diff --git a/apps/readest-app/src/__tests__/services/sync/KOSyncClient.test.ts b/apps/readest-app/src/__tests__/services/sync/KOSyncClient.test.ts
new file mode 100644
index 00000000..d6567045
--- /dev/null
+++ b/apps/readest-app/src/__tests__/services/sync/KOSyncClient.test.ts
@@ -0,0 +1,91 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { KOSyncClient } from '@/services/sync/KOSyncClient';
+import { KOSyncSettings } from '@/types/settings';
+
+// The LAN-server branch of KOSyncClient.request uses window.fetch (mocked
+// per-test); the Tauri HTTP plugin is never invoked here, so stub the import
+// to keep the unit environment free of Tauri internals.
+vi.mock('@tauri-apps/plugin-http', () => ({ fetch: vi.fn() }));
+
+const makeConfig = (overrides: Partial<KOSyncSettings> = {}): KOSyncSettings => ({
+  enabled: true,
+  // A LAN address makes request() take the direct window.fetch path.
+  serverUrl: 'http://192.168.1.50',
+  username: 'alice',
+  userkey: '',
+  password: '',
+  deviceId: 'device-1',
+  deviceName: 'Readest',
+  checksumMethod: 'binary',
+  strategy: 'prompt',
+  ...overrides,
+});
+
+type FetchMock = ReturnType<typeof vi.fn>;
+
+const setFetch = (impl: (...args: unknown[]) => unknown): FetchMock => {
+  const mock = vi.fn(impl) as FetchMock;
+  vi.stubGlobal('fetch', mock);
+  window.fetch = mock as unknown as typeof window.fetch;
+  return mock;
+};
+
+// Minimal Response-like object covering the fields KOSyncClient reads.
+const htmlPage = (status = 200) => ({
+  ok: status >= 200 && status < 300,
+  status,
+  json: async () => {
+    throw new SyntaxError('Unexpected token < in JSON');
+  },
+});
+
+const jsonResponse = (status: number, body: unknown) => ({
+  ok: status >= 200 && status < 300,
+  status,
+  json: async () => body,
+});
+
+describe('KOSyncClient.connect – server validation', () => {
+  beforeEach(() => {
+    vi.restoreAllMocks();
+  });
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it('fails when /users/auth returns 200 with a non-JSON (web UI) page', async () => {
+    // A wrong Server URL that lands on the server's static web UI returns the
+    // HTML index page with 200 OK. That must NOT be treated as a successful
+    // login (it isn't a KOReader sync endpoint).
+    setFetch(() => htmlPage(200));
+
+    const client = new KOSyncClient(makeConfig());
+    const result = await client.connect('alice', 'secret');
+
+    expect(result.success).toBe(false);
+  });
+
+  it('succeeds when /users/auth returns a valid KOReader auth JSON', async () => {
+    setFetch(() => jsonResponse(200, { authorized: 'OK' }));
+
+    const client = new KOSyncClient(makeConfig());
+    const result = await client.connect('alice', 'secret');
+
+    expect(result.success).toBe(true);
+  });
+
+  it('fails when registration (/users/create) returns 200 with a non-JSON page', async () => {
+    // /users/auth → 401 routes connect() into the create path; a web UI that
+    // returns 200 HTML there must not be reported as a successful registration.
+    const mock = setFetch((url: unknown) => {
+      if (String(url).includes('/users/create')) return htmlPage(200);
+      return htmlPage(401); // auth fails -> triggers create
+    });
+
+    const client = new KOSyncClient(makeConfig());
+    const result = await client.connect('alice', 'secret');
+
+    expect(result.success).toBe(false);
+    expect(mock).toHaveBeenCalled();
+  });
+});
diff --git a/apps/readest-app/src/services/sync/KOSyncClient.ts b/apps/readest-app/src/services/sync/KOSyncClient.ts
index e38ea055..636800bd 100644
--- a/apps/readest-app/src/services/sync/KOSyncClient.ts
+++ b/apps/readest-app/src/services/sync/KOSyncClient.ts
@@ -129,6 +129,13 @@ export class KOSyncClient {
       });
 
       if (authResponse.ok) {
+        // A wrong Server URL can land on the host's web UI, which answers 200
+        // with an HTML page. Only treat the response as a successful login when
+        // it's an actual KOReader Sync JSON response, otherwise the user is
+        // silently "connected" to an endpoint that can never sync.
+        if (!(await this.isKoSyncJsonResponse(authResponse))) {
+          return { success: false, message: 'Not a KOReader Sync server. Check the Server URL.' };
+        }
         return { success: true, message: 'Login successful.' };
       }
 
@@ -140,6 +147,9 @@ export class KOSyncClient {
         });
 
         if (registerResponse.ok) {
+          if (!(await this.isKoSyncJsonResponse(registerResponse))) {
+            return { success: false, message: 'Not a KOReader Sync server. Check the Server URL.' };
+          }
           return { success: true, message: 'Registration successful.' };
         }
 
@@ -230,6 +240,17 @@ export class KOSyncClient {
     }
   }
 
+  /**
+   * A genuine KOReader Sync server replies with a JSON object (e.g.
+   * `{ "authorized": "OK" }`). A misconfigured Server URL that hits a static
+   * web UI returns an HTML page instead, which fails JSON parsing — use that to
+   * tell the two apart.
+   */
+  private async isKoSyncJsonResponse(response: Response): Promise<boolean> {
+    const data = await response.json().catch(() => null);
+    return typeof data === 'object' && data !== null;
+  }
+
   getDocumentDigest(book: Book): string {
     if (this.config.checksumMethod === 'filename') {
       console.warn('This is not possible anymore, using md5 instead.');