API client, auth bootstrap and job polling (#42)

Co-authored-by: dev_agent <dev_agent@cubecraftlabs.com>
Co-committed-by: dev_agent <dev_agent@cubecraftlabs.com>

web/src/api/ApiContext.tsx

@@ -0,0 +1,15 @@
+import { createContext, useContext, type ReactNode } from 'react'
+import type { ApiClient } from './client.ts'
+
+const ApiContext = createContext<ApiClient | null>(null)
+
+export function ApiProvider({ client, children }: { client: ApiClient; children: ReactNode }) {
+  return <ApiContext.Provider value={client}>{children}</ApiContext.Provider>
+}
+
+// useApi returns the app's API client. It must be rendered inside ApiProvider.
+export function useApi(): ApiClient {
+  const client = useContext(ApiContext)
+  if (client === null) throw new Error('useApi: no ApiProvider above this component')
+  return client
+}

web/src/api/client.test.ts

@@ -0,0 +1,134 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { errorResponse, fakeLocation, jsonResponse, ME, request, stubFetch } from '../test/fetchStub.ts'
+import { ApiError, createApiClient, CSRF_HEADER } from './index.ts'
+
+afterEach(() => {
+  vi.unstubAllGlobals()
+})
+
+describe('apiFetch', () => {
+  it('sends same-origin JSON and the cached CSRF token on non-GET requests only', async () => {
+    const fetchMock = stubFetch(jsonResponse(200, ME), jsonResponse(200, { ok: 1 }), jsonResponse(200, { ok: 2 }))
+    const api = createApiClient(fakeLocation())
+    await api.loadMe()
+
+    await expect(api.apiFetch('/api/v1/settings/household')).resolves.toEqual({ ok: 1 })
+    await expect(
+      api.apiFetch('/api/v1/settings/household', { method: 'PATCH', body: { dinnerTime: '18:00' } }),
+    ).resolves.toEqual({ ok: 2 })
+
+    const get = request(fetchMock, 1)
+    expect(get).toMatchObject({ path: '/api/v1/settings/household', method: 'GET', credentials: 'same-origin' })
+    expect(get.headers.has(CSRF_HEADER)).toBe(false)
+
+    const patch = request(fetchMock, 2)
+    expect(patch).toMatchObject({ method: 'PATCH', credentials: 'same-origin', body: '{"dinnerTime":"18:00"}' })
+    expect(patch.headers.get(CSRF_HEADER)).toBe('token-1')
+    expect(patch.headers.get('Content-Type')).toBe('application/json')
+  })
+
+  it('refuses a non-GET request before /me has loaded', async () => {
+    const fetchMock = stubFetch()
+    const api = createApiClient(fakeLocation())
+    await expect(api.apiFetch('/api/v1/jobs', { method: 'POST', body: {} })).rejects.toThrow(/no CSRF token/)
+    expect(fetchMock).not.toHaveBeenCalled()
+  })
+
+  it('rejects with a typed ApiError from the error body', async () => {
+    stubFetch(errorResponse(503, 'unavailable', true))
+    const api = createApiClient(fakeLocation())
+    const error = await api.apiFetch('/api/v1/jobs/j1').catch((e: unknown) => e)
+    expect(error).toBeInstanceOf(ApiError)
+    expect(error).toMatchObject({ status: 503, code: 'unavailable', message: 'unavailable message', retryable: true })
+  })
+
+  it('rejects a response without the API error body', async () => {
+    stubFetch(new Response('<html>bad gateway</html>', { status: 502 }))
+    const api = createApiClient(fakeLocation())
+    await expect(api.apiFetch('/api/v1/jobs/j1')).rejects.toThrow('GET /api/v1/jobs/j1: 502 response is not JSON')
+  })
+
+  it('on 401 navigates to sign-in with the current path and search to return to', async () => {
+    stubFetch(errorResponse(401, 'unauthenticated'))
+    const location = fakeLocation('/plan/shopping', '?week=2026-10-05')
+    const api = createApiClient(location)
+
+    await expect(api.apiFetch('/api/v1/settings/household')).rejects.toMatchObject({ status: 401 })
+    expect(location.assign).toHaveBeenCalledExactlyOnceWith(
+      '/auth/login?return=%2Fplan%2Fshopping%3Fweek%3D2026-10-05',
+    )
+  })
+
+  it('on a csrf 403 refreshes /me once and retries with the new token', async () => {
+    const fetchMock = stubFetch(
+      jsonResponse(200, ME),
+      errorResponse(403, 'csrf'),
+      jsonResponse(200, { ...ME, csrfToken: 'token-2' }),
+      jsonResponse(201, { id: 'j1' }),
+    )
+    const api = createApiClient(fakeLocation())
+    await api.loadMe()
+
+    await expect(api.apiFetch('/api/v1/jobs', { method: 'POST', body: { kind: 'noop' } })).resolves.toEqual({
+      id: 'j1',
+    })
+    expect(fetchMock).toHaveBeenCalledTimes(4)
+    expect(request(fetchMock, 1).headers.get(CSRF_HEADER)).toBe('token-1')
+    expect(request(fetchMock, 2)).toMatchObject({ path: '/api/v1/me', method: 'GET' })
+    expect(request(fetchMock, 3)).toMatchObject({ path: '/api/v1/jobs', method: 'POST', body: '{"kind":"noop"}' })
+    expect(request(fetchMock, 3).headers.get(CSRF_HEADER)).toBe('token-2')
+    expect(api.forbidden()).toBe(false)
+  })
+
+  it('does not loop when the retry gets a csrf 403 too', async () => {
+    const fetchMock = stubFetch(
+      jsonResponse(200, ME),
+      errorResponse(403, 'csrf'),
+      jsonResponse(200, { ...ME, csrfToken: 'token-2' }),
+      errorResponse(403, 'csrf'),
+    )
+    const api = createApiClient(fakeLocation())
+    await api.loadMe()
+
+    await expect(api.apiFetch('/api/v1/jobs', { method: 'POST', body: {} })).rejects.toMatchObject({
+      status: 403,
+      code: 'csrf',
+    })
+    expect(fetchMock).toHaveBeenCalledTimes(4)
+    expect(api.forbidden()).toBe(false)
+  })
+
+  it('marks any other 403 as not in this household and notifies subscribers', async () => {
+    const fetchMock = stubFetch(errorResponse(403, 'forbidden'))
+    const api = createApiClient(fakeLocation())
+    const listener = vi.fn()
+    api.subscribe(listener)
+
+    await expect(api.apiFetch('/api/v1/me')).rejects.toMatchObject({ status: 403, code: 'forbidden' })
+    expect(fetchMock).toHaveBeenCalledTimes(1)
+    expect(api.forbidden()).toBe(true)
+    expect(listener).toHaveBeenCalledTimes(1)
+  })
+
+  it('does not take a 403 without the API error body as not in this household', async () => {
+    stubFetch(new Response('<html>denied by proxy</html>', { status: 403 }))
+    const api = createApiClient(fakeLocation())
+    await expect(api.apiFetch('/api/v1/me')).rejects.toThrow('GET /api/v1/me: 403 response is not JSON')
+    expect(api.forbidden()).toBe(false)
+  })
+
+  it('does not refresh /me for a csrf 403 on a GET, which carries no token', async () => {
+    const fetchMock = stubFetch(errorResponse(403, 'csrf'))
+    const api = createApiClient(fakeLocation())
+    await expect(api.loadMe()).rejects.toMatchObject({ status: 403, code: 'csrf' })
+    expect(fetchMock).toHaveBeenCalledTimes(1)
+  })
+})
+
+describe('loadMe', () => {
+  it('rejects a /me response without a CSRF token', async () => {
+    stubFetch(jsonResponse(200, { ...ME, csrfToken: '' }))
+    const api = createApiClient(fakeLocation())
+    await expect(api.loadMe()).rejects.toThrow('response is not a member with a CSRF token')
+  })
+})

web/src/api/client.ts

@@ -0,0 +1,136 @@
+import { ApiError, isRecord, parseApiError } from './errors.ts'
+
+// CSRF_HEADER carries the session's CSRF token on every non-GET request.
+export const CSRF_HEADER = 'X-MealQuest-CSRF'
+
+export const ME_PATH = '/api/v1/me'
+
+// Me is the GET /api/v1/me response: the signed-in member, the household and
+// the session's CSRF token.
+export type Me = {
+  name: string
+  email: string
+  household: { label: string }
+  csrfToken: string
+}
+
+// ApiLocation is the part of window.location the client reads and navigates.
+export type ApiLocation = Pick<Location, 'pathname' | 'search' | 'assign'>
+
+export type ApiRequest = {
+  method?: 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE'
+  // Sent as JSON.
+  body?: unknown
+}
+
+export type ApiClient = {
+  // apiFetch sends a same-origin JSON request to path and resolves with the
+  // decoded response body, or rejects with an ApiError.
+  apiFetch<T>(path: string, request?: ApiRequest): Promise<T>
+  // loadMe fetches GET /api/v1/me and caches its CSRF token.
+  loadMe(): Promise<Me>
+  // forbidden reports whether a request was refused because the signed-in
+  // account is not in this household; subscribe is notified when it changes.
+  forbidden(): boolean
+  subscribe(listener: () => void): () => void
+}
+
+// createApiClient builds the app's API client. location is window.location in
+// the app: a 401 sends it to /auth/login with the current path to return to.
+export function createApiClient(location: ApiLocation): ApiClient {
+  let csrfToken: string | null = null
+  let refreshing: Promise<Me> | null = null
+  let forbidden = false
+  const listeners = new Set<() => void>()
+
+  async function send(method: string, path: string, body: unknown): Promise<Response> {
+    const headers: Record<string, string> = { Accept: 'application/json' }
+    if (body !== undefined) headers['Content-Type'] = 'application/json'
+    if (method !== 'GET') {
+      if (csrfToken === null) {
+        throw new Error(`${method} ${path}: no CSRF token; ${ME_PATH} has not loaded`)
+      }
+      headers[CSRF_HEADER] = csrfToken
+    }
+    return fetch(path, {
+      method,
+      headers,
+      credentials: 'same-origin',
+      body: body === undefined ? undefined : JSON.stringify(body),
+    })
+  }
+
+  // refreshMe shares one in-flight /me request between every request that
+  // got a csrf 403 at the same time.
+  function refreshMe(): Promise<Me> {
+    refreshing ??= loadMe().finally(() => {
+      refreshing = null
+    })
+    return refreshing
+  }
+
+  async function apiFetch<T>(path: string, request: ApiRequest = {}): Promise<T> {
+    const method = request.method ?? 'GET'
+    let response = await send(method, path, request.body)
+    if (response.status === 403) {
+      const error = await parseForbidden(method, path, response)
+      // Only a non-GET carries the token, so only it can be refused for it.
+      if (error.code !== 'csrf' || method === 'GET') throw error
+      // The session's token changed: refresh it once and retry once.
+      await refreshMe()
+      response = await send(method, path, request.body)
+      if (response.status === 403) throw await parseForbidden(method, path, response)
+    }
+    return decode<T>(method, path, response)
+  }
+
+  // parseForbidden reads a 403. Any API error other than csrf means the
+  // account is not in this household.
+  async function parseForbidden(method: string, path: string, response: Response): Promise<ApiError> {
+    const error = await parseApiError(method, path, response)
+    if (error.code !== 'csrf') markForbidden()
+    return error
+  }
+
+  function markForbidden() {
+    if (forbidden) return
+    forbidden = true
+    listeners.forEach((listener) => listener())
+  }
+
+  async function decode<T>(method: string, path: string, response: Response): Promise<T> {
+    if (response.status === 401) {
+      location.assign(`/auth/login?return=${encodeURIComponent(location.pathname + location.search)}`)
+    }
+    if (!response.ok) throw await parseApiError(method, path, response)
+    if (response.status === 204) return undefined as T
+    return (await response.json()) as T
+  }
+
+  async function loadMe(): Promise<Me> {
+    const me = await apiFetch<unknown>(ME_PATH)
+    if (
+      !isRecord(me) ||
+      typeof me.name !== 'string' ||
+      typeof me.email !== 'string' ||
+      !isRecord(me.household) ||
+      typeof me.household.label !== 'string' ||
+      typeof me.csrfToken !== 'string' ||
+      me.csrfToken === ''
+    ) {
+      throw new Error(`GET ${ME_PATH}: response is not a member with a CSRF token`)
+    }
+    csrfToken = me.csrfToken
+    return me as Me
+  }
+
+  return {
+    apiFetch,
+    loadMe,
+    forbidden: () => forbidden,
+    subscribe(listener) {
+      listeners.add(listener)
+      return () => listeners.delete(listener)
+    },
+  }
+}

web/src/api/errors.ts

@@ -0,0 +1,41 @@
+// ApiError is a failed API response carrying the server's
+// {"error":{"code","message","retryable"}} body.
+export class ApiError extends Error {
+  readonly status: number
+  readonly code: string
+  readonly retryable: boolean
+
+  constructor(status: number, code: string, message: string, retryable: boolean) {
+    super(message)
+    this.name = 'ApiError'
+    this.status = status
+    this.code = code
+    this.retryable = retryable
+  }
+}
+
+// parseApiError reads an error response body. A body that is not the API
+// error shape is itself an error: the server always sends one.
+export async function parseApiError(method: string, path: string, response: Response): Promise<ApiError> {
+  const text = await response.text()
+  let body: unknown
+  try {
+    body = JSON.parse(text)
+  } catch {
+    throw new Error(`${method} ${path}: ${response.status} response is not JSON: ${text.slice(0, 200)}`)
+  }
+  const error = isRecord(body) ? body.error : undefined
+  if (
+    !isRecord(error) ||
+    typeof error.code !== 'string' ||
+    typeof error.message !== 'string' ||
+    typeof error.retryable !== 'boolean'
+  ) {
+    throw new Error(`${method} ${path}: ${response.status} response has no API error body`)
+  }
+  return new ApiError(response.status, error.code, error.message, error.retryable)
+}
+
+export function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === 'object' && value !== null && !Array.isArray(value)
+}

web/src/api/index.ts

@@ -0,0 +1,5 @@
+export { ApiProvider, useApi } from './ApiContext.tsx'
+export { createApiClient, CSRF_HEADER, ME_PATH, type ApiClient, type ApiLocation, type ApiRequest, type Me } from './client.ts'
+export { ApiError } from './errors.ts'
+export { createQueryClient, shouldRetry } from './queryClient.ts'
+export { useJob, type Job, type JobState, type JobStatus } from './useJob.ts'

web/src/api/queryClient.test.ts

@@ -0,0 +1,18 @@
+import { describe, expect, it } from 'vitest'
+import { ApiError, createQueryClient, shouldRetry } from './index.ts'
+
+describe('query retries', () => {
+  it('retry only an ApiError the server marked retryable, a bounded number of times', () => {
+    expect(shouldRetry(0, new ApiError(503, 'unavailable', 'try later', true))).toBe(true)
+    expect(shouldRetry(3, new ApiError(503, 'unavailable', 'try later', true))).toBe(false)
+    expect(shouldRetry(0, new ApiError(401, 'unauthenticated', 'sign in required', false))).toBe(false)
+    expect(shouldRetry(0, new ApiError(403, 'csrf', 'bad token', false))).toBe(false)
+    expect(shouldRetry(0, new Error('network down'))).toBe(false)
+  })
+
+  it('are the QueryClient default for queries and mutations', () => {
+    const options = createQueryClient().getDefaultOptions()
+    expect(options.queries?.retry).toBe(shouldRetry)
+    expect(options.mutations?.retry).toBe(shouldRetry)
+  })
+})

web/src/api/queryClient.ts

@@ -0,0 +1,20 @@
+import { QueryClient } from '@tanstack/react-query'
+import { ApiError } from './errors.ts'
+
+// MAX_RETRIES bounds retries of a failure the server marked retryable.
+export const MAX_RETRIES = 3
+
+// shouldRetry retries only an ApiError the server marked retryable — never a
+// 401, a 403 or a malformed response.
+export function shouldRetry(failureCount: number, error: unknown): boolean {
+  return error instanceof ApiError && error.retryable && failureCount < MAX_RETRIES
+}
+
+export function createQueryClient(): QueryClient {
+  return new QueryClient({
+    defaultOptions: {
+      queries: { retry: shouldRetry, staleTime: 30_000 },
+      mutations: { retry: shouldRetry },
+    },
+  })
+}

web/src/api/useJob.test.tsx

@@ -0,0 +1,169 @@
+import { act, renderHook } from '@testing-library/react'
+import type { ReactNode } from 'react'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { errorResponse, fakeLocation, jsonResponse, request, stubFetch } from '../test/fetchStub.ts'
+import { ApiProvider, createApiClient, useJob } from './index.ts'
+
+type Draft = { dinners: string[] }
+
+function job(status: string, result: Draft | null, error: string | null = null) {
+  return jsonResponse(200, { id: 'j1', kind: 'plan.draft', status, result, error })
+}
+
+function renderJob(jobId: string | null) {
+  const api = createApiClient(fakeLocation())
+  const wrapper = ({ children }: { children: ReactNode }) => <ApiProvider client={api}>{children}</ApiProvider>
+  return renderHook(({ id }) => useJob<Draft>(id), { wrapper, initialProps: { id: jobId } })
+}
+
+// advance runs the timers due in ms and lets the polls they start answer.
+async function advance(ms: number) {
+  await act(() => vi.advanceTimersByTimeAsync(ms))
+}
+
+beforeEach(() => {
+  vi.useFakeTimers()
+})
+
+afterEach(() => {
+  vi.clearAllTimers()
+  vi.useRealTimers()
+  vi.unstubAllGlobals()
+})
+
+describe('useJob', () => {
+  it('exposes the partial result while running, then the terminal result, backing off 500 ms to 2 s', async () => {
+    const fetchMock = stubFetch(
+      job('queued', null),
+      job('running', { dinners: ['Tacos'] }),
+      job('running', { dinners: ['Tacos', 'Chili'] }),
+      job('running', { dinners: ['Tacos', 'Chili'] }),
+      job('succeeded', { dinners: ['Tacos', 'Chili', 'Soup'] }),
+    )
+    const { result } = renderJob('j1')
+    expect(result.current).toEqual({ status: undefined, result: null, error: null })
+
+    await advance(0)
+    expect(result.current.status).toBe('queued')
+    expect(request(fetchMock, 0)).toMatchObject({ path: '/api/v1/jobs/j1', method: 'GET' })
+
+    await advance(499)
+    expect(fetchMock).toHaveBeenCalledTimes(1)
+    await advance(1)
+    expect(result.current).toEqual({ status: 'running', result: { dinners: ['Tacos'] }, error: null })
+
+    await advance(999)
+    expect(fetchMock).toHaveBeenCalledTimes(2)
+    await advance(1)
+    expect(result.current.result).toEqual({ dinners: ['Tacos', 'Chili'] })
+
+    await advance(2000)
+    expect(fetchMock).toHaveBeenCalledTimes(4)
+    await advance(2000)
+    expect(fetchMock).toHaveBeenCalledTimes(5)
+    expect(result.current).toEqual({
+      status: 'succeeded',
+      result: { dinners: ['Tacos', 'Chili', 'Soup'] },
+      error: null,
+    })
+
+    // Terminal: no timer left, no further polls.
+    expect(vi.getTimerCount()).toBe(0)
+    await advance(10_000)
+    expect(fetchMock).toHaveBeenCalledTimes(5)
+  })
+
+  it('exposes a failed job as its error and stops', async () => {
+    const fetchMock = stubFetch(job('running', { dinners: ['Tacos'] }), job('failed', null, 'model refused'))
+    const { result } = renderJob('j1')
+
+    await advance(0)
+    await advance(500)
+    expect(result.current.status).toBe('failed')
+    expect(result.current.error?.message).toBe('model refused')
+    expect(vi.getTimerCount()).toBe(0)
+    expect(fetchMock).toHaveBeenCalledTimes(2)
+  })
+
+  it('exposes a failed poll as the error and keeps polling to the terminal status', async () => {
+    const fetchMock = stubFetch(
+      job('running', { dinners: ['Tacos'] }),
+      errorResponse(503, 'unavailable', true),
+      job('succeeded', { dinners: ['Tacos', 'Chili'] }),
+    )
+    const { result } = renderJob('j1')
+
+    await advance(0)
+    await advance(500)
+    expect(fetchMock).toHaveBeenCalledTimes(2)
+    expect(result.current).toMatchObject({ status: 'running', result: { dinners: ['Tacos'] } })
+    expect(result.current.error).toMatchObject({ status: 503, code: 'unavailable' })
+    expect(vi.getTimerCount()).toBe(1)
+
+    // The backoff carries on from the failed poll: the next one is 1 s later.
+    await advance(999)
+    expect(fetchMock).toHaveBeenCalledTimes(2)
+    await advance(1)
+    expect(fetchMock).toHaveBeenCalledTimes(3)
+    expect(result.current).toEqual({ status: 'succeeded', result: { dinners: ['Tacos', 'Chili'] }, error: null })
+    expect(vi.getTimerCount()).toBe(0)
+  })
+
+  it('stops polling on unmount after a failed poll', async () => {
+    const fetchMock = stubFetch(errorResponse(503, 'unavailable', true))
+    const { result, unmount } = renderJob('j1')
+
+    await advance(0)
+    expect(result.current).toMatchObject({ status: undefined, result: null })
+    expect(result.current.error).toMatchObject({ status: 503 })
+    expect(vi.getTimerCount()).toBe(1)
+    unmount()
+    expect(vi.getTimerCount()).toBe(0)
+    await advance(10_000)
+    expect(fetchMock).toHaveBeenCalledTimes(1)
+  })
+
+  it('stops polling the old job when the job id changes', async () => {
+    const fetchMock = stubFetch(job('running', null), job('running', null))
+    const { result, rerender } = renderJob('j1')
+
+    await advance(0)
+    rerender({ id: 'j2' })
+    expect(result.current).toEqual({ status: undefined, result: null, error: null })
+    await advance(0)
+    expect(request(fetchMock, 1).path).toBe('/api/v1/jobs/j2')
+    expect(vi.getTimerCount()).toBe(1)
+  })
+
+  it('stops polling on unmount', async () => {
+    const fetchMock = stubFetch(job('running', null))
+    const { unmount } = renderJob('j1')
+
+    await advance(0)
+    expect(vi.getTimerCount()).toBe(1)
+    unmount()
+    expect(vi.getTimerCount()).toBe(0)
+    await advance(10_000)
+    expect(fetchMock).toHaveBeenCalledTimes(1)
+  })
+
+  it('ignores a poll that answers after unmount', async () => {
+    let answer!: (response: Response) => void
+    const fetchMock = stubFetch()
+    fetchMock.mockImplementationOnce(() => new Promise((resolve) => (answer = resolve)))
+    const { result, unmount } = renderJob('j1')
+
+    unmount()
+    await act(async () => answer(await Promise.resolve(job('running', null))))
+    expect(vi.getTimerCount()).toBe(0)
+    expect(result.current.status).toBeUndefined()
+  })
+
+  it('polls nothing without a job id', async () => {
+    const fetchMock = stubFetch()
+    const { result } = renderJob(null)
+    await advance(5000)
+    expect(fetchMock).not.toHaveBeenCalled()
+    expect(result.current).toEqual({ status: undefined, result: null, error: null })
+  })
+})

web/src/api/useJob.ts

@@ -0,0 +1,83 @@
+import { useEffect, useState } from 'react'
+import { useApi } from './ApiContext.tsx'
+
+export type JobStatus = 'queued' | 'running' | 'succeeded' | 'failed'
+
+// Job is the GET /api/v1/jobs/{id} response. result may be set while the job
+// is running (a partial result); error is set only when it failed.
+export type Job<R> = {
+  id: string
+  kind: string
+  status: JobStatus
+  result: R | null
+  error: string | null
+}
+
+export type JobState<R> = {
+  // undefined until the first poll answers.
+  status: JobStatus | undefined
+  // The latest result, partial while the job runs.
+  result: R | null
+  // The job's failure, or the latest polling request's own failure (cleared
+  // by the next poll that answers).
+  error: Error | null
+}
+
+// Polling starts at FIRST_DELAY_MS and doubles up to MAX_DELAY_MS.
+export const FIRST_DELAY_MS = 500
+export const MAX_DELAY_MS = 2000
+
+const idle = { status: undefined, result: null, error: null }
+
+function isTerminal(status: JobStatus): boolean {
+  return status === 'succeeded' || status === 'failed'
+}
+
+// useJob polls GET /api/v1/jobs/{jobId} until the job succeeds or fails,
+// backing off from 500 ms to 2 s. A failed poll is exposed as the error and
+// polling continues. Polling stops only at a terminal status, when jobId
+// changes and on unmount. A null jobId polls nothing.
+export function useJob<R>(jobId: string | null): JobState<R> {
+  const { apiFetch } = useApi()
+  const [state, setState] = useState<{ jobId: string | null; job: JobState<R> }>({ jobId, job: idle })
+
+  useEffect(() => {
+    if (jobId === null) return
+    let active = true
+    let timer: ReturnType<typeof setTimeout> | undefined
+    let delay = FIRST_DELAY_MS
+
+    function next() {
+      timer = setTimeout(poll, delay)
+      delay = Math.min(delay * 2, MAX_DELAY_MS)
+    }
+
+    async function poll() {
+      try {
+        const job = await apiFetch<Job<R>>(`/api/v1/jobs/${encodeURIComponent(jobId!)}`)
+        if (!active) return
+        const error = job.status === 'failed' ? new Error(job.error ?? `job ${job.id} failed without an error message`) : null
+        setState({ jobId, job: { status: job.status, result: job.result, error } })
+        if (isTerminal(job.status)) return
+      } catch (error) {
+        if (!active) return
+        // A failed poll is not the job's end: keep the last known status and
+        // result, expose the request's error, and poll again.
+        setState((prev) => ({
+          jobId,
+          job: { ...(prev.jobId === jobId ? prev.job : idle), error: error instanceof Error ? error : new Error(String(error)) },
+        }))
+      }
+      next()
+    }
+
+    void poll()
+    return () => {
+      active = false
+      clearTimeout(timer)
+    }
+  }, [apiFetch, jobId])
+
+  // State left from a previous jobId is not this job's.
+  return state.jobId === jobId ? state.job : idle
+}