From 16ea0cee925df1f96accf48b5a09e87f9207f062 Mon Sep 17 00:00:00 2001
From: Antonio Bennett <31296212+Antonio-Bennett@users.noreply.github.com>
Date: Thu, 29 Jan 2026 19:23:34 -0500
Subject: [PATCH] fix: handle renderer.run rejections (#17591)

* fix: handle renderer run rejections

* add test

* changeset

* simplify

* explanatory comment

---------

Co-authored-by: Antonio Bennett <abennett@mabelslabels.com>
Co-authored-by: Rich Harris <rich.harris@vercel.com>
---
 .changeset/tidy-zebras-design.md                           | 5 +++++
 packages/svelte/src/internal/server/renderer.js            | 6 ++++++
 .../runtime-runes/samples/async-error-in-script/_config.js | 6 ++++++
 .../samples/async-error-in-script/main.svelte              | 7 +++++++
 4 files changed, 24 insertions(+)
 create mode 100644 .changeset/tidy-zebras-design.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-error-in-script/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-error-in-script/main.svelte

diff --git a/.changeset/tidy-zebras-design.md b/.changeset/tidy-zebras-design.md
new file mode 100644
index 0000000000..3b18af798f
--- /dev/null
+++ b/.changeset/tidy-zebras-design.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent unhandled exceptions arising from dangling promises in <script>
diff --git a/packages/svelte/src/internal/server/renderer.js b/packages/svelte/src/internal/server/renderer.js
index 4da0b05cfd..65f9bcae6a 100644
--- a/packages/svelte/src/internal/server/renderer.js
+++ b/packages/svelte/src/internal/server/renderer.js
@@ -11,6 +11,7 @@ import { attributes } from './index.js';
 import { get_render_context, with_render_context, init_render_context } from './render-context.js';
 import { sha256 } from './crypto.js';
 import * as devalue from 'devalue';
+import { noop } from '../shared/utils.js';
 
 /** @typedef {'head' | 'body'} RendererType */
 /** @typedef {{ [key in RendererType]: string }} AccumulatedContent */
@@ -162,6 +163,11 @@ export class Renderer {
 			promises.push(promise);
 		}
 
+		// prevent unhandled rejections, and attach the promise to the renderer instance
+		// so that rejections correctly cause rendering to fail
+		promise.catch(noop);
+		this.promise = promise;
+
 		return promises;
 	}
 
diff --git a/packages/svelte/tests/runtime-runes/samples/async-error-in-script/_config.js b/packages/svelte/tests/runtime-runes/samples/async-error-in-script/_config.js
new file mode 100644
index 0000000000..2e0fab473c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-error-in-script/_config.js
@@ -0,0 +1,6 @@
+import { test } from '../../test';
+
+export default test({
+	mode: ['async-server', 'hydrate'],
+	error: 'oops'
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-error-in-script/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-error-in-script/main.svelte
new file mode 100644
index 0000000000..64d9e6ce08
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-error-in-script/main.svelte
@@ -0,0 +1,7 @@
+<script>
+	await 1;
+
+	throw new Error('oops');
+</script>
+
+<h1>hello</h1>