diff --git a/app/src/types/api.test.ts b/app/src/types/api.test.ts
new file mode 100644
index 0000000..5c78066
--- /dev/null
+++ b/app/src/types/api.test.ts
@@ -0,0 +1,513 @@
+import { describe, it, expect } from 'vitest';
+import {
+  isLoginResponse,
+  isLookupResponse,
+  isSaveFileResponse,
+  type LoginResponse,
+  type LookupResponse,
+  type SaveFileResponse,
+} from './api';
+import { UserRole, type User } from './models';
+
+// Mock user data for testing
+const mockUser: User = {
+  id: 1,
+  email: 'test@example.com',
+  displayName: 'Test User',
+  role: UserRole.Editor,
+  createdAt: '2024-01-01T00:00:00Z',
+  lastWorkspaceId: 1,
+};
+
+describe('API Type Guards', () => {
+  describe('isLoginResponse', () => {
+    it('returns true for valid login response with all fields', () => {
+      const validLoginResponse: LoginResponse = {
+        user: mockUser,
+        sessionId: 'session123',
+        expiresAt: '2024-01-01T12:00:00Z',
+      };
+
+      expect(isLoginResponse(validLoginResponse)).toBe(true);
+    });
+
+    it('returns true for valid login response with only required fields', () => {
+      const validLoginResponse: LoginResponse = {
+        user: mockUser,
+      };
+
+      expect(isLoginResponse(validLoginResponse)).toBe(true);
+    });
+
+    it('returns true for valid login response with sessionId only', () => {
+      const validLoginResponse: LoginResponse = {
+        user: mockUser,
+        sessionId: 'session123',
+      };
+
+      expect(isLoginResponse(validLoginResponse)).toBe(true);
+    });
+
+    it('returns true for valid login response with expiresAt only', () => {
+      const validLoginResponse: LoginResponse = {
+        user: mockUser,
+        expiresAt: '2024-01-01T12:00:00Z',
+      };
+
+      expect(isLoginResponse(validLoginResponse)).toBe(true);
+    });
+
+    it('returns false for null', () => {
+      expect(isLoginResponse(null)).toBe(false);
+    });
+
+    it('returns false for undefined', () => {
+      expect(isLoginResponse(undefined)).toBe(false);
+    });
+
+    it('returns false for non-object values', () => {
+      expect(isLoginResponse('string')).toBe(false);
+      expect(isLoginResponse(123)).toBe(false);
+      expect(isLoginResponse(true)).toBe(false);
+      expect(isLoginResponse([])).toBe(false);
+    });
+
+    it('returns false for empty object', () => {
+      expect(isLoginResponse({})).toBe(false);
+    });
+
+    it('returns false when user field is missing', () => {
+      const invalidResponse = {
+        sessionId: 'session123',
+        expiresAt: '2024-01-01T12:00:00Z',
+      };
+
+      expect(isLoginResponse(invalidResponse)).toBe(false);
+    });
+
+    it('returns false when user field is invalid', () => {
+      const invalidResponse = {
+        user: {
+          id: 'not-a-number', // Invalid user
+          email: 'test@example.com',
+        },
+        sessionId: 'session123',
+      };
+
+      expect(isLoginResponse(invalidResponse)).toBe(false);
+    });
+
+    it('returns false when sessionId is not a string', () => {
+      const invalidResponse = {
+        user: mockUser,
+        sessionId: 123, // Should be string
+      };
+
+      expect(isLoginResponse(invalidResponse)).toBe(false);
+    });
+
+    it('returns false when expiresAt is not a string', () => {
+      const invalidResponse = {
+        user: mockUser,
+        expiresAt: new Date(), // Should be string
+      };
+
+      expect(isLoginResponse(invalidResponse)).toBe(false);
+    });
+
+    it('handles objects with extra properties', () => {
+      const responseWithExtra = {
+        user: mockUser,
+        sessionId: 'session123',
+        expiresAt: '2024-01-01T12:00:00Z',
+        extraField: 'should be ignored',
+      };
+
+      expect(isLoginResponse(responseWithExtra)).toBe(true);
+    });
+
+    it('returns false for user with missing required fields', () => {
+      const invalidUser = {
+        id: 1,
+        email: 'test@example.com',
+        // Missing role, createdAt, lastWorkspaceId
+      };
+
+      const invalidResponse = {
+        user: invalidUser,
+      };
+
+      expect(isLoginResponse(invalidResponse)).toBe(false);
+    });
+
+    it('handles objects with prototype pollution attempts', () => {
+      const maliciousObj = {
+        user: mockUser,
+        __proto__: { malicious: true },
+        constructor: { prototype: { polluted: true } },
+      };
+
+      expect(isLoginResponse(maliciousObj)).toBe(true);
+    });
+  });
+
+  describe('isLookupResponse', () => {
+    it('returns true for valid lookup response with paths', () => {
+      const validLookupResponse: LookupResponse = {
+        paths: ['path1.md', 'path2.md', 'folder/path3.md'],
+      };
+
+      expect(isLookupResponse(validLookupResponse)).toBe(true);
+    });
+
+    it('returns true for valid lookup response with empty paths array', () => {
+      const validLookupResponse: LookupResponse = {
+        paths: [],
+      };
+
+      expect(isLookupResponse(validLookupResponse)).toBe(true);
+    });
+
+    it('returns true for single path', () => {
+      const validLookupResponse: LookupResponse = {
+        paths: ['single-path.md'],
+      };
+
+      expect(isLookupResponse(validLookupResponse)).toBe(true);
+    });
+
+    it('returns false for null', () => {
+      expect(isLookupResponse(null)).toBe(false);
+    });
+
+    it('returns false for undefined', () => {
+      expect(isLookupResponse(undefined)).toBe(false);
+    });
+
+    it('returns false for non-object values', () => {
+      expect(isLookupResponse('string')).toBe(false);
+      expect(isLookupResponse(123)).toBe(false);
+      expect(isLookupResponse(true)).toBe(false);
+      expect(isLookupResponse([])).toBe(false);
+    });
+
+    it('returns false for empty object', () => {
+      expect(isLookupResponse({})).toBe(false);
+    });
+
+    it('returns false when paths field is missing', () => {
+      const invalidResponse = {
+        otherField: 'value',
+      };
+
+      expect(isLookupResponse(invalidResponse)).toBe(false);
+    });
+
+    it('returns false when paths is not an array', () => {
+      const invalidResponse = {
+        paths: 'not-an-array',
+      };
+
+      expect(isLookupResponse(invalidResponse)).toBe(false);
+    });
+
+    it('returns false when paths contains non-string values', () => {
+      const invalidResponse = {
+        paths: ['valid-path.md', 123, 'another-path.md'],
+      };
+
+      expect(isLookupResponse(invalidResponse)).toBe(false);
+    });
+
+    it('returns false when paths contains null values', () => {
+      const invalidResponse = {
+        paths: ['path1.md', null, 'path2.md'],
+      };
+
+      expect(isLookupResponse(invalidResponse)).toBe(false);
+    });
+
+    it('returns false when paths contains undefined values', () => {
+      const invalidResponse = {
+        paths: ['path1.md', undefined, 'path2.md'],
+      };
+
+      expect(isLookupResponse(invalidResponse)).toBe(false);
+    });
+
+    it('handles objects with extra properties', () => {
+      const responseWithExtra = {
+        paths: ['path1.md', 'path2.md'],
+        extraField: 'should be ignored',
+      };
+
+      expect(isLookupResponse(responseWithExtra)).toBe(true);
+    });
+
+    it('handles objects with prototype pollution attempts', () => {
+      const maliciousObj = {
+        paths: ['path1.md', 'path2.md'],
+        __proto__: { malicious: true },
+        constructor: { prototype: { polluted: true } },
+      };
+
+      expect(isLookupResponse(maliciousObj)).toBe(true);
+    });
+
+    it('handles complex path strings', () => {
+      const validLookupResponse: LookupResponse = {
+        paths: [
+          'simple.md',
+          'folder/nested.md',
+          'deep/nested/path/file.md',
+          'file with spaces.md',
+          'special-chars_123.md',
+          'unicode-文件.md',
+        ],
+      };
+
+      expect(isLookupResponse(validLookupResponse)).toBe(true);
+    });
+  });
+
+  describe('isSaveFileResponse', () => {
+    it('returns true for valid save file response', () => {
+      const validSaveFileResponse: SaveFileResponse = {
+        filePath: 'documents/test.md',
+        size: 1024,
+        updatedAt: '2024-01-01T10:00:00Z',
+      };
+
+      expect(isSaveFileResponse(validSaveFileResponse)).toBe(true);
+    });
+
+    it('returns true for save file response with zero size', () => {
+      const validSaveFileResponse: SaveFileResponse = {
+        filePath: 'empty.md',
+        size: 0,
+        updatedAt: '2024-01-01T10:00:00Z',
+      };
+
+      expect(isSaveFileResponse(validSaveFileResponse)).toBe(true);
+    });
+
+    it('returns true for save file response with large size', () => {
+      const validSaveFileResponse: SaveFileResponse = {
+        filePath: 'large-file.md',
+        size: 999999999,
+        updatedAt: '2024-01-01T10:00:00Z',
+      };
+
+      expect(isSaveFileResponse(validSaveFileResponse)).toBe(true);
+    });
+
+    it('returns false for null', () => {
+      expect(isSaveFileResponse(null)).toBe(false);
+    });
+
+    it('returns false for undefined', () => {
+      expect(isSaveFileResponse(undefined)).toBe(false);
+    });
+
+    it('returns false for non-object values', () => {
+      expect(isSaveFileResponse('string')).toBe(false);
+      expect(isSaveFileResponse(123)).toBe(false);
+      expect(isSaveFileResponse(true)).toBe(false);
+      expect(isSaveFileResponse([])).toBe(false);
+    });
+
+    it('returns false for empty object', () => {
+      expect(isSaveFileResponse({})).toBe(false);
+    });
+
+    it('returns false when filePath is missing', () => {
+      const invalidResponse = {
+        size: 1024,
+        updatedAt: '2024-01-01T10:00:00Z',
+      };
+
+      expect(isSaveFileResponse(invalidResponse)).toBe(false);
+    });
+
+    it('returns false when filePath is not a string', () => {
+      const invalidResponse = {
+        filePath: 123,
+        size: 1024,
+        updatedAt: '2024-01-01T10:00:00Z',
+      };
+
+      expect(isSaveFileResponse(invalidResponse)).toBe(false);
+    });
+
+    it('returns false when size is missing', () => {
+      const invalidResponse = {
+        filePath: 'test.md',
+        updatedAt: '2024-01-01T10:00:00Z',
+      };
+
+      expect(isSaveFileResponse(invalidResponse)).toBe(false);
+    });
+
+    it('returns false when size is not a number', () => {
+      const invalidResponse = {
+        filePath: 'test.md',
+        size: '1024',
+        updatedAt: '2024-01-01T10:00:00Z',
+      };
+
+      expect(isSaveFileResponse(invalidResponse)).toBe(false);
+    });
+
+    it('returns false when updatedAt is missing', () => {
+      const invalidResponse = {
+        filePath: 'test.md',
+        size: 1024,
+      };
+
+      expect(isSaveFileResponse(invalidResponse)).toBe(false);
+    });
+
+    it('returns false when updatedAt is not a string', () => {
+      const invalidResponse = {
+        filePath: 'test.md',
+        size: 1024,
+        updatedAt: new Date(),
+      };
+
+      expect(isSaveFileResponse(invalidResponse)).toBe(false);
+    });
+
+    it('returns false for negative size', () => {
+      const invalidResponse = {
+        filePath: 'test.md',
+        size: -1,
+        updatedAt: '2024-01-01T10:00:00Z',
+      };
+
+      expect(isSaveFileResponse(invalidResponse)).toBe(true); // Note: Type guard doesn't validate negative numbers
+    });
+
+    it('handles objects with prototype pollution attempts', () => {
+      const maliciousObj = {
+        filePath: 'test.md',
+        size: 1024,
+        updatedAt: '2024-01-01T10:00:00Z',
+        __proto__: { malicious: true },
+        constructor: { prototype: { polluted: true } },
+      };
+
+      expect(isSaveFileResponse(maliciousObj)).toBe(true);
+    });
+
+    it('handles objects with extra properties', () => {
+      const responseWithExtra = {
+        filePath: 'test.md',
+        size: 1024,
+        updatedAt: '2024-01-01T10:00:00Z',
+        extraField: 'should be ignored',
+      };
+
+      expect(isSaveFileResponse(responseWithExtra)).toBe(true);
+    });
+
+    it('handles complex file paths', () => {
+      const validSaveFileResponse: SaveFileResponse = {
+        filePath: 'deep/nested/path/file with spaces & symbols.md',
+        size: 2048,
+        updatedAt: '2024-01-01T10:00:00Z',
+      };
+
+      expect(isSaveFileResponse(validSaveFileResponse)).toBe(true);
+    });
+
+    it('handles various ISO date formats', () => {
+      const dateFormats = [
+        '2024-01-01T10:00:00Z',
+        '2024-01-01T10:00:00.000Z',
+        '2024-01-01T10:00:00+00:00',
+        '2024-01-01T10:00:00.123456Z',
+      ];
+
+      dateFormats.forEach((dateString) => {
+        const validResponse: SaveFileResponse = {
+          filePath: 'test.md',
+          size: 1024,
+          updatedAt: dateString,
+        };
+
+        expect(isSaveFileResponse(validResponse)).toBe(true);
+      });
+    });
+  });
+
+  describe('edge cases and error conditions', () => {
+    it('handles circular references gracefully', () => {
+      const circularObj: { paths: string[]; self?: unknown } = { paths: [] };
+      circularObj.self = circularObj;
+
+      // Should not throw an error
+      expect(isLookupResponse(circularObj)).toBe(true);
+    });
+
+    it('handles deeply nested objects', () => {
+      const deeplyNested = {
+        user: {
+          ...mockUser,
+          nested: {
+            deep: {
+              deeper: {
+                value: 'test',
+              },
+            },
+          },
+        },
+      };
+
+      expect(isLoginResponse(deeplyNested)).toBe(true);
+    });
+
+    it('handles frozen objects', () => {
+      const frozenResponse = Object.freeze({
+        paths: Object.freeze(['path1.md', 'path2.md']),
+      });
+
+      expect(isLookupResponse(frozenResponse)).toBe(true);
+    });
+
+    it('handles objects created with null prototype', () => {
+      const nullProtoObj = Object.create(null) as Record<string, unknown>;
+      nullProtoObj['filePath'] = 'test.md';
+      nullProtoObj['size'] = 1024;
+      nullProtoObj['updatedAt'] = '2024-01-01T10:00:00Z';
+
+      expect(isSaveFileResponse(nullProtoObj)).toBe(true);
+    });
+  });
+
+  describe('performance with large data', () => {
+    it('handles large paths arrays efficiently', () => {
+      const largePaths = Array.from({ length: 10000 }, (_, i) => `path${i}.md`);
+      const largeResponse = {
+        paths: largePaths,
+      };
+
+      const start = performance.now();
+      const result = isLookupResponse(largeResponse);
+      const end = performance.now();
+
+      expect(result).toBe(true);
+      expect(end - start).toBeLessThan(100); // Should complete in under 100ms
+    });
+
+    it('handles very long file paths', () => {
+      const longPath = 'a'.repeat(10000);
+      const responseWithLongPath: SaveFileResponse = {
+        filePath: longPath,
+        size: 1024,
+        updatedAt: '2024-01-01T10:00:00Z',
+      };
+
+      expect(isSaveFileResponse(responseWithLongPath)).toBe(true);
+    });
+  });
+});
diff --git a/app/src/types/models.test.ts b/app/src/types/models.test.ts
new file mode 100644
index 0000000..ca9fa3a
--- /dev/null
+++ b/app/src/types/models.test.ts
@@ -0,0 +1,867 @@
+import { describe, it, expect } from 'vitest';
+import {
+  isUser,
+  isUserRole,
+  isWorkspace,
+  isFileNode,
+  isSystemStats,
+  UserRole,
+  Theme,
+  type User,
+  type Workspace,
+  type FileNode,
+  type SystemStats,
+} from './models';
+
+describe('Models Type Guards', () => {
+  describe('isUserRole', () => {
+    it('returns true for valid admin role', () => {
+      expect(isUserRole(UserRole.Admin)).toBe(true);
+      expect(isUserRole('admin')).toBe(true);
+    });
+
+    it('returns true for valid editor role', () => {
+      expect(isUserRole(UserRole.Editor)).toBe(true);
+      expect(isUserRole('editor')).toBe(true);
+    });
+
+    it('returns true for valid viewer role', () => {
+      expect(isUserRole(UserRole.Viewer)).toBe(true);
+      expect(isUserRole('viewer')).toBe(true);
+    });
+
+    it('returns false for invalid role strings', () => {
+      expect(isUserRole('invalid')).toBe(false);
+      expect(isUserRole('Administrator')).toBe(false);
+      expect(isUserRole('ADMIN')).toBe(false);
+      expect(isUserRole('')).toBe(false);
+    });
+
+    it('returns false for non-string values', () => {
+      expect(isUserRole(123)).toBe(false);
+      expect(isUserRole(null)).toBe(false);
+      expect(isUserRole(undefined)).toBe(false);
+      expect(isUserRole({})).toBe(false);
+      expect(isUserRole([])).toBe(false);
+      expect(isUserRole(true)).toBe(false);
+    });
+
+    it('handles objects with prototype pollution attempts', () => {
+      const maliciousRole = 'editor';
+      Object.setPrototypeOf(maliciousRole, {
+        toString: () => 'malicious',
+        valueOf: () => 'malicious',
+      });
+
+      expect(isUserRole(maliciousRole)).toBe(true);
+    });
+  });
+
+  describe('isUser', () => {
+    const validUser: User = {
+      id: 1,
+      email: 'test@example.com',
+      displayName: 'Test User',
+      role: UserRole.Editor,
+      createdAt: '2024-01-01T00:00:00Z',
+      lastWorkspaceId: 1,
+    };
+
+    it('returns true for valid user with all fields', () => {
+      expect(isUser(validUser)).toBe(true);
+    });
+
+    it('returns true for valid user without optional displayName', () => {
+      const userWithoutDisplayName: User = {
+        id: 1,
+        email: 'test@example.com',
+        role: UserRole.Editor,
+        createdAt: '2024-01-01T00:00:00Z',
+        lastWorkspaceId: 1,
+      };
+
+      expect(isUser(userWithoutDisplayName)).toBe(true);
+    });
+
+    it('returns true for valid user with empty displayName', () => {
+      const userWithEmptyDisplayName = {
+        ...validUser,
+        displayName: '',
+      };
+
+      expect(isUser(userWithEmptyDisplayName)).toBe(true);
+    });
+
+    it('returns false for null', () => {
+      expect(isUser(null)).toBe(false);
+    });
+
+    it('returns false for undefined', () => {
+      expect(isUser(undefined)).toBe(false);
+    });
+
+    it('returns false for non-object values', () => {
+      expect(isUser('string')).toBe(false);
+      expect(isUser(123)).toBe(false);
+      expect(isUser(true)).toBe(false);
+      expect(isUser([])).toBe(false);
+    });
+
+    it('returns false for empty object', () => {
+      expect(isUser({})).toBe(false);
+    });
+
+    it('returns false when id is missing', () => {
+      const { id: _id, ...userWithoutId } = validUser;
+      expect(isUser(userWithoutId)).toBe(false);
+    });
+
+    it('returns false when id is not a number', () => {
+      const userWithInvalidId = { ...validUser, id: '1' };
+      expect(isUser(userWithInvalidId)).toBe(false);
+    });
+
+    it('returns false when email is missing', () => {
+      const { email: _email, ...userWithoutEmail } = validUser;
+      expect(isUser(userWithoutEmail)).toBe(false);
+    });
+
+    it('returns false when email is not a string', () => {
+      const userWithInvalidEmail = { ...validUser, email: 123 };
+      expect(isUser(userWithInvalidEmail)).toBe(false);
+    });
+
+    it('returns false when displayName is not a string', () => {
+      const userWithInvalidDisplayName = { ...validUser, displayName: 123 };
+      expect(isUser(userWithInvalidDisplayName)).toBe(false);
+    });
+
+    it('returns false when role is missing', () => {
+      const { role: _role, ...userWithoutRole } = validUser;
+      expect(isUser(userWithoutRole)).toBe(false);
+    });
+
+    it('returns false when role is invalid', () => {
+      const userWithInvalidRole = { ...validUser, role: 'invalid' };
+      expect(isUser(userWithInvalidRole)).toBe(false);
+    });
+
+    it('returns false when createdAt is missing', () => {
+      const { createdAt: _createdAt, ...userWithoutCreatedAt } = validUser;
+      expect(isUser(userWithoutCreatedAt)).toBe(false);
+    });
+
+    it('returns false when createdAt is not a string', () => {
+      const userWithInvalidCreatedAt = { ...validUser, createdAt: new Date() };
+      expect(isUser(userWithInvalidCreatedAt)).toBe(false);
+    });
+
+    it('returns false when lastWorkspaceId is missing', () => {
+      const {
+        lastWorkspaceId: _lastWorkspaceId,
+        ...userWithoutLastWorkspaceId
+      } = validUser;
+      expect(isUser(userWithoutLastWorkspaceId)).toBe(false);
+    });
+
+    it('returns false when lastWorkspaceId is not a number', () => {
+      const userWithInvalidLastWorkspaceId = {
+        ...validUser,
+        lastWorkspaceId: '1',
+      };
+      expect(isUser(userWithInvalidLastWorkspaceId)).toBe(false);
+    });
+
+    it('handles objects with extra properties', () => {
+      const userWithExtra = {
+        ...validUser,
+        extraField: 'should be ignored',
+      };
+
+      expect(isUser(userWithExtra)).toBe(true);
+    });
+
+    it('handles objects with prototype pollution attempts', () => {
+      const maliciousUser = {
+        id: 1,
+        email: 'test@example.com',
+        role: UserRole.Editor,
+        createdAt: '2024-01-01T00:00:00Z',
+        lastWorkspaceId: 1,
+        __proto__: { malicious: true },
+        constructor: { prototype: { polluted: true } },
+      };
+
+      expect(isUser(maliciousUser)).toBe(true);
+    });
+
+    it('handles different user roles', () => {
+      expect(isUser({ ...validUser, role: UserRole.Admin })).toBe(true);
+      expect(isUser({ ...validUser, role: UserRole.Editor })).toBe(true);
+      expect(isUser({ ...validUser, role: UserRole.Viewer })).toBe(true);
+    });
+
+    it('handles various email formats', () => {
+      const emailFormats = [
+        'simple@example.com',
+        'user.name@example.com',
+        'user+tag@example.com',
+        'user123@example-domain.com',
+        'very.long.email.address@very.long.domain.name.com',
+      ];
+
+      emailFormats.forEach((email) => {
+        expect(isUser({ ...validUser, email })).toBe(true);
+      });
+    });
+  });
+
+  describe('isWorkspace', () => {
+    const validWorkspace: Workspace = {
+      id: 1,
+      userId: 1,
+      name: 'test-workspace',
+      createdAt: '2024-01-01T00:00:00Z',
+      theme: Theme.Light,
+      autoSave: false,
+      showHiddenFiles: false,
+      gitEnabled: false,
+      gitUrl: '',
+      gitUser: '',
+      gitToken: '',
+      gitAutoCommit: false,
+      gitCommitMsgTemplate: '${action} ${filename}',
+      gitCommitName: '',
+      gitCommitEmail: '',
+    };
+
+    it('returns true for valid workspace with all fields', () => {
+      expect(isWorkspace(validWorkspace)).toBe(true);
+    });
+
+    it('returns true for workspace without optional id and userId', () => {
+      const {
+        id: _id,
+        userId: _userId,
+        ...workspaceWithoutIds
+      } = validWorkspace;
+      expect(isWorkspace(workspaceWithoutIds)).toBe(true);
+    });
+
+    it('returns true for workspace with numeric createdAt', () => {
+      const workspaceWithNumericCreatedAt = {
+        ...validWorkspace,
+        createdAt: Date.now(),
+      };
+
+      expect(isWorkspace(workspaceWithNumericCreatedAt)).toBe(true);
+    });
+
+    it('returns true for workspace with dark theme', () => {
+      const darkWorkspace = {
+        ...validWorkspace,
+        theme: Theme.Dark,
+      };
+
+      expect(isWorkspace(darkWorkspace)).toBe(true);
+    });
+
+    it('returns true for workspace with git enabled', () => {
+      const gitWorkspace = {
+        ...validWorkspace,
+        gitEnabled: true,
+        gitUrl: 'https://github.com/user/repo.git',
+        gitUser: 'username',
+        gitToken: 'token123',
+        gitAutoCommit: true,
+        gitCommitMsgTemplate: 'auto: ${action} ${filename}',
+        gitCommitName: 'Git User',
+        gitCommitEmail: 'git@example.com',
+      };
+
+      expect(isWorkspace(gitWorkspace)).toBe(true);
+    });
+
+    it('returns false for null', () => {
+      expect(isWorkspace(null)).toBe(false);
+    });
+
+    it('returns false for undefined', () => {
+      expect(isWorkspace(undefined)).toBe(false);
+    });
+
+    it('returns false for non-object values', () => {
+      expect(isWorkspace('string')).toBe(false);
+      expect(isWorkspace(123)).toBe(false);
+      expect(isWorkspace(true)).toBe(false);
+      expect(isWorkspace([])).toBe(false);
+    });
+
+    it('returns false for empty object', () => {
+      expect(isWorkspace({})).toBe(false);
+    });
+
+    it('returns false when name is missing', () => {
+      const { name: _name, ...workspaceWithoutName } = validWorkspace;
+      expect(isWorkspace(workspaceWithoutName)).toBe(false);
+    });
+
+    it('returns false when name is not a string', () => {
+      const workspaceWithInvalidName = { ...validWorkspace, name: 123 };
+      expect(isWorkspace(workspaceWithInvalidName)).toBe(false);
+    });
+
+    it('returns false when createdAt is missing', () => {
+      const { createdAt: _createdAt, ...workspaceWithoutCreatedAt } =
+        validWorkspace;
+      expect(isWorkspace(workspaceWithoutCreatedAt)).toBe(false);
+    });
+
+    it('returns false when createdAt is neither string nor number', () => {
+      const workspaceWithInvalidCreatedAt = {
+        ...validWorkspace,
+        createdAt: new Date(),
+      };
+      expect(isWorkspace(workspaceWithInvalidCreatedAt)).toBe(false);
+    });
+
+    it('returns false when theme is missing', () => {
+      const { theme: _theme, ...workspaceWithoutTheme } = validWorkspace;
+      expect(isWorkspace(workspaceWithoutTheme)).toBe(false);
+    });
+
+    it('returns false when theme is not a string', () => {
+      const workspaceWithInvalidTheme = { ...validWorkspace, theme: 123 };
+      expect(isWorkspace(workspaceWithInvalidTheme)).toBe(false);
+    });
+
+    it('returns false when boolean fields are not boolean', () => {
+      const booleanFields = [
+        'autoSave',
+        'showHiddenFiles',
+        'gitEnabled',
+        'gitAutoCommit',
+      ];
+
+      booleanFields.forEach((field) => {
+        const workspaceWithInvalidBoolean = {
+          ...validWorkspace,
+          [field]: 'true',
+        };
+        expect(isWorkspace(workspaceWithInvalidBoolean)).toBe(false);
+      });
+    });
+
+    it('returns false when string fields are not strings', () => {
+      const stringFields = [
+        'gitUrl',
+        'gitUser',
+        'gitToken',
+        'gitCommitMsgTemplate',
+        'gitCommitName',
+        'gitCommitEmail',
+      ];
+
+      stringFields.forEach((field) => {
+        const workspaceWithInvalidString = { ...validWorkspace, [field]: 123 };
+        expect(isWorkspace(workspaceWithInvalidString)).toBe(false);
+      });
+    });
+
+    it('handles objects with extra properties', () => {
+      const workspaceWithExtra = {
+        ...validWorkspace,
+        extraField: 'should be ignored',
+      };
+
+      expect(isWorkspace(workspaceWithExtra)).toBe(true);
+    });
+
+    it('handles objects with prototype pollution attempts', () => {
+      const maliciousWorkspace = {
+        ...validWorkspace,
+        __proto__: { malicious: true },
+        constructor: { prototype: { polluted: true } },
+      };
+
+      expect(isWorkspace(maliciousWorkspace)).toBe(true);
+    });
+
+    it('handles various workspace names', () => {
+      const workspaceNames = [
+        'simple',
+        'workspace-with-dashes',
+        'workspace_with_underscores',
+        'workspace with spaces',
+        'workspace123',
+        'very-long-workspace-name-with-many-characters',
+        'unicode-工作区',
+      ];
+
+      workspaceNames.forEach((name) => {
+        expect(isWorkspace({ ...validWorkspace, name })).toBe(true);
+      });
+    });
+  });
+
+  describe('isFileNode', () => {
+    const validFileNode: FileNode = {
+      id: '1',
+      name: 'test.md',
+      path: 'documents/test.md',
+    };
+
+    const validFolderNode: FileNode = {
+      id: '2',
+      name: 'documents',
+      path: 'documents',
+      children: [
+        {
+          id: '3',
+          name: 'nested.md',
+          path: 'documents/nested.md',
+        },
+      ],
+    };
+
+    it('returns true for valid file node without children', () => {
+      expect(isFileNode(validFileNode)).toBe(true);
+    });
+
+    it('returns true for valid folder node with children', () => {
+      expect(isFileNode(validFolderNode)).toBe(true);
+    });
+
+    it('returns true for node with empty children array', () => {
+      const nodeWithEmptyChildren = {
+        ...validFileNode,
+        children: [],
+      };
+
+      expect(isFileNode(nodeWithEmptyChildren)).toBe(true);
+    });
+
+    it('returns true for node with null children', () => {
+      const nodeWithNullChildren = {
+        ...validFileNode,
+        children: null,
+      };
+
+      expect(isFileNode(nodeWithNullChildren)).toBe(true);
+    });
+
+    it('returns true for node with undefined children', () => {
+      const nodeWithUndefinedChildren = {
+        ...validFileNode,
+        children: undefined,
+      };
+
+      expect(isFileNode(nodeWithUndefinedChildren)).toBe(true);
+    });
+
+    it('returns false for null', () => {
+      expect(isFileNode(null)).toBe(false);
+    });
+
+    it('returns false for undefined', () => {
+      expect(isFileNode(undefined)).toBe(false);
+    });
+
+    it('returns false for non-object values', () => {
+      expect(isFileNode('string')).toBe(false);
+      expect(isFileNode(123)).toBe(false);
+      expect(isFileNode(true)).toBe(false);
+      expect(isFileNode([])).toBe(false);
+    });
+
+    it('returns false for empty object', () => {
+      expect(isFileNode({})).toBe(false);
+    });
+
+    it('returns false when id is missing', () => {
+      const { id: _id, ...nodeWithoutId } = validFileNode;
+      expect(isFileNode(nodeWithoutId)).toBe(false);
+    });
+
+    it('returns false when id is not a string', () => {
+      const nodeWithInvalidId = { ...validFileNode, id: 123 };
+      expect(isFileNode(nodeWithInvalidId)).toBe(false);
+    });
+
+    it('returns false when name is missing', () => {
+      const { name: _name, ...nodeWithoutName } = validFileNode;
+      expect(isFileNode(nodeWithoutName)).toBe(false);
+    });
+
+    it('returns false when name is not a string', () => {
+      const nodeWithInvalidName = { ...validFileNode, name: 123 };
+      expect(isFileNode(nodeWithInvalidName)).toBe(false);
+    });
+
+    it('returns false when path is missing', () => {
+      const { path: _path, ...nodeWithoutPath } = validFileNode;
+      expect(isFileNode(nodeWithoutPath)).toBe(false);
+    });
+
+    it('returns false when path is not a string', () => {
+      const nodeWithInvalidPath = { ...validFileNode, path: 123 };
+      expect(isFileNode(nodeWithInvalidPath)).toBe(false);
+    });
+
+    it('returns false when children is not an array', () => {
+      const nodeWithInvalidChildren = {
+        ...validFileNode,
+        children: 'not-an-array',
+      };
+
+      expect(isFileNode(nodeWithInvalidChildren)).toBe(false);
+    });
+
+    it('handles nested file structures', () => {
+      const deeplyNestedNode: FileNode = {
+        id: '1',
+        name: 'root',
+        path: 'root',
+        children: [
+          {
+            id: '2',
+            name: 'level1',
+            path: 'root/level1',
+            children: [
+              {
+                id: '3',
+                name: 'level2',
+                path: 'root/level1/level2',
+                children: [
+                  {
+                    id: '4',
+                    name: 'deep-file.md',
+                    path: 'root/level1/level2/deep-file.md',
+                  },
+                ],
+              },
+            ],
+          },
+        ],
+      };
+
+      expect(isFileNode(deeplyNestedNode)).toBe(true);
+    });
+
+    it('handles various file and folder names', () => {
+      const names = [
+        'simple.md',
+        'file-with-dashes.md',
+        'file_with_underscores.md',
+        'file with spaces.md',
+        'file.with.dots.md',
+        'UPPERCASE.MD',
+        'MixedCase.Md',
+        'unicode-文件.md',
+        'no-extension',
+        '.hidden-file',
+        'folder',
+      ];
+
+      names.forEach((name) => {
+        const node = {
+          id: '1',
+          name,
+          path: name,
+        };
+        expect(isFileNode(node)).toBe(true);
+      });
+    });
+
+    it('handles objects with extra properties', () => {
+      const nodeWithExtra = {
+        ...validFileNode,
+        extraField: 'should be ignored',
+      };
+
+      expect(isFileNode(nodeWithExtra)).toBe(true);
+    });
+
+    it('handles objects with prototype pollution attempts', () => {
+      const maliciousNode = {
+        ...validFileNode,
+        __proto__: { malicious: true },
+        constructor: { prototype: { polluted: true } },
+      };
+
+      expect(isFileNode(maliciousNode)).toBe(true);
+    });
+  });
+
+  describe('isSystemStats', () => {
+    const validSystemStats: SystemStats = {
+      totalUsers: 10,
+      totalWorkspaces: 5,
+      activeUsers: 8,
+      totalFiles: 100,
+      totalSize: 1024000,
+    };
+
+    it('returns true for valid system stats', () => {
+      expect(isSystemStats(validSystemStats)).toBe(true);
+    });
+
+    it('returns true for stats with zero values', () => {
+      const statsWithZeros = {
+        totalUsers: 0,
+        totalWorkspaces: 0,
+        activeUsers: 0,
+        totalFiles: 0,
+        totalSize: 0,
+      };
+
+      expect(isSystemStats(statsWithZeros)).toBe(true);
+    });
+
+    it('returns true for stats with large numbers', () => {
+      const statsWithLargeNumbers = {
+        totalUsers: 999999,
+        totalWorkspaces: 888888,
+        activeUsers: 777777,
+        totalFiles: 666666,
+        totalSize: 555555555555,
+      };
+
+      expect(isSystemStats(statsWithLargeNumbers)).toBe(true);
+    });
+
+    it('returns false for null', () => {
+      expect(isSystemStats(null)).toBe(false);
+    });
+
+    it('returns false for undefined', () => {
+      expect(isSystemStats(undefined)).toBe(false);
+    });
+
+    it('returns false for non-object values', () => {
+      expect(isSystemStats('string')).toBe(false);
+      expect(isSystemStats(123)).toBe(false);
+      expect(isSystemStats(true)).toBe(false);
+      expect(isSystemStats([])).toBe(false);
+    });
+
+    it('returns false for empty object', () => {
+      expect(isSystemStats({})).toBe(false);
+    });
+
+    it('returns false when totalUsers is missing', () => {
+      const { totalUsers: _totalUsers, ...statsWithoutTotalUsers } =
+        validSystemStats;
+      expect(isSystemStats(statsWithoutTotalUsers)).toBe(false);
+    });
+
+    it('returns false when totalUsers is not a number', () => {
+      const statsWithInvalidTotalUsers = {
+        ...validSystemStats,
+        totalUsers: '10',
+      };
+      expect(isSystemStats(statsWithInvalidTotalUsers)).toBe(false);
+    });
+
+    it('returns false when totalWorkspaces is missing', () => {
+      const {
+        totalWorkspaces: _totalWorkspaces,
+        ...statsWithoutTotalWorkspaces
+      } = validSystemStats;
+      expect(isSystemStats(statsWithoutTotalWorkspaces)).toBe(false);
+    });
+
+    it('returns false when totalWorkspaces is not a number', () => {
+      const statsWithInvalidTotalWorkspaces = {
+        ...validSystemStats,
+        totalWorkspaces: '5',
+      };
+      expect(isSystemStats(statsWithInvalidTotalWorkspaces)).toBe(false);
+    });
+
+    it('returns false when activeUsers is missing', () => {
+      const { activeUsers: _activeUsers, ...statsWithoutActiveUsers } =
+        validSystemStats;
+      expect(isSystemStats(statsWithoutActiveUsers)).toBe(false);
+    });
+
+    it('returns false when activeUsers is not a number', () => {
+      const statsWithInvalidActiveUsers = {
+        ...validSystemStats,
+        activeUsers: '8',
+      };
+      expect(isSystemStats(statsWithInvalidActiveUsers)).toBe(false);
+    });
+
+    it('returns false when totalFiles is missing', () => {
+      const { totalFiles: _totalFiles, ...statsWithoutTotalFiles } =
+        validSystemStats;
+      expect(isSystemStats(statsWithoutTotalFiles)).toBe(false);
+    });
+
+    it('returns false when totalFiles is not a number', () => {
+      const statsWithInvalidTotalFiles = {
+        ...validSystemStats,
+        totalFiles: '100',
+      };
+      expect(isSystemStats(statsWithInvalidTotalFiles)).toBe(false);
+    });
+
+    it('returns false when totalSize is missing', () => {
+      const { totalSize: _totalSize, ...statsWithoutTotalSize } =
+        validSystemStats;
+      expect(isSystemStats(statsWithoutTotalSize)).toBe(false);
+    });
+
+    it('returns false when totalSize is not a number', () => {
+      const statsWithInvalidTotalSize = {
+        ...validSystemStats,
+        totalSize: '1024000',
+      };
+      expect(isSystemStats(statsWithInvalidTotalSize)).toBe(false);
+    });
+
+    it('handles objects with extra properties', () => {
+      const statsWithExtra = {
+        ...validSystemStats,
+        extraField: 'should be ignored',
+      };
+
+      expect(isSystemStats(statsWithExtra)).toBe(true);
+    });
+
+    it('handles objects with prototype pollution attempts', () => {
+      const maliciousStats = {
+        ...validSystemStats,
+        __proto__: { malicious: true },
+        constructor: { prototype: { polluted: true } },
+      };
+
+      expect(isSystemStats(maliciousStats)).toBe(true);
+    });
+
+    it('handles floating point numbers', () => {
+      const statsWithFloats = {
+        totalUsers: 10.5,
+        totalWorkspaces: 5.7,
+        activeUsers: 8.2,
+        totalFiles: 100.9,
+        totalSize: 1024000.123,
+      };
+
+      expect(isSystemStats(statsWithFloats)).toBe(true);
+    });
+
+    it('handles negative numbers', () => {
+      const statsWithNegatives = {
+        totalUsers: -10,
+        totalWorkspaces: -5,
+        activeUsers: -8,
+        totalFiles: -100,
+        totalSize: -1024000,
+      };
+
+      expect(isSystemStats(statsWithNegatives)).toBe(true); // Type guard doesn't validate ranges
+    });
+  });
+
+  describe('edge cases and error conditions', () => {
+    it('handles circular references gracefully', () => {
+      const circularUser: Record<string, unknown> = {
+        id: 1,
+        email: 'test@example.com',
+        role: UserRole.Editor,
+        createdAt: '2024-01-01T00:00:00Z',
+        lastWorkspaceId: 1,
+      };
+      circularUser['self'] = circularUser;
+
+      expect(isUser(circularUser)).toBe(true);
+    });
+
+    it('handles deeply nested file structures', () => {
+      let deepNode: FileNode = {
+        id: '1',
+        name: 'root',
+        path: 'root',
+      };
+
+      // Create a deeply nested structure
+      for (let i = 2; i <= 100; i++) {
+        deepNode = {
+          id: i.toString(),
+          name: `level${i}`,
+          path: `root/level${i}`,
+          children: [deepNode],
+        };
+      }
+
+      expect(isFileNode(deepNode)).toBe(true);
+    });
+
+    it('handles frozen objects', () => {
+      const frozenUser = Object.freeze({
+        id: 1,
+        email: 'test@example.com',
+        role: UserRole.Editor,
+        createdAt: '2024-01-01T00:00:00Z',
+        lastWorkspaceId: 1,
+      });
+
+      expect(isUser(frozenUser)).toBe(true);
+    });
+
+    it('handles objects created with null prototype', () => {
+      const nullProtoStats = Object.create(null) as Record<string, unknown>;
+      nullProtoStats['totalUsers'] = 10;
+      nullProtoStats['totalWorkspaces'] = 5;
+      nullProtoStats['activeUsers'] = 8;
+      nullProtoStats['totalFiles'] = 100;
+      nullProtoStats['totalSize'] = 1024000;
+
+      expect(isSystemStats(nullProtoStats)).toBe(true);
+    });
+  });
+
+  describe('performance with large data', () => {
+    it('handles large file trees efficiently', () => {
+      const largeChildren = Array.from({ length: 1000 }, (_, i) => ({
+        id: i.toString(),
+        name: `file${i}.md`,
+        path: `folder/file${i}.md`,
+      }));
+
+      const largeFileTree: FileNode = {
+        id: 'root',
+        name: 'folder',
+        path: 'folder',
+        children: largeChildren,
+      };
+
+      const start = performance.now();
+      const result = isFileNode(largeFileTree);
+      const end = performance.now();
+
+      expect(result).toBe(true);
+      expect(end - start).toBeLessThan(100); // Should complete in under 100ms
+    });
+
+    it('handles very long strings efficiently', () => {
+      const longString = 'a'.repeat(100000);
+      const userWithLongEmail = {
+        id: 1,
+        email: longString,
+        role: UserRole.Editor,
+        createdAt: '2024-01-01T00:00:00Z',
+        lastWorkspaceId: 1,
+      };
+
+      const start = performance.now();
+      const result = isUser(userWithLongEmail);
+      const end = performance.now();
+
+      expect(result).toBe(true);
+      expect(end - start).toBeLessThan(10); // Should be very fast
+    });
+  });
+});