diff --git a/tests/unit/cliproxy/backend-ui-default-ports-sync.test.ts b/tests/unit/cliproxy/backend-ui-default-ports-sync.test.ts
new file mode 100644
index 00000000..751d7624
--- /dev/null
+++ b/tests/unit/cliproxy/backend-ui-default-ports-sync.test.ts
@@ -0,0 +1,23 @@
+/**
+ * Default Port Sync Test
+ *
+ * Keeps backend and UI default ports in sync while allowing independent modules.
+ */
+
+import { describe, expect, test } from 'bun:test';
+import { CLIPROXY_DEFAULT_PORT as BACKEND_CLIPROXY_DEFAULT_PORT } from '../../../src/cliproxy/config/port-manager';
+import { DEFAULT_CURSOR_PORT as BACKEND_CURSOR_DEFAULT_PORT } from '../../../src/cursor/cursor-models';
+import {
+  CLIPROXY_DEFAULT_PORT as UI_CLIPROXY_DEFAULT_PORT,
+  DEFAULT_CURSOR_PORT as UI_CURSOR_DEFAULT_PORT,
+} from '../../../ui/src/lib/default-ports';
+
+describe('Default Port Sync', () => {
+  test('CLIProxy default port is synced between backend and UI', () => {
+    expect(UI_CLIPROXY_DEFAULT_PORT).toBe(BACKEND_CLIPROXY_DEFAULT_PORT);
+  });
+
+  test('Cursor default port is synced between backend and UI', () => {
+    expect(UI_CURSOR_DEFAULT_PORT).toBe(BACKEND_CURSOR_DEFAULT_PORT);
+  });
+});
diff --git a/ui/src/lib/default-ports.ts b/ui/src/lib/default-ports.ts
new file mode 100644
index 00000000..e16a097f
--- /dev/null
+++ b/ui/src/lib/default-ports.ts
@@ -0,0 +1,9 @@
+/**
+ * UI-side default ports.
+ *
+ * Keep UI defaults explicit to preserve frontend decoupling from backend build internals.
+ * Sync is enforced by backend/UI parity tests in `tests/unit/cliproxy`.
+ */
+
+export const CLIPROXY_DEFAULT_PORT = 8317;
+export const DEFAULT_CURSOR_PORT = 20129;
diff --git a/ui/src/lib/preset-utils.ts b/ui/src/lib/preset-utils.ts
index ebdb724d..bf2de02b 100644
--- a/ui/src/lib/preset-utils.ts
+++ b/ui/src/lib/preset-utils.ts
@@ -4,9 +4,11 @@
  */
 
 import { MODEL_CATALOGS } from './model-catalogs';
+import { CLIPROXY_DEFAULT_PORT } from './default-ports';
+export { CLIPROXY_DEFAULT_PORT } from './default-ports';
 
 /** CLIProxy port - should match the backend configuration */
-export const CLIPROXY_PORT = 8317;
+export const CLIPROXY_PORT = CLIPROXY_DEFAULT_PORT;
 
 /** Default fallback API key if fetch fails */
 const DEFAULT_API_KEY = 'ccs-internal-managed';
@@ -53,7 +55,7 @@ export async function applyDefaultPreset(
   // Fetch effective API key (respects user customization)
   const effectiveApiKey = await fetchEffectiveApiKey();
 
-  const effectivePort = port ?? CLIPROXY_PORT;
+  const effectivePort = port ?? CLIPROXY_DEFAULT_PORT;
   const settings = {
     env: {
       ANTHROPIC_BASE_URL: `http://127.0.0.1:${effectivePort}/api/provider/${provider}`,