diff --git a/src/frontend/src/api.ts b/src/frontend/src/api.ts
new file mode 100644
index 0000000..c0ecea3
--- /dev/null
+++ b/src/frontend/src/api.ts
@@ -0,0 +1,118 @@
+// API 客户端
+
+import type {
+  Task,
+  Project,
+  Observation,
+  Event,
+  DaemonStatus,
+} from './types';
+
+const API_BASE = '/api';
+
+async function fetchJSON<T>(url: string, options?: RequestInit): Promise<T> {
+  const resp = await fetch(`${API_BASE}${url}`, {
+    headers: { 'Content-Type': 'application/json' },
+    ...options,
+  });
+  if (!resp.ok) {
+    throw new Error(`API error: ${resp.status} ${resp.statusText}`);
+  }
+  return resp.json();
+}
+
+// ---- Projects ----
+
+export async function listProjects(): Promise<Project[]> {
+  return fetchJSON('/projects');
+}
+
+export async function getProject(projectId: string): Promise<Project> {
+  return fetchJSON(`/projects/${projectId}`);
+}
+
+export async function createProject(data: Partial<Project>): Promise<Project> {
+  return fetchJSON('/projects', {
+    method: 'POST',
+    body: JSON.stringify(data),
+  });
+}
+
+// ---- Tasks ----
+
+export async function listTasks(projectId: string): Promise<Task[]> {
+  return fetchJSON(`/projects/${projectId}/tasks`);
+}
+
+export async function getTask(projectId: string, taskId: string): Promise<Task> {
+  return fetchJSON(`/projects/${projectId}/tasks/${taskId}`);
+}
+
+export async function createTask(
+  projectId: string,
+  data: Partial<Task>
+): Promise<Task> {
+  return fetchJSON(`/projects/${projectId}/tasks`, {
+    method: 'POST',
+    body: JSON.stringify(data),
+  });
+}
+
+export async function updateTask(
+  projectId: string,
+  taskId: string,
+  data: Partial<Task>
+): Promise<Task> {
+  return fetchJSON(`/projects/${projectId}/tasks/${taskId}`, {
+    method: 'PATCH',
+    body: JSON.stringify(data),
+  });
+}
+
+// ---- Observations ----
+
+export async function listObservations(
+  projectId: string,
+  taskId: string
+): Promise<Observation[]> {
+  return fetchJSON(`/projects/${projectId}/tasks/${taskId}/observations`);
+}
+
+// ---- Events ----
+
+export async function listEvents(
+  projectId: string,
+  taskId: string
+): Promise<Event[]> {
+  return fetchJSON(`/projects/${projectId}/tasks/${taskId}/events`);
+}
+
+// ---- Daemon ----
+
+export async function getDaemonStatus(): Promise<DaemonStatus> {
+  return fetchJSON('/daemon/status');
+}
+
+export async function triggerTick(): Promise<{ tick: number }> {
+  return fetchJSON('/daemon/tick', { method: 'POST' });
+}
+
+// ---- SSE ----
+
+export function createEventSource(
+  onEvent: (data: unknown) => void,
+  onError?: () => void
+): EventSource {
+  const es = new EventSource(`${API_BASE}/events`);
+  es.onmessage = (e) => {
+    try {
+      onEvent(JSON.parse(e.data));
+    } catch {
+      onEvent(e.data);
+    }
+  };
+  es.onerror = () => {
+    onError?.();
+  };
+  return es;
+}