fix

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/component.js

@@ -253,11 +253,12 @@ export function build_component(node, component_name, context, anchor = context.
 				}
 			}
 		} else if (attribute.type === 'AttachTag') {
+			// TODO do we need to create a derived here?
 			push_prop(
 				b.prop(
-					'init',
+					'get',
 					b.call('Symbol', b.literal('@attach')),
-					b.thunk(/** @type {Expression} */ (context.visit(attribute.expression))),
+					/** @type {Expression} */ (context.visit(attribute.expression)),
 					true
 				)
 			);

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -409,7 +409,7 @@ export function set_attributes(
 	}
 
 	for (let symbol of Object.getOwnPropertySymbols(next)) {
-		attach(element, next[symbol]);
+		attach(element, () => next[symbol]);
 	}
 
 	return current;

packages/svelte/tests/runtime-runes/samples/attachment-spread/_config.js

@@ -0,0 +1,8 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, logs, target }) {
+		assert.deepEqual(logs, ['hello']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/attachment-spread/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	let stuff = $state({
+		[Symbol()]: () => console.log('hello')
+	});
+</script>
+
+<div {...stuff}></div>