From fe12ffc2a3ebfb4fcf43f283fccb984363c1ea84 Mon Sep 17 00:00:00 2001
From: David <david@hmnd.io>
Date: Sun, 25 Jan 2026 07:16:45 -0800
Subject: [PATCH] fix: avoid unnecessary block effect re-runs after async work
 completes (#17535)

* add samples for async readable freeze

* test updating without changing reference too

* fix: treat block effects as maybe dirty in revive()

* even simpler - put block effects in maybe_dirty_effects

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>
---
 .changeset/wise-emus-hunt.md                  |  5 +++
 .../src/internal/client/reactivity/batch.js   |  2 +-
 .../async-await-store-mutate/_config.js       | 24 ++++++++++++++
 .../async-await-store-mutate/main.svelte      | 26 +++++++++++++++
 .../async-each-await-store-update/_config.js  | 32 +++++++++++++++++++
 .../async-each-await-store-update/main.svelte | 31 ++++++++++++++++++
 6 files changed, 119 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/wise-emus-hunt.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-await-store-mutate/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-await-store-mutate/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-each-await-store-update/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-each-await-store-update/main.svelte

diff --git a/.changeset/wise-emus-hunt.md b/.changeset/wise-emus-hunt.md
new file mode 100644
index 0000000000..c8b3f93de8
--- /dev/null
+++ b/.changeset/wise-emus-hunt.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid unnecessary block effect re-runs after async work completes
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index c4caf8a20b..5647c08ba5 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -243,7 +243,7 @@ export class Batch {
 				} else if (async_mode_flag && (flags & (RENDER_EFFECT | MANAGED_EFFECT)) !== 0) {
 					render_effects.push(effect);
 				} else if (is_dirty(effect)) {
-					if ((flags & BLOCK_EFFECT) !== 0) this.#dirty_effects.add(effect);
+					if ((flags & BLOCK_EFFECT) !== 0) this.#maybe_dirty_effects.add(effect);
 					update_effect(effect);
 				}
 
diff --git a/packages/svelte/tests/runtime-runes/samples/async-await-store-mutate/_config.js b/packages/svelte/tests/runtime-runes/samples/async-await-store-mutate/_config.js
new file mode 100644
index 0000000000..e89065865f
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-await-store-mutate/_config.js
@@ -0,0 +1,24 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+// ensure in-place object mutations stay reactive in async
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const button = /** @type {HTMLElement} */ (target.querySelector('button'));
+
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, `<p>count: 1, computed: 10</p><button>mutate</button>`);
+
+		button.click();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, `<p>count: 2, computed: 20</p><button>mutate</button>`);
+
+		button.click();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, `<p>count: 3, computed: 30</p><button>mutate</button>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-await-store-mutate/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-await-store-mutate/main.svelte
new file mode 100644
index 0000000000..327c775d6e
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-await-store-mutate/main.svelte
@@ -0,0 +1,26 @@
+<script>
+	import { writable } from 'svelte/store';
+
+	const data = writable({ count: 1 });
+
+	function mutate_count() {
+		data.update(d => {
+			d.count++;
+			return d;
+		});
+	}
+
+	async function compute(d) {
+		return Promise.resolve(d.count * 10);
+	}
+</script>
+
+<svelte:boundary>
+	<p>count: {$data.count}, computed: {await compute($data)}</p>
+
+	{#snippet pending()}
+		<p>pending</p>
+	{/snippet}
+</svelte:boundary>
+
+<button onclick={mutate_count}>mutate</button>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-each-await-store-update/_config.js b/packages/svelte/tests/runtime-runes/samples/async-each-await-store-update/_config.js
new file mode 100644
index 0000000000..165577af96
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-each-await-store-update/_config.js
@@ -0,0 +1,32 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const button = /** @type {HTMLElement} */ (target.querySelector('button'));
+
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<ul><li data-item="1"><span>10</span><span>20</span></li></ul><button>add</button>`
+		);
+
+		button.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<ul><li data-item="1"><span>10</span><span>20</span></li><li data-item="2"><span>20</span><span>40</span></li></ul><button>add</button>`
+		);
+
+		button.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<ul><li data-item="1"><span>10</span><span>20</span></li><li data-item="2"><span>20</span><span>40</span></li><li data-item="3"><span>30</span><span>60</span></li></ul><button>add</button>`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-each-await-store-update/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-each-await-store-update/main.svelte
new file mode 100644
index 0000000000..41982b767d
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-each-await-store-update/main.svelte
@@ -0,0 +1,31 @@
+<script>
+	import { writable } from 'svelte/store';
+
+	const items = writable([{ id: 1 }]);
+
+	function add_item() {
+		items.update(arr => [...arr, { id: arr.length + 1 }]);
+	}
+
+	function query(item) {
+		return Promise.resolve([item.id * 10, item.id * 20]);
+	}
+</script>
+
+<svelte:boundary>
+	<ul>
+		{#each $items as item (item.id)}
+			<li data-item={item.id}>
+				{#each await query(item) as value}
+					<span>{value}</span>
+				{/each}
+			</li>
+		{/each}
+	</ul>
+	
+	{#snippet pending()}
+		<p>pending</p>
+	{/snippet}
+</svelte:boundary>
+
+<button onclick={add_item}>add</button>