From bfbb026f2f7db6ced0d86ba0feb40587c0e8f598 Mon Sep 17 00:00:00 2001
From: JY Wey <34165386+JaiWey@users.noreply.github.com>
Date: Sat, 11 Jul 2026 01:42:01 +1200
Subject: [PATCH] fix: skip unnecessary derived effect in earlier batch
 (#18525)

fixes #18438
Currently the `mark` method inside `#merge` for `earlier_batch` will
schedule effect for undirty derived. Add the condition for derived to
prevent unnecessary effect be scheduled.

```
if ((flags & DERIVED) !== 0) {
	mark(/** @type {Derived} */ (reaction));
}
```
---
 .changeset/curly-wasps-hide.md                |  5 ++++
 .../src/internal/client/reactivity/batch.js   |  6 ++++
 .../samples/async-batch-derived/_config.js    | 28 ++++++++++++++++++
 .../samples/async-batch-derived/main.svelte   | 29 +++++++++++++++++++
 4 files changed, 68 insertions(+)
 create mode 100644 .changeset/curly-wasps-hide.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-batch-derived/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-batch-derived/main.svelte

diff --git a/.changeset/curly-wasps-hide.md b/.changeset/curly-wasps-hide.md
new file mode 100644
index 0000000000..7e55d77ba0
--- /dev/null
+++ b/.changeset/curly-wasps-hide.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: skip unnecessary derived effect in earlier batch
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 7d14b80519..5becae2dc9 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -530,6 +530,12 @@ export class Batch {
 		const mark = (value) => {
 			var reactions = value.reactions;
 			if (reactions === null) return;
+			// skip if value is derived and is neither dirty nor maybe dirty. transitive
+			// deriveds (a derived depending on another derived) are only MAYBE_DIRTY, so
+			// we must continue traversing them to reach the effects that depend on them
+			if ((value.f & DERIVED) !== 0 && (value.f & (DIRTY | MAYBE_DIRTY)) === 0) {
+				return;
+			}
 
 			for (const reaction of reactions) {
 				var flags = reaction.f;
diff --git a/packages/svelte/tests/runtime-runes/samples/async-batch-derived/_config.js b/packages/svelte/tests/runtime-runes/samples/async-batch-derived/_config.js
new file mode 100644
index 0000000000..d5286d9d95
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-batch-derived/_config.js
@@ -0,0 +1,28 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [increment, pop] = target.querySelectorAll('button');
+
+		increment.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>increment</button> <button>pop</button> <p>Loading...</p>`
+		);
+		increment.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>increment</button> <button>pop</button> <p>Loading...</p>`
+		);
+		pop.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<button>increment</button> <button>pop</button> 2 2 1`);
+
+		pop.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<button>increment</button> <button>pop</button> 2 2 1`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-batch-derived/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-batch-derived/main.svelte
new file mode 100644
index 0000000000..af7f0f468c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-batch-derived/main.svelte
@@ -0,0 +1,29 @@
+<script>
+	let count = $state(0);
+	let other = $state(0);
+
+	const queue = [];
+  	let pending = $derived(defaultPending);
+	function push(v) {
+		return new Promise((resolve) => queue.push(() => resolve(v)));
+	}
+</script>
+
+<button onclick={() => {
+	if (count === 0) {
+		other++;
+		count++;
+	} else {
+		count++
+	}
+}}>increment</button>
+<button onclick={() => queue.pop()?.()}>pop</button>
+{#snippet defaultPending()}
+  <p>Loading...</p>
+{/snippet}
+
+{#if count > 0}
+	<svelte:boundary {pending}>
+		{await push(count)} {count} {other}
+	</svelte:boundary> 
+{/if}