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/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

@@ -10,29 +10,41 @@
  */
 import { useQueryClient } from '@tanstack/react-query'
 import { useCallback } from 'react'
-import { useSSE, type SSEMessage, type SSEStatus } from './useSSE'
+import { useSSE, type SSEStatus } from './useSSE'
+import type { SSEMessage } from '../services/sse'
 
 export function useRealtimeSync(): { sseStatus: SSEStatus } {
   const queryClient = useQueryClient()
 
   const handleMessage = useCallback(
-    (msg: SSEMessage) => {
+    (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'] })

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

@@ -18,12 +18,18 @@ export interface UseSSEOptions {
   onMessage?: (msg: SSEMessage) => void
   /** Called when connection opens or reconnects */
   onOpen?: () => void
-  /** Called on unrecoverable error */
+  /** 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
 }
@@ -35,9 +41,14 @@ const SSE_EVENTS = ['agent.status', 'agent.task', 'agent.progress', 'fleet.updat
  *
  * Handles:
  *  - Initial connection on mount
- *  - Exponential back-off reconnection on drop
+ *  - 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 four event types: agent.status, agent.task, agent.progress, fleet.update
+ *  - 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',
@@ -46,6 +57,7 @@ export function useSSE({
   onError,
   reconnectBaseMs = 1_000,
   reconnectMaxMs = 30_000,
+  reconnectLimit,
   enabled = true,
 }: UseSSEOptions = {}): { status: SSEStatus } {
   const [status, setStatus] = useState<SSEStatus>('connecting')
@@ -100,12 +112,20 @@ export function useSSE({
 
       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,
+        reconnectBaseMs * 2 ** (reconnectAttemptRef.current - 1),
         reconnectMaxMs,
       )
-      reconnectAttemptRef.current += 1
       setStatus('reconnecting')
 
       clearReconnectTimer()
@@ -128,7 +148,8 @@ export function useSSE({
       })
     }
 
-    // Catch-all for unnamed events (type == 'message')
+    // 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
@@ -139,7 +160,7 @@ export function useSSE({
       }
       onMessageRef.current?.({ type: 'message', data })
     }
-  }, [url, enabled, reconnectBaseMs, reconnectMaxMs, clearReconnectTimer])
+  }, [url, enabled, reconnectBaseMs, reconnectMaxMs, reconnectLimit, clearReconnectTimer])
 
   useEffect(() => {
     mountedRef.current = true

frontend/src/services/sse.ts

@@ -4,6 +4,11 @@
  * 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'
@@ -53,3 +58,15 @@ export type SSEPayloadMap = {
   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'],
+  },
+})