diff --git a/src/Url/MetadataUrlTransformer.php b/src/Url/MetadataUrlTransformer.php
index f160338..89ea04a 100644
--- a/src/Url/MetadataUrlTransformer.php
+++ b/src/Url/MetadataUrlTransformer.php
@@ -25,12 +25,19 @@ final class MetadataUrlTransformer {
 			return $value;
 		}
 
-		if ( $this->isSerialized( $value ) ) {
-			// phpcs:ignore -- Required to transform serialized metadata while preserving valid string lengths.
-			$unserialized = unserialize( $value, array( 'allowed_classes' => false ) );
+		if ( $this->looksSerialized( trim( $value ) ) && trim( $value ) !== $value ) {
+			return $value;
+		}
+
+		if ( $this->looksSerialized( $value ) ) {
+			$unserialized = $this->unserializeValue( $value );
+
+			if ( ! $unserialized['valid'] || is_object( $unserialized['value'] ) ) {
+				return $value;
+			}
 
 			// phpcs:ignore -- Required to reserialize metadata with updated string lengths.
-			return serialize( $this->transformValue( $unserialized, $mappings ) );
+			return serialize( $this->transformValue( $unserialized['value'], $mappings ) );
 		}
 
 		if ( $this->isJsonObjectOrArray( $value ) ) {
@@ -43,6 +50,8 @@ final class MetadataUrlTransformer {
 					return $encoded;
 				}
 			}
+
+			return $value;
 		}
 
 		return $this->url_transformer->transformString( $value, $mappings );
@@ -65,23 +74,43 @@ final class MetadataUrlTransformer {
 		return $transformed;
 	}
 
-	private function isSerialized( string $value ): bool {
-		$trimmed = trim( $value );
+	private function looksSerialized( string $value ): bool {
+		return 'N;' === $value || 'b:0;' === $value || 1 === preg_match( '/^(?:a|O|s|i|d|b):/', $value );
+	}
 
-		if ( 'N;' === $trimmed ) {
-			return true;
+	/**
+	 * @return array{valid: bool, value: mixed}
+	 */
+	private function unserializeValue( string $value ): array {
+		if ( 'N;' === $value ) {
+			return array(
+				'valid' => true,
+				'value' => null,
+			);
 		}
 
-		if ( 'b:0;' === $trimmed ) {
-			return true;
+		// phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged, WordPress.PHP.DiscouragedPHPFunctions.serialize_unserialize -- Metadata may be stored in PHP serialized format.
+		$unserialized = @unserialize( $value, array( 'allowed_classes' => false ) );
+
+		if ( false === $unserialized && 'b:0;' !== $value ) {
+			return array(
+				'valid' => false,
+				'value' => null,
+			);
 		}
 
-		if ( ! preg_match( '/^(?:a|O|s|i|d|b):/', $trimmed ) ) {
-			return false;
+		// phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.serialize_serialize -- Strict validation avoids transforming malformed serialized-looking strings.
+		if ( serialize( $unserialized ) !== $value ) {
+			return array(
+				'valid' => false,
+				'value' => null,
+			);
 		}
 
-		// phpcs:ignore -- Validation must parse PHP serialized metadata.
-		return false !== @unserialize( $trimmed, array( 'allowed_classes' => false ) );
+		return array(
+			'valid' => true,
+			'value' => $unserialized,
+		);
 	}
 
 	private function isJsonObjectOrArray( string $value ): bool {
diff --git a/tests/Unit/Url/MetadataUrlTransformerTest.php b/tests/Unit/Url/MetadataUrlTransformerTest.php
index 32562a6..3995a65 100644
--- a/tests/Unit/Url/MetadataUrlTransformerTest.php
+++ b/tests/Unit/Url/MetadataUrlTransformerTest.php
@@ -71,6 +71,51 @@ class MetadataUrlTransformerTest extends TestCase {
 		);
 	}
 
+	public function test_it_leaves_invalid_serialized_strings_unchanged(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+		$value       = 'a:1:{s:3:"url";s:37:"https://example.test/uploads/hero.jpg";} trailing';
+
+		self::assertSame( $value, $transformer->transformValue( $value, $this->mappings() ) );
+	}
+
+	public function test_it_leaves_whitespace_wrapped_serialized_strings_unchanged(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+		// phpcs:ignore -- Test fixture requires native serialized metadata.
+		$value       = ' ' . serialize( array( 'url' => 'https://example.test/uploads/hero.jpg' ) ) . ' ';
+
+		self::assertSame( $value, $transformer->transformValue( $value, $this->mappings() ) );
+	}
+
+	public function test_it_preserves_serialized_false_and_null_values(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+
+		self::assertSame( 'b:0;', $transformer->transformValue( 'b:0;', $this->mappings() ) );
+		self::assertSame( 'N;', $transformer->transformValue( 'N;', $this->mappings() ) );
+	}
+
+	public function test_it_leaves_serialized_objects_unchanged(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+		$value       = 'O:8:"stdClass":1:{s:3:"url";s:28:"https://example.test/inside";}';
+
+		self::assertSame( $value, $transformer->transformValue( $value, $this->mappings() ) );
+	}
+
+	public function test_it_leaves_invalid_json_strings_unchanged(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+		$value       = '{"url":"https://example.test/missing"';
+
+		self::assertSame( $value, $transformer->transformValue( $value, $this->mappings() ) );
+	}
+
+	public function test_it_transforms_nested_json_arrays(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+
+		self::assertSame(
+			'[{"url":"https:\/\/staging.example.test\/one"},{"count":2}]',
+			$transformer->transformValue( '[{"url":"https:\/\/example.test\/one"},{"count":2}]', $this->mappings() )
+		);
+	}
+
 	public function test_it_transforms_plain_string_metadata(): void {
 		$transformer = new MetadataUrlTransformer( new UrlTransformer() );