diff --git a/src/background/diagnostic-log-store.ts b/src/background/diagnostic-log-store.ts
new file mode 100644
index 0000000..1d586d1
--- /dev/null
+++ b/src/background/diagnostic-log-store.ts
@@ -0,0 +1,33 @@
+import { type DiagnosticLogEntry, DIAGNOSTIC_STORAGE_KEYS } from '../types/diagnostic';
+import { type StorageManager, storage } from '../utils/storage';
+
+const MAX_RECENT_LOGS = 300;
+
+type DiagnosticStoreStorage = Pick<StorageManager, 'get' | 'set' | 'remove'>;
+
+export class DiagnosticLogStore {
+  constructor(private readonly storageManager: DiagnosticStoreStorage = storage) {}
+
+  async append(entry: DiagnosticLogEntry): Promise<void> {
+    const recent = await this.getRecent();
+    const next = [...recent, entry];
+
+    if (next.length > MAX_RECENT_LOGS) {
+      next.splice(0, next.length - MAX_RECENT_LOGS);
+    }
+
+    await this.storageManager.set(DIAGNOSTIC_STORAGE_KEYS.RECENT_LOGS, next);
+  }
+
+  async getRecent(): Promise<DiagnosticLogEntry[]> {
+    const stored = await this.storageManager.get<DiagnosticLogEntry[]>(
+      DIAGNOSTIC_STORAGE_KEYS.RECENT_LOGS
+    );
+
+    return Array.isArray(stored) ? stored : [];
+  }
+
+  async clear(): Promise<void> {
+    await this.storageManager.remove(DIAGNOSTIC_STORAGE_KEYS.RECENT_LOGS);
+  }
+}
diff --git a/src/types/diagnostic.ts b/src/types/diagnostic.ts
new file mode 100644
index 0000000..2d98cdc
--- /dev/null
+++ b/src/types/diagnostic.ts
@@ -0,0 +1,54 @@
+/**
+ * 诊断日志级别
+ */
+export type DiagnosticLogLevel = 'DEBUG' | 'INFO' | 'WARN' | 'ERROR';
+
+/**
+ * 单条诊断日志
+ */
+export interface DiagnosticLogEntry {
+  id: string;
+  level: DiagnosticLogLevel;
+  message: string;
+  timestamp?: number;
+  context?: Record<string, unknown>;
+}
+
+/**
+ * 单次失败记录
+ */
+export interface DiagnosticFailureEntry {
+  id: string;
+  occurredAt: number;
+  reason: string;
+  message?: string;
+}
+
+/**
+ * 最近一次失败的诊断快照
+ */
+export interface LastFailureDiagnosticSnapshot {
+  failure: DiagnosticFailureEntry;
+  recentLogs: DiagnosticLogEntry[];
+}
+
+/**
+ * 导出诊断载荷
+ */
+export interface DiagnosticExportPayload {
+  exportedAt: number;
+  lastFailure: LastFailureDiagnosticSnapshot | null;
+}
+
+/**
+ * 诊断提示状态
+ */
+export interface DiagnosticPromptState {
+  shouldPrompt: boolean;
+  lastPromptedAt?: number;
+}
+
+export const DIAGNOSTIC_STORAGE_KEYS = {
+  RECENT_LOGS: 'diagnostic_recent_logs',
+  LAST_FAILURE: 'diagnostic_last_failure',
+} as const;
diff --git a/tests/unit/diagnostic-log-store.test.ts b/tests/unit/diagnostic-log-store.test.ts
new file mode 100644
index 0000000..eee73f1
--- /dev/null
+++ b/tests/unit/diagnostic-log-store.test.ts
@@ -0,0 +1,64 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { StorageManager } from '../../src/utils/storage';
+import { DiagnosticLogStore } from '../../src/background/diagnostic-log-store';
+import { DIAGNOSTIC_STORAGE_KEYS, type DiagnosticLogEntry } from '../../src/types/diagnostic';
+
+describe('DiagnosticLogStore', () => {
+  let mockChromeStorage: {
+    get: ReturnType<typeof vi.fn>;
+    set: ReturnType<typeof vi.fn>;
+    remove: ReturnType<typeof vi.fn>;
+  };
+
+  beforeEach(() => {
+    mockChromeStorage = {
+      get: vi.fn(() => Promise.resolve({})),
+      set: vi.fn(() => Promise.resolve()),
+      remove: vi.fn(() => Promise.resolve()),
+    };
+
+    global.chrome = {
+      storage: {
+        local: mockChromeStorage,
+      },
+    } as any;
+
+    global.localStorage = {
+      getItem: vi.fn(),
+      setItem: vi.fn(),
+      removeItem: vi.fn(),
+      clear: vi.fn(),
+      key: vi.fn(),
+      length: 0,
+    } as any;
+  });
+
+  it('keeps only the latest 300 diagnostic log entries', async () => {
+    let currentLogs: DiagnosticLogEntry[] = [];
+    mockChromeStorage.get.mockImplementation(async () => ({
+      [DIAGNOSTIC_STORAGE_KEYS.RECENT_LOGS]: currentLogs,
+    }));
+    mockChromeStorage.set.mockImplementation(async (payload: Record<string, DiagnosticLogEntry[]>) => {
+      currentLogs = payload[DIAGNOSTIC_STORAGE_KEYS.RECENT_LOGS] ?? [];
+    });
+
+    const store = new DiagnosticLogStore(new StorageManager());
+
+    for (let i = 0; i < 305; i++) {
+      await store.append({ id: `log-${i}`, level: 'INFO', message: `m-${i}` } as DiagnosticLogEntry);
+    }
+
+    const logs = await store.getRecent();
+    expect(logs).toHaveLength(300);
+    expect(logs[0].id).toBe('log-5');
+    expect(logs.at(-1)?.id).toBe('log-304');
+  });
+
+  it('clears the buffer without touching unrelated keys', async () => {
+    const store = new DiagnosticLogStore(new StorageManager());
+
+    await store.clear();
+
+    expect(mockChromeStorage.remove).toHaveBeenCalledWith(DIAGNOSTIC_STORAGE_KEYS.RECENT_LOGS);
+  });
+});