diff --git a/web/src/composables/useAcpStream.test.ts b/web/src/composables/useAcpStream.test.ts
new file mode 100644
index 0000000..03b28bb
--- /dev/null
+++ b/web/src/composables/useAcpStream.test.ts
@@ -0,0 +1,141 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
+
+vi.mock('@/stores/auth', () => ({
+  useAuthStore: () => ({ token: 'jwt-token' }),
+}))
+
+import { useAcpStream } from './useAcpStream'
+
+class MockWebSocket {
+  static CONNECTING = 0
+  static OPEN = 1
+  static CLOSING = 2
+  static CLOSED = 3
+  static instances: MockWebSocket[] = []
+  static last(): MockWebSocket {
+    return this.instances[this.instances.length - 1]
+  }
+
+  url: string
+  readyState = 0
+  onopen: ((ev: unknown) => void) | null = null
+  onmessage: ((ev: { data: string }) => void) | null = null
+  onerror: ((ev: unknown) => void) | null = null
+  onclose: ((ev: unknown) => void) | null = null
+  sent: string[] = []
+
+  constructor(url: string) {
+    this.url = url
+    MockWebSocket.instances.push(this)
+    queueMicrotask(() => {
+      this.readyState = 1
+      this.onopen?.({})
+    })
+  }
+
+  send(data: string) {
+    this.sent.push(data)
+  }
+
+  close() {
+    this.readyState = 3
+    this.onclose?.({})
+  }
+
+  emit(data: unknown) {
+    this.onmessage?.({ data: JSON.stringify(data) })
+  }
+}
+
+beforeEach(() => {
+  MockWebSocket.instances = []
+  vi.stubGlobal('WebSocket', MockWebSocket)
+  vi.stubGlobal('crypto', { randomUUID: () => 'fake-uuid' })
+  vi.stubGlobal('window', { location: { protocol: 'http:', host: 'test.local' } })
+})
+
+afterEach(() => {
+  vi.unstubAllGlobals()
+  vi.restoreAllMocks()
+})
+
+describe('useAcpStream', () => {
+  it('connects with ?since=0 on mount', async () => {
+    const stream = useAcpStream('sess-1')
+    await new Promise((r) => setTimeout(r, 10))
+    const ws = MockWebSocket.last()
+    expect(ws.url).toContain('/sessions/sess-1/ws')
+    expect(ws.url).toContain('since=0')
+    expect(ws.url).toContain('token=jwt-token')
+    expect(stream.status.value).toBe('streaming')
+    stream.close()
+  })
+
+  it('updates sessionStatus from state event', async () => {
+    const stream = useAcpStream('s1')
+    await new Promise((r) => setTimeout(r, 10))
+    MockWebSocket.last().emit({ kind: 'state', status: 'running' })
+    expect(stream.sessionStatus.value).toBe('running')
+    stream.close()
+  })
+
+  it('accumulates events and tracks lastEventID', async () => {
+    const stream = useAcpStream('s2')
+    await new Promise((r) => setTimeout(r, 10))
+    MockWebSocket.last().emit({
+      id: 1,
+      direction: 'out',
+      kind: 'notification',
+      method: 'session/update',
+      payload: {},
+    })
+    MockWebSocket.last().emit({
+      id: 2,
+      direction: 'out',
+      kind: 'notification',
+      method: 'session/update',
+      payload: {},
+    })
+    expect(stream.events.value).toHaveLength(2)
+    stream.close()
+  })
+
+  it('reconnects with ?since=<last> after close', async () => {
+    const stream = useAcpStream('s3')
+    await new Promise((r) => setTimeout(r, 10))
+    MockWebSocket.last().emit({ id: 5, direction: 'out', kind: 'notification' })
+
+    MockWebSocket.last().close()
+    await new Promise((r) => setTimeout(r, 1100))
+    const ws2 = MockWebSocket.last()
+    expect(ws2.url).toContain('since=5')
+    stream.close()
+  })
+
+  it('sends session/prompt with client-<uuid> ID', async () => {
+    const stream = useAcpStream('s4')
+    await new Promise((r) => setTimeout(r, 10))
+    stream.prompt('hello', 'agent-sid')
+    const sent = MockWebSocket.last().sent
+    expect(sent).toHaveLength(1)
+    const parsed = JSON.parse(sent[0]) as {
+      method: string
+      id: string
+      params: { sessionId: string; prompt: { text: string }[] }
+    }
+    expect(parsed.method).toBe('session/prompt')
+    expect(parsed.id).toMatch(/^client-/)
+    expect(parsed.params.sessionId).toBe('agent-sid')
+    expect(parsed.params.prompt[0].text).toBe('hello')
+    stream.close()
+  })
+
+  it('manual close prevents reconnect', async () => {
+    const stream = useAcpStream('s5')
+    await new Promise((r) => setTimeout(r, 10))
+    stream.close()
+    expect(stream.status.value).toBe('closed')
+    await new Promise((r) => setTimeout(r, 1500))
+    expect(MockWebSocket.instances).toHaveLength(1)
+  })
+})