 }
 
+/* Sidebar collapse toggle in the brand header (#396). */
+.ctv-sidebar-toggle {
+  margin-left: auto;
+  display: inline-flex;
+  flex: 0 0 auto;
+  align-items: center;
+  justify-content: center;
+  width: 28px;
+  height: 28px;
+  border: 0;
+  border-radius: var(--radius-sm, 5px);
+  background: transparent;
+  color: var(--text-disabled);
+  cursor: pointer;
+  transition:
+    background var(--dur-fast) var(--ease-standard),
+    color var(--dur-fast) var(--ease-standard);
+}
+
+.ctv-sidebar-toggle:hover {
+  background: var(--ctv-accent-soft);
+  color: var(--text-primary);
+}
+
+.ctv-sidebar-toggle:focus-visible {
+  outline: none;
+  box-shadow: var(--ring-focus);
+}
+
+/* Hairline divider shown between nav groups in the collapsed rail (accordions don't apply
+   there, so the group label/chevron is replaced by a plain separator — #396). */
+.ctv-nav-divider {
+  height: 1px;
+  margin: var(--space-4, 8px) var(--space-4, 8px);
+  background: var(--border-hairline);
+}
+
+/* ---- Collapsed (icon-rail) sidebar overrides (#396) ---- */
+.ctv-app-shell-collapsed .ctv-brand {
+  padding: 0;
+  justify-content: center;
+}
+
+.ctv-app-shell-collapsed .ctv-brand img,
+.ctv-app-shell-collapsed .ctv-brand-wordmark {
+  display: none;
+}
+
+.ctv-app-shell-collapsed .ctv-sidebar-toggle {
+  margin-left: 0;
+}
+
+/* Footer: hide the version text block and the health label, keep the status dot centered. */
+.ctv-app-shell-collapsed .ctv-sidebar-health {
+  justify-content: center;
+  padding: var(--space-6, 12px) 0;
+}
+
+.ctv-app-shell-collapsed .ctv-sidebar-health > div {
+  display: none;
+}
+
+.ctv-app-shell-collapsed .ctv-sidebar-health .ctv-status-dot > span:last-child {
+  display: none;
+}
+
 .ctv-shell-body {
   min-width: 0;
   height: 100vh;
@@ -3730,7 +3811,8 @@ body {
   }
 
   .ctv-nav,
-  .ctv-sidebar-health {
+  .ctv-sidebar-health,
+  .ctv-sidebar-toggle {
     display: none;
   }