diff --git a/psalm.baseline.xml b/psalm.baseline.xml
index 7c03f1d..8487a77 100644
--- a/psalm.baseline.xml
+++ b/psalm.baseline.xml
@@ -883,6 +883,12 @@
       <code><![CDATA[$slug]]></code>
     </PossiblyUnusedProperty>
   </file>
+  <file src="tests/unit/AssetQueueTest.php">
+    <PossiblyUndefinedIntArrayOffset>
+      <code><![CDATA[$existingAssets[1]]]></code>
+      <code><![CDATA[$processedAssets[0]]]></code>
+    </PossiblyUndefinedIntArrayOffset>
+  </file>
   <file src="web/index.php">
     <MixedArgument>
       <code><![CDATA[$GLOBALS['YASSG_BASEDIR'] ?? throw new LogicException('YASSG base dir not found')]]></code>
diff --git a/src/AssetQueue.php b/src/AssetQueue.php
index d1ee935..2e3ee00 100644
--- a/src/AssetQueue.php
+++ b/src/AssetQueue.php
@@ -42,7 +42,7 @@ public function add(AssetCopy|AssetFetch $specification): void
     public function flush(?callable $callable = null): array
     {
         foreach ($this->queue as $specification) {
-            $destination = $this->buildDir.'/'.mb_ltrim($specification->destination, '/');
+            $destination = $this->buildDir.'/'.\mb_ltrim($specification->destination, '/');
             if (file_exists($destination)) {
                 continue;
             }
diff --git a/tests/unit/AssetQueueTest.php b/tests/unit/AssetQueueTest.php
new file mode 100644
index 0000000..c3ff439
--- /dev/null
+++ b/tests/unit/AssetQueueTest.php
@@ -0,0 +1,198 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of the Sigwin Yassg project.
+ *
+ * (c) sigwin.hr
+ *
+ * This source file is subject to the MIT license that is bundled
+ * with this source code in the file LICENSE.
+ */
+
+namespace Sigwin\YASSG\Test;
+
+use PHPUnit\Framework\TestCase;
+use Sigwin\YASSG\Asset\AssetCopy;
+use Sigwin\YASSG\Asset\AssetFetch;
+use Sigwin\YASSG\AssetQueue;
+use Symfony\Component\Filesystem\Filesystem;
+use Symfony\Contracts\HttpClient\HttpClientInterface;
+
+/**
+ * @internal
+ *
+ * @small
+ */
+#[\PHPUnit\Framework\Attributes\CoversClass(AssetQueue::class)]
+#[\PHPUnit\Framework\Attributes\UsesClass(AssetCopy::class)]
+#[\PHPUnit\Framework\Attributes\UsesClass(AssetFetch::class)]
+final class AssetQueueTest extends TestCase
+{
+    private string $tempDir;
+    private AssetQueue $assetQueue;
+    private Filesystem $filesystem;
+    private HttpClientInterface $httpClient;
+
+    #[\Override]
+    protected function setUp(): void
+    {
+        $this->tempDir = sys_get_temp_dir().'/yassg_test_'.uniqid();
+        mkdir($this->tempDir, 0777, true);
+
+        $this->filesystem = new Filesystem();
+        $this->httpClient = $this->createMock(HttpClientInterface::class);
+        $this->assetQueue = new AssetQueue($this->tempDir, $this->filesystem, $this->httpClient);
+    }
+
+    #[\Override]
+    protected function tearDown(): void
+    {
+        if (is_dir($this->tempDir)) {
+            $this->filesystem->remove($this->tempDir);
+        }
+    }
+
+    public function testFlushReturnsAllAssetsIncludingExisting(): void
+    {
+        // Create a temporary source file
+        $sourceFile = $this->tempDir.'/source.txt';
+        file_put_contents($sourceFile, 'test content');
+
+        // Create existing destination file
+        $existingDestination = '/existing/asset.txt';
+        $existingDestinationPath = $this->tempDir.$existingDestination;
+        $this->filesystem->mkdir(\dirname($existingDestinationPath));
+        file_put_contents($existingDestinationPath, 'existing content');
+
+        // Create new destination path (doesn't exist yet)
+        $newDestination = '/new/asset.txt';
+
+        // Add assets to queue
+        $existingAsset = new AssetCopy($sourceFile, $existingDestination);
+        $newAsset = new AssetCopy($sourceFile, $newDestination);
+
+        $this->assetQueue->add($existingAsset);
+        $this->assetQueue->add($newAsset);
+
+        // Track which assets were processed via callback
+        $processedAssets = [];
+
+        // Flush the queue
+        $returnedAssets = $this->assetQueue->flush(static function ($asset) use (&$processedAssets): void {
+            $processedAssets[] = $asset;
+        });
+
+        /**
+         * Added for mutation tests.
+         *
+         * @psalm-suppress RedundantCondition
+         *
+         * @phpstan-ignore-next-line
+         */
+        self::assertTrue(array_is_list($returnedAssets), 'Returned assets should be a list');
+
+        // Should return ALL assets (both existing and new) for incremental build support
+        self::assertCount(2, $returnedAssets, 'Should return all assets including existing ones');
+        self::assertContains($existingAsset, $returnedAssets, 'Should include existing asset in return');
+        self::assertContains($newAsset, $returnedAssets, 'Should include new asset in return');
+
+        // Verify callback was only called for new asset (existing assets are skipped from processing)
+        self::assertCount(1, $processedAssets, 'Callback should only be called for newly processed assets');
+        self::assertSame($newAsset, $processedAssets[0], 'Callback should be called with the new asset only');
+
+        // Verify the new asset was actually copied
+        self::assertFileExists($this->tempDir.$newDestination, 'New asset should be copied');
+        self::assertSame('test content', file_get_contents($this->tempDir.$newDestination));
+
+        // Verify existing file was not modified
+        self::assertSame('existing content', file_get_contents($existingDestinationPath));
+    }
+
+    public function testFlushWithNoExistingAssets(): void
+    {
+        // Create a temporary source file
+        $sourceFile = $this->tempDir.'/source.txt';
+        file_put_contents($sourceFile, 'test content');
+
+        // Create assets that don't exist yet
+        $asset1 = new AssetCopy($sourceFile, '/new/asset1.txt');
+        $asset2 = new AssetCopy($sourceFile, '/new/asset2.txt');
+
+        $this->assetQueue->add($asset1);
+        $this->assetQueue->add($asset2);
+
+        // Track which assets were processed via callback
+        $processedAssets = [];
+
+        // Flush the queue
+        $returnedAssets = $this->assetQueue->flush(static function ($asset) use (&$processedAssets): void {
+            $processedAssets[] = $asset;
+        });
+
+        // Should return all assets since none existed
+        self::assertCount(2, $returnedAssets, 'Should return all processed assets');
+        self::assertContains($asset1, $returnedAssets);
+        self::assertContains($asset2, $returnedAssets);
+
+        // Verify callback was called for all assets
+        self::assertCount(2, $processedAssets, 'Callback should be called for all processed assets');
+        self::assertContains($asset1, $processedAssets);
+        self::assertContains($asset2, $processedAssets);
+
+        // Verify both assets were copied
+        self::assertFileExists($this->tempDir.'/new/asset1.txt');
+        self::assertFileExists($this->tempDir.'/new/asset2.txt');
+    }
+
+    public function testFlushWithAllExistingAssets(): void
+    {
+        // Create a temporary source file
+        $sourceFile = $this->tempDir.'/source.txt';
+        file_put_contents($sourceFile, 'test content');
+
+        // Create multiple existing destination files
+        $existingDestinations = ['/existing1/asset.txt', '/existing2/asset.txt'];
+        $existingAssets = [];
+
+        foreach ($existingDestinations as $destination) {
+            $path = $this->tempDir.$destination;
+            $this->filesystem->mkdir(\dirname($path));
+            file_put_contents($path, 'existing content');
+
+            $asset = new AssetCopy($sourceFile, $destination);
+            $existingAssets[] = $asset;
+            $this->assetQueue->add($asset);
+        }
+
+        // Track which assets were processed via callback
+        $processedAssets = [];
+
+        // Flush the queue
+        $returnedAssets = $this->assetQueue->flush(static function ($asset) use (&$processedAssets): void {
+            $processedAssets[] = $asset;
+        });
+
+        // Should return all assets even though they all exist (for incremental build support)
+        self::assertCount(2, $returnedAssets, 'Should return all assets even when they all exist');
+        self::assertContains($existingAssets[0], $returnedAssets);
+        self::assertContains($existingAssets[1], $returnedAssets);
+
+        // Verify callback was never called since no assets were actually processed
+        self::assertEmpty($processedAssets, 'Callback should not be called when all assets already exist');
+
+        // Verify all existing files were not modified
+        foreach ($existingDestinations as $destination) {
+            $path = $this->tempDir.$destination;
+            self::assertSame('existing content', file_get_contents($path));
+        }
+    }
+
+    public function testEmptyQueue(): void
+    {
+        $returnedAssets = $this->assetQueue->flush();
+
+        self::assertEmpty($returnedAssets, 'Empty queue should return empty array');
+    }
+}