diff --git a/src/Http/Controllers/CP/Concerns/ResolvesUserId.php b/src/Http/Controllers/CP/Concerns/ResolvesUserId.php
index 26752a5..538f8be 100644
--- a/src/Http/Controllers/CP/Concerns/ResolvesUserId.php
+++ b/src/Http/Controllers/CP/Concerns/ResolvesUserId.php
@@ -15,9 +15,9 @@ private function resolveUserId(): string
     {
         $user = User::current();
 
-        /** @var string $userId */
-        $userId = $user ? $user->id() : '';
-
-        return $userId;
+        // Cast to string: Statamic's User->id() returns string under the file
+        // users driver but int under the Eloquent driver. The trait's return
+        // type contract (and downstream token storage) requires string.
+        return $user ? (string) $user->id() : '';
     }
 }
diff --git a/tests/Unit/Http/Controllers/CP/Concerns/ResolvesUserIdTest.php b/tests/Unit/Http/Controllers/CP/Concerns/ResolvesUserIdTest.php
new file mode 100644
index 0000000..5ffc576
--- /dev/null
+++ b/tests/Unit/Http/Controllers/CP/Concerns/ResolvesUserIdTest.php
@@ -0,0 +1,48 @@
+<?php
+
+declare(strict_types=1);
+
+use Cboxdk\StatamicMcp\Http\Controllers\CP\Concerns\ResolvesUserId;
+use Illuminate\Contracts\Auth\Authenticatable;
+use Statamic\Contracts\Auth\User as StatamicUserContract;
+use Statamic\Facades\User;
+
+/**
+ * Concrete subject that exposes the private trait method for direct testing.
+ */
+class ResolvesUserIdSubject
+{
+    use ResolvesUserId {
+        resolveUserId as public;
+    }
+}
+
+it('returns the user id as a string when User->id() returns an int (Eloquent driver)', function () {
+    $user = Mockery::mock(StatamicUserContract::class, Authenticatable::class);
+    $user->shouldReceive('id')->andReturn(42);
+
+    User::shouldReceive('current')->andReturn($user);
+
+    $result = (new ResolvesUserIdSubject)->resolveUserId();
+
+    expect($result)->toBe('42');
+});
+
+it('returns the user id as a string when User->id() returns a string (file driver)', function () {
+    $user = Mockery::mock(StatamicUserContract::class, Authenticatable::class);
+    $user->shouldReceive('id')->andReturn('019357a0-7f6a-7000-a1b2-deadbeef0000');
+
+    User::shouldReceive('current')->andReturn($user);
+
+    $result = (new ResolvesUserIdSubject)->resolveUserId();
+
+    expect($result)->toBe('019357a0-7f6a-7000-a1b2-deadbeef0000');
+});
+
+it('returns an empty string when no user is authenticated', function () {
+    User::shouldReceive('current')->andReturn(null);
+
+    $result = (new ResolvesUserIdSubject)->resolveUserId();
+
+    expect($result)->toBe('');
+});