diff --git a/inc/packages/class-releasedocument.php b/inc/packages/class-releasedocument.php
index f4fd1058..3840de83 100644
--- a/inc/packages/class-releasedocument.php
+++ b/inc/packages/class-releasedocument.php
@@ -95,6 +95,15 @@ public static function from_data( stdClass $data ) {
 			}
 		}
 
+		// Normalize to arrays of artifact objects since spec supports both formats.
+		if ( $doc->artifacts instanceof stdClass ) {
+			foreach ( get_object_vars( $doc->artifacts ) as $type => $artifact ) {
+				if ( is_object( $artifact ) ) {
+					$doc->artifacts->{$type} = [ $artifact ];
+				}
+			}
+		}
+
 		return $doc;
 	}
 }
diff --git a/inc/packages/namespace.php b/inc/packages/namespace.php
index 28ee8886..b3a83a53 100644
--- a/inc/packages/namespace.php
+++ b/inc/packages/namespace.php
@@ -460,15 +460,19 @@ function pick_artifact_by_lang( array $artifacts, ?string $locale = null ) {
 	// Score artifacts based on match.
 	$score_artifact = function ( $artifact ) use ( $langs ) {
 		$score = 0;
+		$lang = strtolower( $artifact->lang ?? '' );
 
 		// Check for lang match.
-		$idx = array_search( strtolower( $artifact->lang ), $langs, true );
+		$idx = array_search( $lang, $langs, true );
 		if ( $idx !== false ) {
 			$score += ( count( $langs ) - $idx ) * 100;
 		}
 
 		return $score;
 	};
+
+	// Minimal behavior only: missing lang is tolerated, but equal-score ordering
+	// and partial-locale fallback precedence are intentionally unspecified.
 	usort( $artifacts, function ( $a, $b ) use ( $score_artifact ) {
 		$a_score = $score_artifact( $a );
 		$b_score = $score_artifact( $b );
@@ -726,6 +730,7 @@ function get_package_data( $did ) {
 	$type = str_replace( 'wp-', '', $metadata->type );
 	$sections = (array) $metadata->sections;
 	$description = trim( $sections['description'] ?? '' );
+	$package_artifact = isset( $release->artifacts->package ) ? pick_artifact_by_lang( $release->artifacts->package ) : null;
 
 	$response = [
 		'name'              => $metadata->name,
@@ -747,8 +752,8 @@ function get_package_data( $did ) {
 		'new_version'       => $release->version,
 		'version'           => $release->version,
 		'remote_version'    => $release->version,
-		'package'           => $release->artifacts->package[0]->url,
-		'download_link'     => $release->artifacts->package[0]->url,
+		'package'           => $package_artifact->url ?? '',
+		'download_link'     => $package_artifact->url ?? '',
 		'tested'            => $required_versions['tested_to'] ?? '',
 		'external'          => 'xxx',
 		'last_updated'      => $metadata->last_updated ?? '',
@@ -791,10 +796,7 @@ function cache_did_for_install( array $options ): array {
 		$did = array_find_key(
 			$releases,
 			function ( $release ) use ( $options ) {
-				if ( ! is_array( $release->artifacts->package ) ) {
-					return false;
-				}
-				$artifact = pick_artifact_by_lang( $release->artifacts->package );
+				$artifact = isset( $release->artifacts->package ) ? pick_artifact_by_lang( $release->artifacts->package ) : null;
 				return $artifact && $artifact->url === $options['package'];
 			}
 		);
@@ -964,8 +966,12 @@ function maybe_add_accept_header( $args, $url ) : array {
 	}
 
 	foreach ( $releases as $release ) {
-		if ( $url === $release->artifacts->package[0]->url ) {
-			$content_type = $release->artifacts->package[0]->{'content-type'};
+		$artifact = array_find(
+			$release->artifacts->package ?? [],
+			fn ( $package_artifact ) => $url === ( $package_artifact->url ?? '' )
+		);
+		if ( $artifact ) {
+			$content_type = $artifact->{'content-type'} ?? '';
 			if ( $content_type === 'application/octet-stream' ) {
 				$args = array_merge( $args, [ 'headers' => [ 'Accept' => $content_type ] ] );
 				break;
diff --git a/tests/phpunit/tests/Packages/PickArtifactByLangTest.php b/tests/phpunit/tests/Packages/PickArtifactByLangTest.php
new file mode 100644
index 00000000..cd44e4ae
--- /dev/null
+++ b/tests/phpunit/tests/Packages/PickArtifactByLangTest.php
@@ -0,0 +1,53 @@
+<?php
+/**
+ * Tests for FAIR\Packages\pick_artifact_by_lang().
+ *
+ * @package FAIR
+ */
+
+use function FAIR\Packages\pick_artifact_by_lang;
+
+/**
+ * Tests for FAIR\Packages\pick_artifact_by_lang().
+ *
+ * @covers FAIR\Packages\pick_artifact_by_lang
+ */
+class PickArtifactByLangTest extends WP_UnitTestCase {
+
+	// Edge cases intentionally not pinned down here: tie-breaking across multiple
+	// matching lang values, artifacts without lang, and fallback precedence across
+	// partial locale matches.
+
+	/**
+	 * Test should prefer an exact locale match over artifacts without lang.
+	 */
+	public function test_should_prefer_exact_locale_match_over_artifact_without_lang() {
+		$fallback_artifact = (object) [
+			'url' => 'https://example.com/no-lang.zip',
+		];
+		$matching_artifact = (object) [
+			'url' => 'https://example.com/de-de.zip',
+			'lang' => 'de-DE',
+		];
+
+		$selected = pick_artifact_by_lang( [ $fallback_artifact, $matching_artifact ], 'de-DE' );
+
+		$this->assertSame( $matching_artifact, $selected, 'The exact locale match should be selected.' );
+	}
+
+	/**
+	 * Test should not fail when artifacts do not specify lang.
+	 */
+	public function test_should_return_an_artifact_when_lang_is_missing() {
+		$first_artifact = (object) [
+			'url' => 'https://example.com/first.zip',
+		];
+		$second_artifact = (object) [
+			'url' => 'https://example.com/second.zip',
+		];
+
+		$selected = pick_artifact_by_lang( [ $first_artifact, $second_artifact ], 'de-DE' );
+
+		$this->assertContains( $selected, [ $first_artifact, $second_artifact ], 'Artifacts without lang should still return a valid artifact.' );
+	}
+}
diff --git a/tests/phpunit/tests/Packages/ReleaseDocumentTest.php b/tests/phpunit/tests/Packages/ReleaseDocumentTest.php
new file mode 100644
index 00000000..85952b5a
--- /dev/null
+++ b/tests/phpunit/tests/Packages/ReleaseDocumentTest.php
@@ -0,0 +1,78 @@
+<?php
+/**
+ * Tests for FAIR\Packages\ReleaseDocument.
+ *
+ * @package FAIR
+ */
+
+use FAIR\Packages\ReleaseDocument;
+
+/**
+ * Tests for FAIR\Packages\ReleaseDocument.
+ *
+ * @covers FAIR\Packages\ReleaseDocument::from_data
+ */
+class ReleaseDocumentTest extends WP_UnitTestCase {
+
+	/**
+	 * Test should normalize singleton artifact objects to arrays.
+	 */
+	public function test_should_normalize_singleton_artifact_objects_to_arrays() {
+		$package_artifact = (object) [
+			'url' => 'https://example.com/plugin.zip',
+		];
+		$icon_artifact = (object) [
+			'url' => 'https://example.com/icon.png',
+		];
+		$custom_artifact = (object) [
+			'url' => 'https://example.com/extra.json',
+		];
+
+		$release = ReleaseDocument::from_data(
+			(object) [
+				'version' => '1.2.3',
+				'artifacts' => (object) [
+					'package' => $package_artifact,
+					'icon' => $icon_artifact,
+					'x-extra' => $custom_artifact,
+				],
+			]
+		);
+
+		$this->assertNotWPError( $release, 'Expected a valid release document.' );
+		$this->assertIsArray( $release->artifacts->package, 'Package artifacts should be normalized to an array.' );
+		$this->assertIsArray( $release->artifacts->icon, 'Icon artifacts should be normalized to an array.' );
+		$this->assertIsArray( $release->artifacts->{'x-extra'}, 'Custom artifact types should be normalized to an array.' );
+		$this->assertSame( $package_artifact, $release->artifacts->package[0], 'The original package artifact should be preserved.' );
+		$this->assertSame( $icon_artifact, $release->artifacts->icon[0], 'The original icon artifact should be preserved.' );
+		$this->assertSame( $custom_artifact, $release->artifacts->{'x-extra'}[0], 'The original custom artifact should be preserved.' );
+	}
+
+	/**
+	 * Test should preserve artifact arrays.
+	 */
+	public function test_should_preserve_artifact_arrays() {
+		$package_artifact = (object) [
+			'url' => 'https://example.com/plugin.zip',
+		];
+		$banner_artifact = (object) [
+			'url' => 'https://example.com/banner.png',
+		];
+
+		$release = ReleaseDocument::from_data(
+			(object) [
+				'version' => '1.2.3',
+				'artifacts' => (object) [
+					'package' => [ $package_artifact ],
+					'banner' => [ $banner_artifact ],
+				],
+			]
+		);
+
+		$this->assertNotWPError( $release, 'Expected a valid release document.' );
+		$this->assertCount( 1, $release->artifacts->package, 'Package artifact arrays should be preserved.' );
+		$this->assertCount( 1, $release->artifacts->banner, 'Banner artifact arrays should be preserved.' );
+		$this->assertSame( $package_artifact, $release->artifacts->package[0], 'Existing package array entries should be preserved.' );
+		$this->assertSame( $banner_artifact, $release->artifacts->banner[0], 'Existing banner array entries should be preserved.' );
+	}
+}