From 1c150a460fdb67703aeba9eb515c0a30e2348ccb Mon Sep 17 00:00:00 2001
From: Simon H <5968653+dummdidumm@users.noreply.github.com>
Date: Wed, 6 May 2026 22:37:46 +0200
Subject: [PATCH] fix: abort running obsolete async branches (#18118)

We shouldn't continue executing async work where we know the surrounding
branch is destroyed already, it can leave to noisy "derived inter"
warnings or even runtime errors ("cannot stringify symbol" when running
a template effect with an uninitialized source). Neither should we warn
about waterfalls on an already-destroyed async effect.

Fixes #18097 (though strictly speaking that particular instance is also
fixed by #18117 which fixes the underlying cause for the reruns; this
one is necessary in itself though, as shown by the new test)

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>
---
 .changeset/stupid-baboons-fall.md             |  5 +++
 .../src/internal/client/reactivity/async.js   |  8 +++--
 .../internal/client/reactivity/deriveds.js    |  2 +-
 .../Child.svelte                              |  6 ++++
 .../_config.js                                | 27 ++++++++++++++++
 .../main.svelte                               | 31 +++++++++++++++++++
 6 files changed, 75 insertions(+), 4 deletions(-)
 create mode 100644 .changeset/stupid-baboons-fall.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-obsolete-branch-no-effect-runs/Child.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-obsolete-branch-no-effect-runs/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-obsolete-branch-no-effect-runs/main.svelte

diff --git a/.changeset/stupid-baboons-fall.md b/.changeset/stupid-baboons-fall.md
new file mode 100644
index 0000000000..66895ad015
--- /dev/null
+++ b/.changeset/stupid-baboons-fall.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: abort running obsolete async branches
diff --git a/packages/svelte/src/internal/client/reactivity/async.js b/packages/svelte/src/internal/client/reactivity/async.js
index 61fff31f8a..5e418d81a1 100644
--- a/packages/svelte/src/internal/client/reactivity/async.js
+++ b/packages/svelte/src/internal/client/reactivity/async.js
@@ -55,14 +55,16 @@ export function flatten(blockers, sync, async, fn) {
 
 	/** @param {Value[]} values */
 	function finish(values) {
+		if ((parent.f & DESTROYED) !== 0) {
+			return;
+		}
+
 		restore();
 
 		try {
 			fn(values);
 		} catch (error) {
-			if ((parent.f & DESTROYED) === 0) {
-				invoke_error_boundary(error, parent);
-			}
+			invoke_error_boundary(error, parent);
 		}
 
 		unset_context();
diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index eb934d96ff..070dfc8ff3 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -239,7 +239,7 @@ export function async_derived(fn, label, location) {
 					recent_async_deriveds.add(signal);
 
 					setTimeout(() => {
-						if (recent_async_deriveds.has(signal)) {
+						if (recent_async_deriveds.has(signal) && (effect.f & DESTROYED) === 0) {
 							w.await_waterfall(/** @type {string} */ (signal.label), location);
 							recent_async_deriveds.delete(signal);
 						}
diff --git a/packages/svelte/tests/runtime-runes/samples/async-obsolete-branch-no-effect-runs/Child.svelte b/packages/svelte/tests/runtime-runes/samples/async-obsolete-branch-no-effect-runs/Child.svelte
new file mode 100644
index 0000000000..1d9bdfada2
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-obsolete-branch-no-effect-runs/Child.svelte
@@ -0,0 +1,6 @@
+<script>
+	let { count } = $props();
+	let double = $derived(count * 2);
+
+	$effect.pre(() => console.log(count, double));
+</script>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-obsolete-branch-no-effect-runs/_config.js b/packages/svelte/tests/runtime-runes/samples/async-obsolete-branch-no-effect-runs/_config.js
new file mode 100644
index 0000000000..83364706e5
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-obsolete-branch-no-effect-runs/_config.js
@@ -0,0 +1,27 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs, warnings }) {
+		const [increment, resolve] = target.querySelectorAll('button');
+
+		increment.click();
+		await tick();
+		assert.deepEqual(logs, []);
+
+		resolve.click();
+		await tick();
+		assert.deepEqual(logs, []);
+
+		resolve.click();
+		await tick();
+		assert.deepEqual(logs, []);
+
+		resolve.click();
+		await tick();
+		assert.deepEqual(logs, [1, 2]);
+
+		// no await waterfall / inert derived warnings
+		assert.deepEqual(warnings, []);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-obsolete-branch-no-effect-runs/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-obsolete-branch-no-effect-runs/main.svelte
new file mode 100644
index 0000000000..fe01ae457e
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-obsolete-branch-no-effect-runs/main.svelte
@@ -0,0 +1,31 @@
+<script>
+	import Child from "./Child.svelte";
+	let count = $state(0);
+
+	let deferreds = [];
+
+	function push(v) {
+		return new Promise((resolve, reject) => {
+			deferreds.push({ resolve: () => resolve(v), reject });
+		});
+	}
+</script>
+
+<button onclick={() => count += 1}>increment</button>
+<button onclick={() => deferreds.shift()?.resolve()}>resolve</button>
+
+<svelte:boundary>
+	{#if count % 2 === 0}
+		{@const double = count * 2}
+		<p>true</p>
+		{await push(count)} {double}
+		<Child count={await push(count)} />
+	{:else}
+		<p>false</p>
+		<Child count={await push(count)} />
+	{/if}
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>