From 04eadbc8a9ea905878fec90e61b5a62a2f9a4045 Mon Sep 17 00:00:00 2001
From: Simon H <5968653+dummdidumm@users.noreply.github.com>
Date: Sun, 29 Mar 2026 21:56:27 +0200
Subject: [PATCH] fix: correctly handle bindings on the server (#18009)

`item.subsume(item)` did nothing, honestly kinda weird how this only
turned up now

Fixes #17981
---
 .changeset/eight-trees-occur.md                        |  5 +++++
 packages/svelte/src/internal/server/renderer.js        | 10 +++++++---
 .../samples/async-hydration-binding/Async.svelte       |  7 +++++++
 .../samples/async-hydration-binding/Binding.svelte     |  7 +++++++
 .../samples/async-hydration-binding/Bound.svelte       |  0
 .../samples/async-hydration-binding/_config.js         | 10 ++++++++++
 .../samples/async-hydration-binding/main.svelte        |  7 +++++++
 7 files changed, 43 insertions(+), 3 deletions(-)
 create mode 100644 .changeset/eight-trees-occur.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-hydration-binding/Async.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-hydration-binding/Binding.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-hydration-binding/Bound.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-hydration-binding/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-hydration-binding/main.svelte

diff --git a/.changeset/eight-trees-occur.md b/.changeset/eight-trees-occur.md
new file mode 100644
index 0000000000..cc200b39ba
--- /dev/null
+++ b/.changeset/eight-trees-occur.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly handle bindings on the server
diff --git a/packages/svelte/src/internal/server/renderer.js b/packages/svelte/src/internal/server/renderer.js
index da65811eca..4bcb45512e 100644
--- a/packages/svelte/src/internal/server/renderer.js
+++ b/packages/svelte/src/internal/server/renderer.js
@@ -468,10 +468,14 @@ export class Renderer {
 		}
 
 		this.local = other.local;
-		this.#out = other.#out.map((item) => {
-			if (item instanceof Renderer) {
-				item.subsume(item);
+		this.#out = other.#out.map((item, i) => {
+			const current = this.#out[i];
+
+			if (current instanceof Renderer && item instanceof Renderer) {
+				current.subsume(item);
+				return current;
 			}
+
 			return item;
 		});
 		this.promise = other.promise;
diff --git a/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/Async.svelte b/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/Async.svelte
new file mode 100644
index 0000000000..6e7dbc82bb
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/Async.svelte
@@ -0,0 +1,7 @@
+<script>
+	let data = $derived(await Promise.resolve('test'));
+</script>
+
+<div data-resolved={data ? 'true' : 'false'}>
+	{data}
+</div>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/Binding.svelte b/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/Binding.svelte
new file mode 100644
index 0000000000..3deba21acc
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/Binding.svelte
@@ -0,0 +1,7 @@
+<script>
+	import Bound from './Bound.svelte';
+
+	let open;
+</script>
+
+<Bound bind:open />
diff --git a/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/Bound.svelte b/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/Bound.svelte
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/_config.js b/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/_config.js
new file mode 100644
index 0000000000..d4d36b007f
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/_config.js
@@ -0,0 +1,10 @@
+import { test } from '../../test';
+
+// Tests that renderer.subsume (which is used when bindings are present) works correctly
+export default test({
+	mode: ['hydrate'],
+	html: '<div data-resolved="true">test</div>',
+	async test({ assert, warnings }) {
+		assert.deepEqual(warnings, []);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/main.svelte
new file mode 100644
index 0000000000..a38806be7c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-hydration-binding/main.svelte
@@ -0,0 +1,7 @@
+<script lang="ts">
+	import Async from './Async.svelte';
+	import Binding from './Binding.svelte';
+</script>
+
+<Async />
+<Binding />