diff --git a/plugins/performance-lab/includes/site-health/audit-enqueued-assets/helper.php b/plugins/performance-lab/includes/site-health/audit-enqueued-assets/helper.php
index b528821a28..393b2dae00 100644
--- a/plugins/performance-lab/includes/site-health/audit-enqueued-assets/helper.php
+++ b/plugins/performance-lab/includes/site-health/audit-enqueued-assets/helper.php
@@ -524,9 +524,30 @@ function perflab_aea_get_asset_size( string $resource_url ) {
 		);
 	}
 
-	// TODO: A non-cacheable response should also be considered an error.
-	// TODO: A size of zero could be considered an error too.
-	return strlen( wp_remote_retrieve_body( $response ) );
+	$body = wp_remote_retrieve_body( $response );
+
+	if ( strlen( $body ) === 0 ) {
+		return new WP_Error(
+			'zero_size',
+			esc_html__( 'The asset returned an empty response body.', 'performance-lab' )
+		);
+	}
+
+	$cache_control = wp_remote_retrieve_header( $response, 'cache-control' );
+	if ( is_array( $cache_control ) ) {
+		$cache_control = implode( ', ', $cache_control );
+	}
+	if ( '' !== $cache_control ) {
+		$directives = array_map( 'trim', explode( ',', strtolower( $cache_control ) ) );
+		if ( in_array( 'no-store', $directives, true ) ) {
+			return new WP_Error(
+				'not_cacheable',
+				esc_html__( 'The asset response has a Cache-Control: no-store directive and will not be cached by browsers.', 'performance-lab' )
+			);
+		}
+	}
+
+	return strlen( $body );
 }
 
 /**
diff --git a/plugins/performance-lab/tests/data/class-audit-assets-mock-assets.php b/plugins/performance-lab/tests/data/class-audit-assets-mock-assets.php
index 10b9b793ff..f4406d7bbc 100644
--- a/plugins/performance-lab/tests/data/class-audit-assets-mock-assets.php
+++ b/plugins/performance-lab/tests/data/class-audit-assets-mock-assets.php
@@ -156,6 +156,7 @@ static function ( $preempt, $parsed_args, $url ) {
 					return array(
 						'response' => self::$mocked_responses[ $url ],
 						'body'     => self::$mocked_responses[ $url ]['body'] ?? '',
+						'headers'  => self::$mocked_responses[ $url ]['headers'] ?? array(),
 					);
 				}
 				return $preempt;
diff --git a/plugins/performance-lab/tests/includes/site-health/audit-enqueued-assets/test-audit-enqueued-assets-helper.php b/plugins/performance-lab/tests/includes/site-health/audit-enqueued-assets/test-audit-enqueued-assets-helper.php
index e3939b8190..68efda134f 100644
--- a/plugins/performance-lab/tests/includes/site-health/audit-enqueued-assets/test-audit-enqueued-assets-helper.php
+++ b/plugins/performance-lab/tests/includes/site-health/audit-enqueued-assets/test-audit-enqueued-assets-helper.php
@@ -520,10 +520,84 @@ public function test_perflab_aea_get_asset_size(): void {
 
 		$this->assertWPError( perflab_aea_get_asset_size( 'https://example.com/script1.js' ) );
 		$this->assertWPError( perflab_aea_get_asset_size( 'https://example.com/script2.js' ) );
-		$this->assertEquals( 0, perflab_aea_get_asset_size( 'https://example.com/script3.js' ) );
+		$zero_size_result = perflab_aea_get_asset_size( 'https://example.com/script3.js' );
+		$this->assertWPError( $zero_size_result );
+		$this->assertSame( 'zero_size', $zero_size_result->get_error_code() );
 		$this->assertEquals( 1000, perflab_aea_get_asset_size( 'https://example.com/script4.js' ) );
 	}
 
+	/**
+	 * Tests perflab_aea_get_asset_size() with a Cache-Control: no-store response.
+	 *
+	 * @covers ::perflab_aea_get_asset_size
+	 */
+	public function test_perflab_aea_get_asset_size_not_cacheable(): void {
+		Audit_Assets_Mock_Assets::clear_mocked();
+		Audit_Assets_Mock_Assets::mock_requests(
+			array(
+				array(
+					'url'      => 'https://example.com/no-store.js',
+					'response' => array(
+						'code'    => 200,
+						'body'    => str_repeat( 'A', 500 ),
+						'headers' => array( 'cache-control' => 'no-store' ),
+					),
+				),
+				array(
+					'url'      => 'https://example.com/no-store-with-extras.js',
+					'response' => array(
+						'code'    => 200,
+						'body'    => str_repeat( 'A', 500 ),
+						'headers' => array( 'cache-control' => 'no-store, must-revalidate' ),
+					),
+				),
+				array(
+					'url'      => 'https://example.com/no-cache.js',
+					'response' => array(
+						'code'    => 200,
+						'body'    => str_repeat( 'A', 500 ),
+						'headers' => array( 'cache-control' => 'no-cache' ),
+					),
+				),
+				array(
+					'url'      => 'https://example.com/cacheable.js',
+					'response' => array(
+						'code'    => 200,
+						'body'    => str_repeat( 'A', 500 ),
+						'headers' => array( 'cache-control' => 'max-age=3600' ),
+					),
+				),
+				array(
+					'url'      => 'https://example.com/no-store-array-header.js',
+					'response' => array(
+						'code'    => 200,
+						'body'    => str_repeat( 'A', 500 ),
+						'headers' => array( 'cache-control' => array( 'no-store', 'must-revalidate' ) ),
+					),
+				),
+			)
+		);
+
+		$no_store_result = perflab_aea_get_asset_size( 'https://example.com/no-store.js' );
+		$this->assertWPError( $no_store_result );
+		$this->assertSame( 'not_cacheable', $no_store_result->get_error_code() );
+
+		$no_store_extras_result = perflab_aea_get_asset_size( 'https://example.com/no-store-with-extras.js' );
+		$this->assertWPError( $no_store_extras_result );
+		$this->assertSame( 'not_cacheable', $no_store_extras_result->get_error_code() );
+
+		// no-cache means revalidate, not no-store — should not be flagged.
+		$this->assertEquals( 500, perflab_aea_get_asset_size( 'https://example.com/no-cache.js' ) );
+
+		// Normal cacheable response should return size.
+		$this->assertEquals( 500, perflab_aea_get_asset_size( 'https://example.com/cacheable.js' ) );
+
+		// Cache-Control header returned as an array (multiple headers) should still be detected.
+		$no_store_array_result = perflab_aea_get_asset_size( 'https://example.com/no-store-array-header.js' );
+		$this->assertWPError( $no_store_array_result );
+		$this->assertSame( 'not_cacheable', $no_store_array_result->get_error_code() );
+	}
+
 	/**
 	 * Tests perflab_aea_copy_basic_auth_headers() with various scenarios.
 	 *