frontend/package.json

@@ -7,7 +7,9 @@
     "dev": "vite",
     "build": "tsc -b && vite build",
     "lint": "eslint .",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "dependencies": {
     "@tanstack/react-query": "^5.100.9",
@@ -20,6 +22,8 @@
   "devDependencies": {
     "@eslint/js": "^10.0.1",
     "@tailwindcss/vite": "^4.2.4",
+    "@testing-library/jest-dom": "^6.9.1",
+    "@testing-library/react": "^16.3.2",
     "@types/node": "^24.12.2",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
@@ -29,10 +33,12 @@
     "eslint-plugin-react-hooks": "^7.1.1",
     "eslint-plugin-react-refresh": "^0.5.2",
     "globals": "^17.5.0",
+    "jsdom": "^29.1.1",
     "postcss": "^8.5.14",
     "tailwindcss": "^4.2.4",
     "typescript": "~6.0.2",
     "typescript-eslint": "^8.58.2",
-    "vite": "^8.0.10"
+    "vite": "^8.0.10",
+    "vitest": "^4.1.7"
   }
 }

frontend/src/components/Layout.tsx

@@ -1,6 +1,8 @@
 import { useState } from 'react'
 import { NavLink } from 'react-router-dom'
-import { Command, Activity, FolderKanban, Monitor, Settings, Menu, X } from 'lucide-react'
+import { Command, Activity, FolderKanban, Monitor, Settings, Menu, X, Wifi, WifiOff, Loader } from 'lucide-react'
+import { useSSEContext } from '../contexts/SSEContext'
+import type { SSEStatus } from '../hooks/useSSE'
 
 const navItems = [
   { to: '/', icon: Command, label: 'Hub' },
@@ -10,9 +12,29 @@ const navItems = [
   { to: '/settings', icon: Settings, label: 'Settings' },
 ]
 
+/** Small status pill shown in the sidebar footer and mobile header. */
+function SSEStatusBadge({ status, showLabel = false }: { status: SSEStatus; showLabel?: boolean }) {
+  const cfg = {
+    connected: { icon: Wifi, color: 'text-green-500', label: 'Live' },
+    connecting: { icon: Loader, color: 'text-yellow-500 animate-spin', label: 'Connecting' },
+    reconnecting: { icon: Loader, color: 'text-yellow-500 animate-spin', label: 'Reconnecting' },
+    error: { icon: WifiOff, color: 'text-red-500', label: 'Disconnected' },
+  }[status]
+
+  const Icon = cfg.icon
+
+  return (
+    <div className="flex items-center gap-1.5" title={cfg.label}>
+      <Icon size={14} className={cfg.color} />
+      {showLabel && <span className={`text-xs ${cfg.color}`}>{cfg.label}</span>}
+    </div>
+  )
+}
+
 export default function Layout({ children }: { children: React.ReactNode }) {
   const [expanded, setExpanded] = useState(false)
   const [mobileOpen, setMobileOpen] = useState(false)
+  const { sseStatus } = useSSEContext()
 
   return (
     <div className="flex min-h-screen bg-surface-darkest text-on-surface">
@@ -46,6 +68,15 @@ export default function Layout({ children }: { children: React.ReactNode }) {
             </NavLink>
           ))}
         </nav>
+        {/* SSE connection status — footer of sidebar */}
+        <div className="px-4 py-3 border-t border-surface-light flex items-center gap-2">
+          <SSEStatusBadge status={sseStatus} />
+          {expanded && (
+            <span className="text-xs text-on-surface-muted whitespace-nowrap">
+              {sseStatus === 'connected' ? 'Live updates on' : sseStatus}
+            </span>
+          )}
+        </div>
       </aside>
 
       {/* Mobile Header + Bottom Nav */}
@@ -54,6 +85,7 @@ export default function Layout({ children }: { children: React.ReactNode }) {
           <div className="flex items-center gap-2">
             <Command size={22} className="text-primary" />
             <span className="font-bold">Control Center</span>
+            <SSEStatusBadge status={sseStatus} />
           </div>
           <button onClick={() => setMobileOpen(!mobileOpen)} className="p-2">
             {mobileOpen ? <X size={22} /> : <Menu size={22} />}

frontend/src/contexts/SSEContext.tsx

@@ -0,0 +1,23 @@
+/**
+ * SSEContext — provides SSE connection status throughout the component tree.
+ * Mount <SSEProvider> once inside QueryClientProvider.
+ */
+import { createContext, useContext, type ReactNode } from 'react'
+import { useRealtimeSync } from '../hooks/useRealtimeSync'
+import type { SSEStatus } from '../hooks/useSSE'
+
+interface SSEContextValue {
+  sseStatus: SSEStatus
+}
+
+const SSEContext = createContext<SSEContextValue>({ sseStatus: 'connecting' })
+
+export function SSEProvider({ children }: { children: ReactNode }) {
+  const { sseStatus } = useRealtimeSync()
+  return <SSEContext.Provider value={{ sseStatus }}>{children}</SSEContext.Provider>
+}
+
+/** Access the SSE connection status from any component. */
+export function useSSEContext(): SSEContextValue {
+  return useContext(SSEContext)
+}

frontend/src/hooks/useRealtimeSync.test.tsx

@@ -0,0 +1,129 @@
+/**
+ * Tests for useRealtimeSync — event → query invalidation mapping.
+ *
+ * Uses .tsx extension so Vite/OXC can parse JSX in the wrapper component.
+ */
+import { renderHook } from '@testing-library/react'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
+import * as useSSEModule from './useSSE'
+import { useRealtimeSync } from './useRealtimeSync'
+import React from 'react'
+import type { SSEMessage } from '../services/sse'
+
+describe('useRealtimeSync', () => {
+  let queryClient: QueryClient
+  let mockSSEOnMessage: ((msg: { type: string; data: unknown }) => void) | null = null
+
+  beforeEach(() => {
+    queryClient = new QueryClient({
+      defaultOptions: { queries: { retry: false } },
+    })
+    mockSSEOnMessage = null
+
+    // Spy on useSSE to capture the onMessage callback
+    vi.spyOn(useSSEModule, 'useSSE').mockImplementation((opts) => {
+      mockSSEOnMessage = opts?.onMessage ?? null
+      return { status: 'connected' }
+    })
+  })
+
+  afterEach(() => {
+    vi.restoreAllMocks()
+  })
+
+  function render() {
+    return renderHook(() => useRealtimeSync(), {
+      wrapper: ({ children }: { children: React.ReactNode }) => (
+        React.createElement(QueryClientProvider, { client: queryClient }, children)
+      ),
+    })
+  }
+
+  it('invalidates ["agents"] on agent.status event', async () => {
+    const invalidateSpy = vi.spyOn(queryClient, 'invalidateQueries')
+    render()
+
+    const msg: SSEMessage = {
+      type: 'agent.status',
+      data: { agentId: 'a1', status: 'active' },
+    }
+    mockSSEOnMessage!(msg)
+
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['agents'] })
+    expect(invalidateSpy).toHaveBeenCalledTimes(1)
+  })
+
+  it('invalidates ["tasks"] and ["agents"] on agent.task event', async () => {
+    const invalidateSpy = vi.spyOn(queryClient, 'invalidateQueries')
+    render()
+
+    const msg: SSEMessage = {
+      type: 'agent.task',
+      data: { agentId: 'a1', taskId: 't1', title: 'Test', action: 'assigned' },
+    }
+    mockSSEOnMessage!(msg)
+
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['tasks'] })
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['agents'] })
+    expect(invalidateSpy).toHaveBeenCalledTimes(2)
+  })
+
+  it('invalidates ["tasks"] and ["agents"] on agent.progress event', async () => {
+    const invalidateSpy = vi.spyOn(queryClient, 'invalidateQueries')
+    render()
+
+    const msg: SSEMessage = {
+      type: 'agent.progress',
+      data: { agentId: 'a1', taskId: 't1', progress: 50, message: 'working' },
+    }
+    mockSSEOnMessage!(msg)
+
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['tasks'] })
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['agents'] })
+    expect(invalidateSpy).toHaveBeenCalledTimes(2)
+  })
+
+  it('invalidates ["agents"], ["sessions"], ["tasks"] on fleet.update event', async () => {
+    const invalidateSpy = vi.spyOn(queryClient, 'invalidateQueries')
+    render()
+
+    const msg: SSEMessage = {
+      type: 'fleet.update',
+      data: { timestamp: '2026-05-20T12:00:00Z', agentCount: 5 },
+    }
+    mockSSEOnMessage!(msg)
+
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['agents'] })
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['sessions'] })
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['tasks'] })
+    expect(invalidateSpy).toHaveBeenCalledTimes(3)
+  })
+
+  it('does nothing on connected event', async () => {
+    const invalidateSpy = vi.spyOn(queryClient, 'invalidateQueries')
+    render()
+
+    const msg: SSEMessage = {
+      type: 'connected',
+      data: { clientCount: 1 },
+    }
+    mockSSEOnMessage!(msg)
+
+    expect(invalidateSpy).not.toHaveBeenCalled()
+  })
+
+  it('does nothing on unknown event types', async () => {
+    const invalidateSpy = vi.spyOn(queryClient, 'invalidateQueries')
+    render()
+
+    mockSSEOnMessage!({ type: 'unknown.event', data: {} })
+
+    expect(invalidateSpy).not.toHaveBeenCalled()
+  })
+
+  it('returns sseStatus from useSSE', () => {
+    const { result } = render()
+    expect(result.current.sseStatus).toBe('connected')
+  })
+})

frontend/src/hooks/useRealtimeSync.ts

@@ -0,0 +1,64 @@
+/**
+ * useRealtimeSync — mounts the SSE connection once at the app level and
+ * wires incoming events to React Query cache invalidation.
+ *
+ * Event → query key mapping:
+ *   agent.status   → ['agents']
+ *   agent.task     → ['tasks'], ['agents']
+ *   agent.progress → ['tasks'], ['agents']
+ *   fleet.update   → ['agents'], ['sessions'], ['tasks']
+ */
+import { useQueryClient } from '@tanstack/react-query'
+import { useCallback } from 'react'
+import { useSSE, type SSEStatus } from './useSSE'
+import type { SSEMessage } from '../services/sse'
+
+export function useRealtimeSync(): { sseStatus: SSEStatus } {
+  const queryClient = useQueryClient()
+
+  const handleMessage = useCallback(
+    (raw: { type: string; data: unknown }) => {
+      // Cast to discriminated union — the backend contract guarantees these shapes
+      const msg = raw as SSEMessage
+
+      switch (msg.type) {
+        case 'agent.status':
+          // msg.data: AgentStatusEvent { agentId, status, reason? }
+          void msg.data.agentId // retained for type-narrowing — ensures payload matches contract
+          queryClient.invalidateQueries({ queryKey: ['agents'] })
+          break
+
+        case 'agent.task':
+          // msg.data: AgentTaskEvent { agentId, taskId, title, action }
+          void msg.data.agentId
+          queryClient.invalidateQueries({ queryKey: ['tasks'] })
+          queryClient.invalidateQueries({ queryKey: ['agents'] })
+          break
+
+        case 'agent.progress':
+          // msg.data: AgentProgressEvent { agentId, taskId, progress, message? }
+          void msg.data.agentId
+          queryClient.invalidateQueries({ queryKey: ['tasks'] })
+          queryClient.invalidateQueries({ queryKey: ['agents'] })
+          break
+
+        case 'fleet.update':
+          // msg.data: FleetUpdateEvent { timestamp, agentCount }
+          void msg.data.agentCount
+          queryClient.invalidateQueries({ queryKey: ['agents'] })
+          queryClient.invalidateQueries({ queryKey: ['sessions'] })
+          queryClient.invalidateQueries({ queryKey: ['tasks'] })
+          break
+
+        default:
+          // 'connected' and unknown events — no action needed
+          break
+      }
+    },
+    [queryClient],
+  )
+
+  const { status: sseStatus } = useSSE({ onMessage: handleMessage })
+
+  return { sseStatus }
+}

frontend/src/hooks/useSSE.test.ts

@@ -0,0 +1,267 @@
+/**
+ * Tests for useSSE — SSE connection lifecycle, back-off, event parsing, and cleanup.
+ *
+ * jsdom does not include EventSource, so we mock it completely.
+ */
+import { renderHook, act, waitFor } from '@testing-library/react'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { useSSE } from './useSSE'
+
+// ---------------------------------------------------------------------------
+// Mock EventSource — defined as a plain class so `new EventSource()` works
+// ---------------------------------------------------------------------------
+class MockEventSource {
+  url: string
+  onopen: (() => void) | null = null
+  onerror: ((evt: Event) => void) | null = null
+  onmessage: ((evt: MessageEvent) => void) | null = null
+  private listeners: Map<string, Array<(evt: Event) => void>> = new Map()
+  readyState: number = 0
+
+  constructor(url: string) {
+    this.url = url
+  }
+
+  addEventListener(type: string, handler: (evt: Event) => void) {
+    if (!this.listeners.has(type)) this.listeners.set(type, [])
+    this.listeners.get(type)!.push(handler)
+  }
+
+  removeEventListener() { /* no-op for tests */ }
+
+  close() {
+    this.readyState = 2
+    this.onopen = null
+    this.onerror = null
+    this.onmessage = null
+    this.listeners.clear()
+  }
+
+  // Test helpers
+  _simulateOpen() { this.onopen?.() }
+  _simulateError() { this.onerror?.(new Event('error')) }
+  _simulateNamedEvent(type: string, data: string) {
+    const handlers = this.listeners.get(type)
+    if (handlers) {
+      const evt = new MessageEvent(type, { data }) as Event
+      handlers.forEach((h) => h(evt))
+    }
+  }
+  _simulateMessage(data: string) {
+    this.onmessage?.(new MessageEvent('message', { data }) as MessageEvent)
+  }
+
+  static readonly CONNECTING = 0
+  static readonly OPEN = 1
+  static readonly CLOSED = 2
+}
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+let esInstances: MockEventSource[]
+
+describe('useSSE', () => {
+  beforeEach(() => {
+    esInstances = []
+    // Replace global EventSource with our mock class
+    Object.defineProperty(globalThis, 'EventSource', {
+      // The mock must use a class for `new EventSource()` to work
+      value: class extends MockEventSource {
+        constructor(url: string) {
+          super(url)
+          esInstances.push(this)
+        }
+      },
+      writable: true,
+      configurable: true,
+    })
+    vi.useFakeTimers({ shouldAdvanceTime: true })
+  })
+
+  afterEach(() => {
+    vi.restoreAllMocks()
+    vi.useRealTimers()
+  })
+
+  // ── Initial connection ──────────────────────────────────────────────────
+  it('starts in "connecting" state and creates an EventSource', () => {
+    const { result } = renderHook(() => useSSE({ url: '/api/events' }))
+
+    expect(result.current.status).toBe('connecting')
+    expect(esInstances.length).toBeGreaterThanOrEqual(1)
+    expect(esInstances[0].url).toBe('/api/events')
+  })
+
+  it('transitions to "connected" on open', async () => {
+    const onOpen = vi.fn()
+    const { result } = renderHook(() => useSSE({ url: '/api/events', onOpen }))
+
+    act(() => { esInstances[0]._simulateOpen() })
+
+    await waitFor(() => {
+      expect(result.current.status).toBe('connected')
+    })
+    expect(onOpen).toHaveBeenCalledTimes(1)
+  })
+
+  // ── Reconnection with exponential back-off ──────────────────────────────
+  it('retries after error with exponential back-off', async () => {
+    const { result } = renderHook(() =>
+      useSSE({ url: '/api/events', reconnectBaseMs: 1000, reconnectMaxMs: 30000 }),
+    )
+
+    // First error → reconnecting, retry at 1s
+    act(() => { esInstances[0]._simulateError() })
+    await waitFor(() => { expect(result.current.status).toBe('reconnecting') })
+    expect(esInstances).toHaveLength(1)
+
+    // Advance 1000ms → second EventSource created
+    act(() => { vi.advanceTimersByTime(1000) })
+    expect(esInstances).toHaveLength(2)
+
+    // Second error → reconnecting, retry at 2s
+    act(() => { esInstances[1]._simulateError() })
+    await waitFor(() => { expect(result.current.status).toBe('reconnecting') })
+    act(() => { vi.advanceTimersByTime(2000) })
+    expect(esInstances).toHaveLength(3)
+
+    // Third error → reconnecting, retry at 4s
+    act(() => { esInstances[2]._simulateError() })
+    act(() => { vi.advanceTimersByTime(4000) })
+    expect(esInstances).toHaveLength(4)
+  })
+
+  it('caps reconnect delay at reconnectMaxMs', async () => {
+    renderHook(() =>
+      useSSE({ url: '/api/events', reconnectBaseMs: 1000, reconnectMaxMs: 10000 }),
+    )
+
+    // Force 5 errors to push the exponent past the cap
+    for (let i = 0; i < 5; i++) {
+      act(() => { esInstances[i]._simulateError() })
+      const expectedDelay = Math.min(1000 * 2 ** i, 10000)
+      act(() => { vi.advanceTimersByTime(expectedDelay) })
+    }
+
+    // 6 ES instances created (initial + 5 retries)
+    expect(esInstances).toHaveLength(6)
+  })
+
+  // ── Circuit-breaker (max retries) ───────────────────────────────────────
+  it('transitions to "error" after reconnectLimit is exceeded', async () => {
+    const { result } = renderHook(() =>
+      useSSE({ url: '/api/events', reconnectBaseMs: 100, reconnectLimit: 2 }),
+    )
+
+    // First error → reconnecting
+    act(() => { esInstances[0]._simulateError() })
+    await waitFor(() => { expect(result.current.status).toBe('reconnecting') })
+
+    // Advance → retry
+    act(() => { vi.advanceTimersByTime(100) })
+
+    // Second error → reconnecting (attempt 2, still ≤ limit)
+    act(() => { esInstances[1]._simulateError() })
+    await waitFor(() => { expect(result.current.status).toBe('reconnecting') })
+    act(() => { vi.advanceTimersByTime(200) })
+
+    // Third error → limit exceeded (3 > 2) → error
+    act(() => { esInstances[2]._simulateError() })
+    await waitFor(() => { expect(result.current.status).toBe('error') })
+  })
+
+  it('resets reconnect counter on successful connection', async () => {
+    const { result } = renderHook(() =>
+      useSSE({ url: '/api/events', reconnectBaseMs: 100, reconnectLimit: 3 }),
+    )
+
+    // Two errors then a successful connect
+    act(() => { esInstances[0]._simulateError() })
+    act(() => { vi.advanceTimersByTime(100) })
+
+    act(() => { esInstances[1]._simulateOpen() })
+    await waitFor(() => { expect(result.current.status).toBe('connected') })
+
+    // Now error again — counter should be reset, so we get fresh attempts
+    act(() => { esInstances[1]._simulateError() })
+    await waitFor(() => { expect(result.current.status).toBe('reconnecting') })
+    expect(result.current.status).toBe('reconnecting')
+  })
+
+  // ── Cleanup on unmount ───────────────────────────────────────────────────
+  it('closes EventSource on unmount', () => {
+    const closeSpy = vi.spyOn(MockEventSource.prototype, 'close')
+    const { unmount } = renderHook(() => useSSE({ url: '/api/events' }))
+
+    unmount()
+    expect(closeSpy).toHaveBeenCalled()
+  })
+
+  it('does not update state after unmount', async () => {
+    const { result, unmount } = renderHook(() => useSSE({ url: '/api/events' }))
+
+    unmount()
+
+    // These should be no-ops after unmount (mountedRef guards)
+    act(() => { esInstances[0]._simulateOpen() })
+    act(() => { esInstances[0]._simulateError() })
+
+    // State should not have changed
+    expect(result.current.status).toBe('connecting')
+  })
+
+  // ── Event parsing ───────────────────────────────────────────────────────
+  it('parses valid JSON data into objects', async () => {
+    const onMessage = vi.fn()
+    renderHook(() => useSSE({ url: '/api/events', onMessage }))
+
+    act(() => {
+      esInstances[0]._simulateNamedEvent('agent.status', JSON.stringify({ agentId: 'a1', status: 'active' }))
+    })
+
+    expect(onMessage).toHaveBeenCalledWith({
+      type: 'agent.status',
+      data: { agentId: 'a1', status: 'active' },
+    })
+  })
+
+  it('passes invalid JSON through as raw string', async () => {
+    const onMessage = vi.fn()
+    renderHook(() => useSSE({ url: '/api/events', onMessage }))
+
+    act(() => {
+      esInstances[0]._simulateNamedEvent('agent.status', 'not valid json {{{')
+    })
+
+    expect(onMessage).toHaveBeenCalledWith({
+      type: 'agent.status',
+      data: 'not valid json {{{',
+    })
+  })
+
+  // ── enabled=false skips connection ──────────────────────────────────────
+  it('does not create EventSource when enabled=false', () => {
+    const { result } = renderHook(() => useSSE({ url: '/api/events', enabled: false }))
+
+    expect(esInstances).toHaveLength(0)
+    expect(result.current.status).toBe('connecting')
+  })
+
+  // ── onError callback ────────────────────────────────────────────────────
+  it('calls onError on connection failure', async () => {
+    const onError = vi.fn()
+    renderHook(() =>
+      useSSE({ url: '/api/events', onError, reconnectBaseMs: 100 }),
+    )
+
+    act(() => { esInstances[0]._simulateError() })
+    expect(onError).toHaveBeenCalledTimes(1)
+  })
+
+  // ── Default URL ─────────────────────────────────────────────────────────
+  it('uses /api/events as default URL', () => {
+    renderHook(() => useSSE())
+    expect(esInstances[0].url).toBe('/api/events')
+  })
+})

frontend/src/hooks/useSSE.ts

@@ -0,0 +1,180 @@
+import { useEffect, useRef, useCallback, useState } from 'react'
+
+/** SSE connection state reported to consumers. */
+export type SSEStatus = 'connecting' | 'connected' | 'reconnecting' | 'error'
+
+/** Typed SSE event received from the backend. */
+export interface SSEMessage {
+  /** event: field from the SSE frame */
+  type: string
+  /** parsed JSON from the data: field */
+  data: unknown
+}
+
+export interface UseSSEOptions {
+  /** Endpoint URL — defaults to /api/events */
+  url?: string
+  /** Called for every SSE message (all event types) */
+  onMessage?: (msg: SSEMessage) => void
+  /** Called when connection opens or reconnects */
+  onOpen?: () => void
+  /** Called on every connection error (both transient and terminal) */
+  onError?: (err: Event) => void
+  /** Base delay in ms before the first reconnect attempt (default 1 000) */
+  reconnectBaseMs?: number
+  /** Maximum reconnect delay in ms (default 30 000) */
+  reconnectMaxMs?: number
+  /**
+   * Maximum number of consecutive reconnect attempts before giving up.
+   * When the limit is reached, status transitions to 'error'.
+   * Default undefined (unlimited).
+   */
+  reconnectLimit?: number
+  /** Set false to disable auto-connect (useful in tests) */
+  enabled?: boolean
+}
+
+const SSE_EVENTS = ['agent.status', 'agent.task', 'agent.progress', 'fleet.update', 'connected'] as const
+
+/**
+ * useSSE — mounts a persistent SSE connection to the Control Center backend.
+ *
+ * Handles:
+ *  - Initial connection on mount
+ *  - Exponential back-off reconnection on drop (1s → 2s → 4s … capped at reconnectMaxMs)
+ *  - Circuit-breaker: after reconnectLimit consecutive failures, transitions to 'error'
+ *  - Cleanup on unmount
+ *  - All five event types: agent.status, agent.task, agent.progress, fleet.update, connected
+ *
+ * The 'connected' SSE event is an application-level handshake sent by the backend
+ * after the transport opens. This is distinct from onOpen, which fires at the
+ * transport level when the EventSource HTTP connection is established.
+ */
+export function useSSE({
+  url = '/api/events',
+  onMessage,
+  onOpen,
+  onError,
+  reconnectBaseMs = 1_000,
+  reconnectMaxMs = 30_000,
+  reconnectLimit,
+  enabled = true,
+}: UseSSEOptions = {}): { status: SSEStatus } {
+  const [status, setStatus] = useState<SSEStatus>('connecting')
+
+  // Stable refs so the effect doesn't need to re-run when callbacks change
+  const onMessageRef = useRef(onMessage)
+  const onOpenRef = useRef(onOpen)
+  const onErrorRef = useRef(onError)
+  onMessageRef.current = onMessage
+  onOpenRef.current = onOpen
+  onErrorRef.current = onError
+
+  const reconnectAttemptRef = useRef(0)
+  const reconnectTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null)
+  const esRef = useRef<EventSource | null>(null)
+  const mountedRef = useRef(true)
+
+  const clearReconnectTimer = useCallback(() => {
+    if (reconnectTimerRef.current !== null) {
+      clearTimeout(reconnectTimerRef.current)
+      reconnectTimerRef.current = null
+    }
+  }, [])
+
+  const connect = useCallback(() => {
+    if (!mountedRef.current || !enabled) return
+
+    // Clean up any existing connection
+    if (esRef.current) {
+      esRef.current.close()
+      esRef.current = null
+    }
+
+    setStatus(reconnectAttemptRef.current === 0 ? 'connecting' : 'reconnecting')
+
+    const es = new EventSource(url)
+    esRef.current = es
+
+    es.onopen = () => {
+      if (!mountedRef.current) return
+      reconnectAttemptRef.current = 0
+      setStatus('connected')
+      onOpenRef.current?.()
+    }
+
+    es.onerror = (evt) => {
+      if (!mountedRef.current) return
+
+      // EventSource auto-retries but we manage our own to get back-off control
+      es.close()
+      esRef.current = null
+
+      onErrorRef.current?.(evt)
+
+      reconnectAttemptRef.current += 1
+
+      // Circuit-breaker: give up after reconnectLimit consecutive failures
+      if (reconnectLimit !== undefined && reconnectAttemptRef.current > reconnectLimit) {
+        setStatus('error')
+        return
+      }
+
+      // Exponential back-off: 1s, 2s, 4s … capped at reconnectMaxMs
+      // Note: attempt is 1-based here (already incremented), so we use attempt-1 for the exponent
+      const delay = Math.min(
+        reconnectBaseMs * 2 ** (reconnectAttemptRef.current - 1),
+        reconnectMaxMs,
+      )
+      setStatus('reconnecting')
+
+      clearReconnectTimer()
+      reconnectTimerRef.current = setTimeout(() => {
+        if (mountedRef.current) connect()
+      }, delay)
+    }
+
+    // Register listeners for all known event types
+    for (const eventType of SSE_EVENTS) {
+      es.addEventListener(eventType, (evt: MessageEvent) => {
+        if (!mountedRef.current) return
+        let data: unknown = evt.data
+        try {
+          data = JSON.parse(evt.data as string)
+        } catch {
+          // leave as raw string
+        }
+        onMessageRef.current?.({ type: eventType, data })
+      })
+    }
+
+    // Catch-all for unnamed events (type == 'message').
+    // Won't fire for the named events registered via addEventListener above.
+    es.onmessage = (evt: MessageEvent) => {
+      if (!mountedRef.current) return
+      let data: unknown = evt.data
+      try {
+        data = JSON.parse(evt.data as string)
+      } catch {
+        // leave as raw string
+      }
+      onMessageRef.current?.({ type: 'message', data })
+    }
+  }, [url, enabled, reconnectBaseMs, reconnectMaxMs, reconnectLimit, clearReconnectTimer])
+
+  useEffect(() => {
+    mountedRef.current = true
+    if (enabled) connect()
+
+    return () => {
+      mountedRef.current = false
+      clearReconnectTimer()
+      if (esRef.current) {
+        esRef.current.close()
+        esRef.current = null
+      }
+    }
+  }, [connect, enabled, clearReconnectTimer])
+
+  return { status }
+}

frontend/src/main.tsx

@@ -4,13 +4,16 @@ import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
 import { BrowserRouter } from 'react-router-dom'
 import ErrorBoundary from './components/ErrorBoundary'
 import { ThemeProvider } from './hooks/useTheme'
+import { SSEProvider } from './contexts/SSEContext'
 import './index.css'
 import App from './App'
 
 const queryClient = new QueryClient({
   defaultOptions: {
     queries: {
-      staleTime: 30_000,
+      // No polling — real-time updates come through SSE.
+      // staleTime is kept high; data is pushed, not pulled.
+      staleTime: 60_000,
       refetchOnWindowFocus: false,
       retry: 1,
     },
@@ -22,9 +25,13 @@ createRoot(document.getElementById('root')!).render(
     <ErrorBoundary>
       <ThemeProvider>
         <QueryClientProvider client={queryClient}>
+          {/* SSEProvider must live inside QueryClientProvider so it can call
+              useQueryClient() to invalidate caches on incoming events. */}
+          <SSEProvider>
             <BrowserRouter>
               <App />
             </BrowserRouter>
+          </SSEProvider>
         </QueryClientProvider>
       </ThemeProvider>
     </ErrorBoundary>

frontend/src/pages/SettingsPage.tsx

@@ -1,18 +1,36 @@
 import { useTheme } from '../hooks/useTheme'
 import { useLocalStorage } from '../hooks/useLocalStorage'
-import { Sun, Moon, Monitor, Zap, Clock } from 'lucide-react'
+import { useSSEContext } from '../contexts/SSEContext'
+import { Sun, Moon, Monitor, Zap, Radio } from 'lucide-react'
 
-const REFRESH_PRESETS = [
-  { label: '5s', value: 5_000 },
-  { label: '10s', value: 10_000 },
-  { label: '30s', value: 30_000 },
-  { label: '60s', value: 60_000 },
-]
+const SSE_STATUS_COPY: Record<string, { label: string; description: string; color: string }> = {
+  connected: {
+    label: 'Connected',
+    description: 'Real-time updates are active. Agent status, tasks, and progress stream live.',
+    color: 'text-green-500',
+  },
+  connecting: {
+    label: 'Connecting…',
+    description: 'Establishing SSE connection to the backend.',
+    color: 'text-yellow-500',
+  },
+  reconnecting: {
+    label: 'Reconnecting…',
+    description: 'Connection lost. Retrying with exponential back-off.',
+    color: 'text-yellow-500',
+  },
+  error: {
+    label: 'Disconnected',
+    description: 'Could not connect to the SSE endpoint. Check that the backend is reachable.',
+    color: 'text-red-500',
+  },
+}
 
 export default function SettingsPage() {
   const { isDark, toggleTheme } = useTheme()
   const [gatewayUrl, setGatewayUrl] = useLocalStorage('cc-gateway-url', '')
-  const [refreshInterval, setRefreshInterval] = useLocalStorage('cc-refresh-interval', 30_000)
+  const { sseStatus } = useSSEContext()
+  const sseInfo = SSE_STATUS_COPY[sseStatus] ?? SSE_STATUS_COPY.error
 
   return (
     <div className="space-y-8 max-w-2xl">
@@ -80,45 +98,31 @@ export default function SettingsPage() {
         </div>
       </section>
 
-      {/* Refresh */}
+      {/* Real-time connection status */}
       <section className="space-y-4">
         <h2 className="text-lg font-semibold flex items-center gap-2">
-          <Clock size={20} className="text-primary" />
-          Auto Refresh
+          <Radio size={20} className="text-primary" />
+          Real-time Updates
         </h2>
 
         <div className="p-5 rounded-xl border border-surface-light bg-surface-dark space-y-3">
-          <p className="text-sm text-on-surface-variant">Data refresh interval for agent status and logs</p>
-
-          <div className="flex flex-col gap-2">
-            <input
-              type="range"
-              min="0"
-              max="3"
-              step="1"
-              value={REFRESH_PRESETS.findIndex((p) => p.value === refreshInterval)}
-              onChange={(e) => {
-                const idx = parseInt(e.target.value)
-                setRefreshInterval(REFRESH_PRESETS[idx].value)
-              }}
-              className="w-full accent-primary"
-            />
-            <div className="flex justify-between text-xs text-on-surface-muted">
-              {REFRESH_PRESETS.map((p) => (
-                <button
-                  key={p.label}
-                  onClick={() => setRefreshInterval(p.value)}
-                  className={`px-3 py-1 rounded-lg transition-colors ${
-                    refreshInterval === p.value
-                      ? 'bg-primary/10 text-primary'
-                      : 'hover:bg-surface-light'
-                  }`}
-                >
-                  {p.label}
-                </button>
-              ))}
+          <div className="flex items-center justify-between">
+            <div>
+              <p className="font-medium">SSE Connection</p>
+              <p className="text-sm text-on-surface-variant mt-0.5">{sseInfo.description}</p>
             </div>
+            <span className={`text-sm font-semibold whitespace-nowrap ${sseInfo.color}`}>
+              {sseInfo.label}
+            </span>
           </div>
+          <p className="text-xs text-on-surface-muted">
+            Endpoint: <code className="bg-surface-light px-1.5 py-0.5 rounded text-on-surface-variant">/api/events</code>
+            &nbsp;·&nbsp;Events: agent.status, agent.task, agent.progress, fleet.update
+          </p>
+          <p className="text-xs text-on-surface-muted">
+            Polling is disabled. All status updates are pushed from the server over a persistent SSE connection.
+            The client reconnects automatically with exponential back-off on drop.
+          </p>
         </div>
       </section>
     </div>

frontend/src/services/sse.ts

@@ -0,0 +1,72 @@
+/**
+ * SSE event payload types matching the Go backend (internal/handler/sse.go).
+ *
+ * Event format on the wire:
+ *   event: <eventType>
+ *   data: <json>
+ *
+ * The types below define the backend contract. The SSEPayloadMap maps
+ * each event type string to its expected payload shape. SSEMessage is a
+ * discriminated union on `type` — when you switch on msg.type, TypeScript
+ * narrows msg.data to the correct payload interface automatically.
+ */
+
+import type { AgentStatus } from '../types'
+
+/** agent.status — agent came online, went offline, changed state */
+export interface AgentStatusEvent {
+  agentId: string
+  status: AgentStatus
+  /** Optional human-readable reason (e.g. error message) */
+  reason?: string
+}
+
+/** agent.task — a task was assigned to or completed by an agent */
+export interface AgentTaskEvent {
+  agentId: string
+  taskId: string
+  title: string
+  action: 'assigned' | 'completed' | 'failed'
+}
+
+/** agent.progress — incremental progress update for a running task */
+export interface AgentProgressEvent {
+  agentId: string
+  taskId: string
+  progress: number
+  /** Optional description of what is currently happening */
+  message?: string
+}
+
+/**
+ * fleet.update — bulk refresh of all agents (e.g. after a deployment).
+ * The backend may send partial or complete agent state.
+ */
+export interface FleetUpdateEvent {
+  /** ISO timestamp of when the snapshot was taken */
+  timestamp: string
+  /** Number of agents in the fleet */
+  agentCount: number
+}
+
+/** Union of all SSE data payloads keyed by event type. */
+export type SSEPayloadMap = {
+  'agent.status': AgentStatusEvent
+  'agent.task': AgentTaskEvent
+  'agent.progress': AgentProgressEvent
+  'fleet.update': FleetUpdateEvent
+  connected: { clientCount: number }
+  message: unknown
+}
+
+/**
+ * Discriminated SSE message — the `type` field narrows `data` via SSEPayloadMap.
+ *
+ * Usage:
+ *   if (msg.type === 'agent.status') {
+ *     msg.data.agentId  // ✅ TypeScript knows this is AgentStatusEvent
+ *   }
+ */
+export type SSEMessage = {
+  [K in keyof SSEPayloadMap]: { type: K; data: SSEPayloadMap[K] }
+}[keyof SSEPayloadMap]

frontend/src/test-setup.ts

@@ -0,0 +1 @@
+import '@testing-library/jest-dom'

frontend/src/types/index.ts

@@ -1,4 +1,4 @@
-export type AgentStatus = 'active' | 'idle' | 'thinking' | 'error'
+export type AgentStatus = 'active' | 'idle' | 'thinking' | 'error' | 'offline'
 
 export interface Agent {
   id: string

frontend/tsconfig.app.json

@@ -4,7 +4,7 @@
     "target": "es2023",
     "lib": ["ES2023", "DOM"],
     "module": "esnext",
-    "types": ["vite/client"],
+    "types": ["vite/client", "vitest/globals"],
     "skipLibCheck": true,
 
     /* Bundler mode */

frontend/tsconfig.node.json

@@ -20,5 +20,5 @@
     "erasableSyntaxOnly": true,
     "noFallthroughCasesInSwitch": true
   },
-  "include": ["vite.config.ts"]
+  "include": ["vite.config.ts", "vitest.config.ts"]
 }

frontend/vitest.config.ts

@@ -0,0 +1,11 @@
+import { defineConfig } from 'vitest/config'
+import react from '@vitejs/plugin-react'
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./src/test-setup.ts'],
+  },
+})