diff --git a/frontend/project_plans/PHASE_F0_foundation.json b/frontend/project_plans/PHASE_F0_foundation.json
index a04dacf..0461230 100644
--- a/frontend/project_plans/PHASE_F0_foundation.json
+++ b/frontend/project_plans/PHASE_F0_foundation.json
@@ -6,7 +6,7 @@
     "created": "2026-01-30",
     "lastUpdated": "2026-01-30",
     "totalTasks": 8,
-    "completedTasks": 5,
+    "completedTasks": 6,
     "status": "in_progress"
   },
   "tasks": [
@@ -101,8 +101,8 @@
       "description": "HTTP client with auth token injection and refresh",
       "category": "api",
       "priority": 5,
-      "completed": false,
-      "tested": false,
+      "completed": true,
+      "tested": true,
       "dependencies": ["F0-001", "F0-004"],
       "files": [
         {"path": "src/api/client.ts", "status": "create"},
diff --git a/frontend/src/api/client.spec.ts b/frontend/src/api/client.spec.ts
new file mode 100644
index 0000000..993c355
--- /dev/null
+++ b/frontend/src/api/client.spec.ts
@@ -0,0 +1,304 @@
+import { describe, it, expect, beforeEach, vi, afterEach } from 'vitest'
+import { setActivePinia, createPinia } from 'pinia'
+
+import { useAuthStore } from '@/stores/auth'
+import { apiClient } from './client'
+import { ApiError } from './types'
+
+// Mock fetch globally
+const mockFetch = vi.fn()
+vi.stubGlobal('fetch', mockFetch)
+
+describe('apiClient', () => {
+  beforeEach(() => {
+    setActivePinia(createPinia())
+    mockFetch.mockReset()
+  })
+
+  afterEach(() => {
+    vi.restoreAllMocks()
+  })
+
+  describe('get', () => {
+    it('makes GET request to correct URL', async () => {
+      /**
+       * Test that GET requests are sent to the correct URL.
+       *
+       * The API client must correctly build URLs from the base URL
+       * and path, which is fundamental to all API operations.
+       */
+      mockFetch.mockResolvedValueOnce({
+        ok: true,
+        status: 200,
+        json: async () => ({ id: '1', name: 'Test' }),
+      })
+
+      const result = await apiClient.get('/api/users/me')
+
+      expect(mockFetch).toHaveBeenCalledWith(
+        expect.stringContaining('/api/users/me'),
+        expect.objectContaining({ method: 'GET' })
+      )
+      expect(result).toEqual({ id: '1', name: 'Test' })
+    })
+
+    it('adds query parameters to URL', async () => {
+      /**
+       * Test that query parameters are correctly appended.
+       *
+       * Many API endpoints accept query parameters for filtering,
+       * pagination, etc. These must be properly encoded.
+       */
+      mockFetch.mockResolvedValueOnce({
+        ok: true,
+        status: 200,
+        json: async () => ({ items: [] }),
+      })
+
+      await apiClient.get('/api/cards', { params: { page: 1, limit: 20 } })
+
+      const calledUrl = mockFetch.mock.calls[0][0] as string
+      expect(calledUrl).toContain('page=1')
+      expect(calledUrl).toContain('limit=20')
+    })
+
+    it('skips undefined query parameters', async () => {
+      /**
+       * Test that undefined parameters are not included in URL.
+       *
+       * Optional parameters should be omitted entirely rather than
+       * being sent as "undefined" strings.
+       */
+      mockFetch.mockResolvedValueOnce({
+        ok: true,
+        status: 200,
+        json: async () => ({}),
+      })
+
+      await apiClient.get('/api/cards', {
+        params: { page: 1, search: undefined },
+      })
+
+      const calledUrl = mockFetch.mock.calls[0][0] as string
+      expect(calledUrl).toContain('page=1')
+      expect(calledUrl).not.toContain('search')
+    })
+  })
+
+  describe('post', () => {
+    it('makes POST request with JSON body', async () => {
+      /**
+       * Test that POST requests include JSON body.
+       *
+       * POST requests typically send data to create resources.
+       * The body must be JSON serialized.
+       */
+      mockFetch.mockResolvedValueOnce({
+        ok: true,
+        status: 201,
+        json: async () => ({ id: '1' }),
+      })
+
+      const body = { name: 'New Deck', cards: [] }
+      await apiClient.post('/api/decks', body)
+
+      expect(mockFetch).toHaveBeenCalledWith(
+        expect.any(String),
+        expect.objectContaining({
+          method: 'POST',
+          body: JSON.stringify(body),
+        })
+      )
+    })
+  })
+
+  describe('authentication', () => {
+    it('adds Authorization header when authenticated', async () => {
+      /**
+       * Test that auth header is automatically added.
+       *
+       * Authenticated requests must include the Bearer token
+       * so the backend can identify the user.
+       */
+      const auth = useAuthStore()
+      auth.setTokens({
+        accessToken: 'test-token',
+        refreshToken: 'refresh-token',
+        expiresAt: Date.now() + 3600000,
+      })
+
+      mockFetch.mockResolvedValueOnce({
+        ok: true,
+        status: 200,
+        json: async () => ({}),
+      })
+
+      await apiClient.get('/api/users/me')
+
+      expect(mockFetch).toHaveBeenCalledWith(
+        expect.any(String),
+        expect.objectContaining({
+          headers: expect.objectContaining({
+            Authorization: 'Bearer test-token',
+          }),
+        })
+      )
+    })
+
+    it('skips auth header when skipAuth is true', async () => {
+      /**
+       * Test that skipAuth option prevents auth header.
+       *
+       * Some endpoints (like login/register) don't need auth
+       * and should not send the token.
+       */
+      const auth = useAuthStore()
+      auth.setTokens({
+        accessToken: 'test-token',
+        refreshToken: 'refresh-token',
+        expiresAt: Date.now() + 3600000,
+      })
+
+      mockFetch.mockResolvedValueOnce({
+        ok: true,
+        status: 200,
+        json: async () => ({}),
+      })
+
+      await apiClient.get('/api/public/health', { skipAuth: true })
+
+      const calledOptions = mockFetch.mock.calls[0][1] as RequestInit
+      expect(calledOptions.headers).not.toHaveProperty('Authorization')
+    })
+
+    it('does not add auth header when not authenticated', async () => {
+      /**
+       * Test that unauthenticated requests have no auth header.
+       *
+       * Without a token, there's nothing to send.
+       */
+      mockFetch.mockResolvedValueOnce({
+        ok: true,
+        status: 200,
+        json: async () => ({}),
+      })
+
+      await apiClient.get('/api/public/status')
+
+      const calledOptions = mockFetch.mock.calls[0][1] as RequestInit
+      const headers = calledOptions.headers as Record<string, string>
+      expect(headers['Authorization']).toBeUndefined()
+    })
+  })
+
+  describe('error handling', () => {
+    it('throws ApiError on non-2xx response', async () => {
+      /**
+       * Test that API errors are properly thrown.
+       *
+       * Non-successful responses should throw ApiError with
+       * status code and message for proper error handling.
+       */
+      mockFetch.mockResolvedValueOnce({
+        ok: false,
+        status: 404,
+        statusText: 'Not Found',
+        json: async () => ({ detail: 'Deck not found' }),
+      })
+
+      try {
+        await apiClient.get('/api/decks/999')
+        expect.fail('Should have thrown')
+      } catch (e) {
+        expect(e).toBeInstanceOf(ApiError)
+        const error = e as ApiError
+        expect(error.status).toBe(404)
+        expect(error.detail).toBe('Deck not found')
+        expect(error.isNotFound).toBe(true)
+      }
+    })
+
+    it('handles non-JSON error responses', async () => {
+      /**
+       * Test that non-JSON errors are handled gracefully.
+       *
+       * Some server errors may return HTML or plain text.
+       * The client should still create a proper ApiError.
+       */
+      mockFetch.mockResolvedValueOnce({
+        ok: false,
+        status: 500,
+        statusText: 'Internal Server Error',
+        json: async () => {
+          throw new Error('Not JSON')
+        },
+      })
+
+      await expect(apiClient.get('/api/crash')).rejects.toThrow(ApiError)
+    })
+
+    it('handles 204 No Content responses', async () => {
+      /**
+       * Test that 204 responses return undefined.
+       *
+       * DELETE and some PUT/PATCH endpoints return 204 with no body.
+       * The client should handle this without trying to parse JSON.
+       */
+      mockFetch.mockResolvedValueOnce({
+        ok: true,
+        status: 204,
+      })
+
+      const result = await apiClient.delete('/api/decks/1')
+
+      expect(result).toBeUndefined()
+    })
+  })
+
+  describe('token refresh', () => {
+    it('retries request after successful token refresh on 401', async () => {
+      /**
+       * Test that 401 triggers token refresh and retry.
+       *
+       * When the access token expires, the client should automatically
+       * refresh it and retry the failed request transparently.
+       */
+      const auth = useAuthStore()
+      auth.setTokens({
+        accessToken: 'expired-token',
+        refreshToken: 'valid-refresh-token',
+        expiresAt: Date.now() + 3600000,
+      })
+
+      // First call returns 401
+      mockFetch.mockResolvedValueOnce({
+        ok: false,
+        status: 401,
+        statusText: 'Unauthorized',
+        json: async () => ({ detail: 'Token expired' }),
+      })
+
+      // Refresh token call succeeds
+      mockFetch.mockResolvedValueOnce({
+        ok: true,
+        status: 200,
+        json: async () => ({
+          access_token: 'new-token',
+          expires_in: 3600,
+        }),
+      })
+
+      // Retry succeeds
+      mockFetch.mockResolvedValueOnce({
+        ok: true,
+        status: 200,
+        json: async () => ({ id: '1', name: 'Test User' }),
+      })
+
+      const result = await apiClient.get('/api/users/me')
+
+      expect(mockFetch).toHaveBeenCalledTimes(3)
+      expect(result).toEqual({ id: '1', name: 'Test User' })
+    })
+  })
+})
diff --git a/frontend/src/api/client.ts b/frontend/src/api/client.ts
new file mode 100644
index 0000000..a8e3a44
--- /dev/null
+++ b/frontend/src/api/client.ts
@@ -0,0 +1,177 @@
+/**
+ * HTTP API client with authentication and error handling.
+ *
+ * Provides typed fetch wrapper that automatically:
+ * - Injects Authorization header from auth store
+ * - Refreshes tokens on 401 responses
+ * - Parses JSON responses
+ * - Throws typed ApiError on failures
+ */
+import { config } from '@/config'
+import { useAuthStore } from '@/stores/auth'
+import { ApiError } from './types'
+import type { RequestOptions, ErrorResponse } from './types'
+
+/**
+ * Build URL with query parameters.
+ */
+function buildUrl(path: string, params?: Record<string, string | number | boolean | undefined>): string {
+  const base = config.apiBaseUrl.replace(/\/$/, '')
+  const cleanPath = path.startsWith('/') ? path : `/${path}`
+  const url = new URL(`${base}${cleanPath}`)
+
+  if (params) {
+    Object.entries(params).forEach(([key, value]) => {
+      if (value !== undefined) {
+        url.searchParams.set(key, String(value))
+      }
+    })
+  }
+
+  return url.toString()
+}
+
+/**
+ * Parse error response from backend.
+ */
+async function parseErrorResponse(response: Response): Promise<ApiError> {
+  let detail: string | undefined
+  let code: string | undefined
+
+  try {
+    const data: ErrorResponse = await response.json()
+    detail = data.detail || data.message
+    code = data.code
+  } catch {
+    // Response body is not JSON or empty
+  }
+
+  return new ApiError(response.status, response.statusText, detail, code)
+}
+
+/**
+ * Make an authenticated API request.
+ *
+ * @param method - HTTP method
+ * @param path - API path (e.g., '/api/users/me')
+ * @param options - Request options
+ * @returns Parsed JSON response
+ * @throws ApiError on non-2xx responses
+ */
+async function request<T>(
+  method: string,
+  path: string,
+  options: RequestOptions = {}
+): Promise<T> {
+  const { skipAuth = false, headers = {}, body, params, signal } = options
+  const auth = useAuthStore()
+
+  // Build headers
+  const requestHeaders: Record<string, string> = {
+    'Content-Type': 'application/json',
+    ...headers,
+  }
+
+  // Add auth header if authenticated and not skipped
+  if (!skipAuth && auth.isAuthenticated) {
+    const token = await auth.getValidToken()
+    if (token) {
+      requestHeaders['Authorization'] = `Bearer ${token}`
+    }
+  }
+
+  // Make request
+  const url = buildUrl(path, params)
+  const response = await fetch(url, {
+    method,
+    headers: requestHeaders,
+    body: body !== undefined ? JSON.stringify(body) : undefined,
+    signal,
+  })
+
+  // Handle 401 - try to refresh and retry once
+  if (response.status === 401 && !skipAuth && auth.refreshToken) {
+    const refreshed = await auth.refreshAccessToken()
+    if (refreshed) {
+      // Retry with new token
+      const newToken = await auth.getValidToken()
+      if (newToken) {
+        requestHeaders['Authorization'] = `Bearer ${newToken}`
+        const retryResponse = await fetch(url, {
+          method,
+          headers: requestHeaders,
+          body: body !== undefined ? JSON.stringify(body) : undefined,
+          signal,
+        })
+
+        if (!retryResponse.ok) {
+          throw await parseErrorResponse(retryResponse)
+        }
+
+        // Handle 204 No Content
+        if (retryResponse.status === 204) {
+          return undefined as T
+        }
+
+        return retryResponse.json()
+      }
+    }
+
+    // Refresh failed, throw the original 401
+    throw await parseErrorResponse(response)
+  }
+
+  // Handle other errors
+  if (!response.ok) {
+    throw await parseErrorResponse(response)
+  }
+
+  // Handle 204 No Content
+  if (response.status === 204) {
+    return undefined as T
+  }
+
+  return response.json()
+}
+
+/**
+ * API client with typed methods for each HTTP verb.
+ */
+export const apiClient = {
+  /**
+   * Make a GET request.
+   */
+  get<T>(path: string, options?: RequestOptions): Promise<T> {
+    return request<T>('GET', path, options)
+  },
+
+  /**
+   * Make a POST request.
+   */
+  post<T>(path: string, body?: unknown, options?: RequestOptions): Promise<T> {
+    return request<T>('POST', path, { ...options, body })
+  },
+
+  /**
+   * Make a PUT request.
+   */
+  put<T>(path: string, body?: unknown, options?: RequestOptions): Promise<T> {
+    return request<T>('PUT', path, { ...options, body })
+  },
+
+  /**
+   * Make a PATCH request.
+   */
+  patch<T>(path: string, body?: unknown, options?: RequestOptions): Promise<T> {
+    return request<T>('PATCH', path, { ...options, body })
+  },
+
+  /**
+   * Make a DELETE request.
+   */
+  delete<T>(path: string, options?: RequestOptions): Promise<T> {
+    return request<T>('DELETE', path, options)
+  },
+}
+
+export default apiClient
diff --git a/frontend/src/api/index.ts b/frontend/src/api/index.ts
new file mode 100644
index 0000000..6475b55
--- /dev/null
+++ b/frontend/src/api/index.ts
@@ -0,0 +1,11 @@
+/**
+ * API module exports.
+ */
+export { apiClient, default } from './client'
+export { ApiError } from './types'
+export type {
+  RequestOptions,
+  ApiResponse,
+  PaginatedResponse,
+  ErrorResponse,
+} from './types'
diff --git a/frontend/src/api/types.ts b/frontend/src/api/types.ts
new file mode 100644
index 0000000..607d98e
--- /dev/null
+++ b/frontend/src/api/types.ts
@@ -0,0 +1,100 @@
+/**
+ * API client types and error classes.
+ */
+
+/**
+ * Custom error class for API errors.
+ *
+ * Provides structured error information including HTTP status code,
+ * error message, and optional detail from the backend.
+ */
+export class ApiError extends Error {
+  constructor(
+    public readonly status: number,
+    public readonly statusText: string,
+    public readonly detail?: string,
+    public readonly code?: string
+  ) {
+    super(detail || statusText)
+    this.name = 'ApiError'
+  }
+
+  /**
+   * Check if this is an authentication error (401).
+   */
+  get isUnauthorized(): boolean {
+    return this.status === 401
+  }
+
+  /**
+   * Check if this is a forbidden error (403).
+   */
+  get isForbidden(): boolean {
+    return this.status === 403
+  }
+
+  /**
+   * Check if this is a not found error (404).
+   */
+  get isNotFound(): boolean {
+    return this.status === 404
+  }
+
+  /**
+   * Check if this is a validation error (422).
+   */
+  get isValidationError(): boolean {
+    return this.status === 422
+  }
+
+  /**
+   * Check if this is a server error (5xx).
+   */
+  get isServerError(): boolean {
+    return this.status >= 500
+  }
+}
+
+/**
+ * Options for API requests.
+ */
+export interface RequestOptions {
+  /** Skip automatic auth header injection */
+  skipAuth?: boolean
+  /** Custom headers to merge with defaults */
+  headers?: Record<string, string>
+  /** Request body (will be JSON serialized) */
+  body?: unknown
+  /** Query parameters */
+  params?: Record<string, string | number | boolean | undefined>
+  /** AbortSignal for request cancellation */
+  signal?: AbortSignal
+}
+
+/**
+ * Standard API response wrapper from backend.
+ */
+export interface ApiResponse<T> {
+  data: T
+  message?: string
+}
+
+/**
+ * Paginated response from backend.
+ */
+export interface PaginatedResponse<T> {
+  items: T[]
+  total: number
+  page: number
+  pageSize: number
+  totalPages: number
+}
+
+/**
+ * Backend error response shape.
+ */
+export interface ErrorResponse {
+  detail?: string
+  message?: string
+  code?: string
+}