diff --git a/frontend/src/components/SessionSidebar.tsx b/frontend/src/components/SessionSidebar.tsx
new file mode 100644
index 0000000..9813a23
--- /dev/null
+++ b/frontend/src/components/SessionSidebar.tsx
@@ -0,0 +1,78 @@
+import { IconPlus, IconLogout } from '@tabler/icons-react'
+import type { Session } from '../lib/types'
+
+interface SessionSidebarProps {
+  sessions: Session[]
+  activeSessionId: string | null
+  onSelect: (id: string) => void
+  onCreate: () => void
+  onLogout: () => void
+}
+
+export function SessionSidebar({
+  sessions,
+  activeSessionId,
+  onSelect,
+  onCreate,
+  onLogout,
+}: SessionSidebarProps) {
+  return (
+    <div className='sidebar'>
+      <div className='sidebar-header'>
+        <h3>Sessions</h3>
+        <div style={{ display: 'flex' }}>
+          <button
+            title='New session'
+            onClick={onCreate}
+            style={{
+              border: 'none',
+              background: 'transparent',
+              cursor: 'pointer',
+              padding: '4px',
+            }}
+          >
+            <IconPlus size={18} color='#7aa2f7' />
+          </button>
+          <button
+            title='Log out'
+            onClick={onLogout}
+            style={{
+              border: 'none',
+              background: 'transparent',
+              cursor: 'pointer',
+              padding: '4px',
+            }}
+          >
+            <IconLogout size={18} color='#8888aa' />
+          </button>
+        </div>
+      </div>
+      <div className='session-list'>
+        {sessions.length === 0 ? (
+          <div
+            style={{
+              color: '#6a6a8a',
+              fontSize: '0.85rem',
+              padding: '1rem',
+            }}
+          >
+            No sessions yet. Click + to start researching.
+          </div>
+        ) : (
+          sessions.map(s => (
+            <div
+              key={s.session_id}
+              className={`session-item ${s.session_id === activeSessionId ? 'active' : ''}`}
+              onClick={() => onSelect(s.session_id)}
+            >
+              {s.session_id.slice(0, 12)}...
+              <div className='session-meta'>
+                {s.total_messages ?? 0} msgs · {s.status}
+              </div>
+            </div>
+          ))
+        )}
+      </div>
+    </div>
+  )
+}
diff --git a/frontend/src/test/SessionSidebar.test.tsx b/frontend/src/test/SessionSidebar.test.tsx
new file mode 100644
index 0000000..0685334
--- /dev/null
+++ b/frontend/src/test/SessionSidebar.test.tsx
@@ -0,0 +1,260 @@
+import { describe, it, expect, vi } from 'vitest'
+import { render, screen } from '@testing-library/react'
+import userEvent from '@testing-library/user-event'
+import { SessionSidebar } from '../components/SessionSidebar'
+import type { Session } from '../lib/types'
+
+const makeSession = (overrides: Partial<Session> = {}): Session => ({
+  session_id: 'abcdefghijklmnop',
+  status: 'active',
+  bundle: 'default',
+  created_at: '2024-01-01T00:00:00Z',
+  last_activity: '2024-01-01T00:00:00Z',
+  total_messages: 5,
+  tool_invocations: 2,
+  stale: false,
+  ...overrides,
+})
+
+describe('SessionSidebar', () => {
+  it('renders a sidebar div with sidebar-header containing h3 Sessions', () => {
+    const { container } = render(
+      <SessionSidebar
+        sessions={[]}
+        activeSessionId={null}
+        onSelect={vi.fn()}
+        onCreate={vi.fn()}
+        onLogout={vi.fn()}
+      />
+    )
+
+    expect(container.querySelector('.sidebar')).toBeTruthy()
+    expect(container.querySelector('.sidebar-header')).toBeTruthy()
+    expect(screen.getByRole('heading', { name: 'Sessions' })).toBeTruthy()
+  })
+
+  it('renders a create button with title "New session"', () => {
+    render(
+      <SessionSidebar
+        sessions={[]}
+        activeSessionId={null}
+        onSelect={vi.fn()}
+        onCreate={vi.fn()}
+        onLogout={vi.fn()}
+      />
+    )
+
+    const createBtn = screen.getByTitle('New session')
+    expect(createBtn).toBeTruthy()
+  })
+
+  it('renders a logout button with title "Log out"', () => {
+    render(
+      <SessionSidebar
+        sessions={[]}
+        activeSessionId={null}
+        onSelect={vi.fn()}
+        onCreate={vi.fn()}
+        onLogout={vi.fn()}
+      />
+    )
+
+    const logoutBtn = screen.getByTitle('Log out')
+    expect(logoutBtn).toBeTruthy()
+  })
+
+  it('calls onCreate when create button is clicked', async () => {
+    const onCreate = vi.fn()
+    render(
+      <SessionSidebar
+        sessions={[]}
+        activeSessionId={null}
+        onSelect={vi.fn()}
+        onCreate={onCreate}
+        onLogout={vi.fn()}
+      />
+    )
+
+    await userEvent.click(screen.getByTitle('New session'))
+    expect(onCreate).toHaveBeenCalledOnce()
+  })
+
+  it('calls onLogout when logout button is clicked', async () => {
+    const onLogout = vi.fn()
+    render(
+      <SessionSidebar
+        sessions={[]}
+        activeSessionId={null}
+        onSelect={vi.fn()}
+        onCreate={vi.fn()}
+        onLogout={onLogout}
+      />
+    )
+
+    await userEvent.click(screen.getByTitle('Log out'))
+    expect(onLogout).toHaveBeenCalledOnce()
+  })
+
+  it('shows empty-state placeholder when sessions array is empty', () => {
+    render(
+      <SessionSidebar
+        sessions={[]}
+        activeSessionId={null}
+        onSelect={vi.fn()}
+        onCreate={vi.fn()}
+        onLogout={vi.fn()}
+      />
+    )
+
+    expect(
+      screen.getByText('No sessions yet. Click + to start researching.')
+    ).toBeTruthy()
+  })
+
+  it('renders .session-list container', () => {
+    const { container } = render(
+      <SessionSidebar
+        sessions={[]}
+        activeSessionId={null}
+        onSelect={vi.fn()}
+        onCreate={vi.fn()}
+        onLogout={vi.fn()}
+      />
+    )
+
+    expect(container.querySelector('.session-list')).toBeTruthy()
+  })
+
+  it('does not show placeholder when sessions are present', () => {
+    render(
+      <SessionSidebar
+        sessions={[makeSession()]}
+        activeSessionId={null}
+        onSelect={vi.fn()}
+        onCreate={vi.fn()}
+        onLogout={vi.fn()}
+      />
+    )
+
+    expect(
+      screen.queryByText('No sessions yet. Click + to start researching.')
+    ).toBeNull()
+  })
+
+  it('renders session items with truncated session_id', () => {
+    const session = makeSession({ session_id: 'abcdefghijklmnopqrst' })
+    render(
+      <SessionSidebar
+        sessions={[session]}
+        activeSessionId={null}
+        onSelect={vi.fn()}
+        onCreate={vi.fn()}
+        onLogout={vi.fn()}
+      />
+    )
+
+    // First 12 chars + '...'
+    expect(screen.getByText('abcdefghijkl...')).toBeTruthy()
+  })
+
+  it('renders session-meta with message count and status', () => {
+    const session = makeSession({ total_messages: 7, status: 'idle' })
+    render(
+      <SessionSidebar
+        sessions={[session]}
+        activeSessionId={null}
+        onSelect={vi.fn()}
+        onCreate={vi.fn()}
+        onLogout={vi.fn()}
+      />
+    )
+
+    expect(screen.getByText('7 msgs · idle')).toBeTruthy()
+  })
+
+  it('shows 0 msgs when total_messages is null/undefined', () => {
+    // Session with total_messages as 0 via override
+    const session = makeSession({ total_messages: 0 })
+    render(
+      <SessionSidebar
+        sessions={[session]}
+        activeSessionId={null}
+        onSelect={vi.fn()}
+        onCreate={vi.fn()}
+        onLogout={vi.fn()}
+      />
+    )
+
+    expect(screen.getByText('0 msgs · active')).toBeTruthy()
+  })
+
+  it('adds "active" class to session item when it matches activeSessionId', () => {
+    const session = makeSession({ session_id: 'sess-abc-123' })
+    const { container } = render(
+      <SessionSidebar
+        sessions={[session]}
+        activeSessionId='sess-abc-123'
+        onSelect={vi.fn()}
+        onCreate={vi.fn()}
+        onLogout={vi.fn()}
+      />
+    )
+
+    const item = container.querySelector('.session-item')
+    expect(item?.classList.contains('active')).toBe(true)
+  })
+
+  it('does not add "active" class when session is not the active one', () => {
+    const session = makeSession({ session_id: 'sess-abc-123' })
+    const { container } = render(
+      <SessionSidebar
+        sessions={[session]}
+        activeSessionId='other-session'
+        onSelect={vi.fn()}
+        onCreate={vi.fn()}
+        onLogout={vi.fn()}
+      />
+    )
+
+    const item = container.querySelector('.session-item')
+    expect(item?.classList.contains('active')).toBe(false)
+  })
+
+  it('calls onSelect with session_id when session item is clicked', async () => {
+    const onSelect = vi.fn()
+    const session = makeSession({ session_id: 'sess-xyz-789' })
+    render(
+      <SessionSidebar
+        sessions={[session]}
+        activeSessionId={null}
+        onSelect={onSelect}
+        onCreate={vi.fn()}
+        onLogout={vi.fn()}
+      />
+    )
+
+    await userEvent.click(screen.getByText('sess-xyz-789...'))
+    expect(onSelect).toHaveBeenCalledWith('sess-xyz-789')
+  })
+
+  it('renders multiple sessions', () => {
+    // 'session-one-abc' -> first 12 chars -> 'session-one-'
+    // 'session-two-xyz' -> first 12 chars -> 'session-two-'
+    const sessions = [
+      makeSession({ session_id: 'session-one-abc' }),
+      makeSession({ session_id: 'session-two-xyz' }),
+    ]
+    render(
+      <SessionSidebar
+        sessions={sessions}
+        activeSessionId={null}
+        onSelect={vi.fn()}
+        onCreate={vi.fn()}
+        onLogout={vi.fn()}
+      />
+    )
+
+    expect(screen.getByText('session-one-...')).toBeTruthy()
+    expect(screen.getByText('session-two-...')).toBeTruthy()
+  })
+})