diff --git a/packages/svelte/tests/runtime-runes/samples/flush-sync-inside-attachment/_config.js b/packages/svelte/tests/runtime-runes/samples/flush-sync-inside-attachment/_config.js
index b34a90e901..ec8858b2c6 100644
--- a/packages/svelte/tests/runtime-runes/samples/flush-sync-inside-attachment/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/flush-sync-inside-attachment/_config.js
@@ -1,8 +1,12 @@
+import { async_mode } from '../../../helpers';
 import { test } from '../../test';
 
 export default test({
+	// In legacy mode this succeeds and logs 'hello'
+	// In async mode this throws an error because flushSync is called inside an effect
 	async test({ assert, target, logs }) {
 		assert.htmlEqual(target.innerHTML, `<button>show</button> <div>hello</div>`);
 		assert.deepEqual(logs, ['hello']);
-	}
+	},
+	runtime_error: async_mode ? 'flush_sync_in_effect' : undefined
 });