From 70bb7f42959a7cc8381fbe81e4b0d5f667630a8f Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 13 Oct 2026 12:07:21 -0400
Subject: [PATCH] restore capturing deriveds in previous_batch before
 current_batch

The committing batch needs the correct previous value, and the new batch must record that its
value came from the committing batch's world.
---
 .../internal/client/reactivity/deriveds.js    |  2 +-
 .../_config.js                                | 33 ++++++++++++++++
 .../main.svelte                               | 38 +++++++++++++++++++
 3 files changed, 72 insertions(+), 1 deletion(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/main.svelte

diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index 2ae137c238..1692762ae4 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -421,8 +421,8 @@ export function update_derived(derived) {
 			// to the previous batch, not the new one (which can already exist if an earlier
 			// effect wrote to a source). Not adding it to either can cause bugs, so we add it to both.
 			// See https://github.com/sveltejs/svelte/pull/18117 for more details.
-			current_batch?.capture(derived, value, true);
 			previous_batch?.capture(derived, value, true);
+			current_batch?.capture(derived, value, true);
 		} else {
 			derived.v = value;
 			derived.wv = increment_write_version();
diff --git a/packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/_config.js b/packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/_config.js
new file mode 100644
index 0000000000..8f021032e0
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/_config.js
@@ -0,0 +1,33 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+// Batch B (x) commits while an earlier batch A (y) is still pending, but B stays around because
+// of the async work in the new boundary. While B flushes its effects, an effect creates a new batch C,
+// and `d` is computed for the first time afterwards (so it's captured in both B and C). When A
+// commits later, it sees B's previous values, which must be consistent (`d` must be `x * 2`).
+export default test({
+	async test({ assert, target, logs }) {
+		await tick();
+		const [y, x, shift] = target.querySelectorAll('button');
+		assert.deepEqual(logs, [0, 0, 0]);
+
+		// A
+		y.click();
+		await tick();
+
+		// B
+		x.click();
+		await tick();
+		assert.deepEqual(logs, [0, 0, 0, 0, 1, 2]);
+
+		// A commits
+		shift.click();
+		await tick();
+
+		for (let i = 0; i < logs.length; i += 3) {
+			assert.equal(logs[i + 2], logs[i + 1] * 2, `torn read: ${logs.slice(i, i + 3)}`);
+		}
+
+		assert.deepEqual(logs.slice(-3), [1, 1, 2]);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/main.svelte
new file mode 100644
index 0000000000..5cd0876884
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/main.svelte
@@ -0,0 +1,38 @@
+<script>
+	import { untrack } from 'svelte';
+
+	let x = $state(0);
+	let y = $state(0);
+	let z = $state(0);
+	let d = $derived(x * 2);
+
+	const queue = [];
+	function push(v) {
+		return new Promise((r) => queue.push(() => r(v)));
+	}
+
+	// creates a new batch while the batch that changed `x` flushes its effects...
+	$effect(() => {
+		if (x) untrack(() => z++);
+	});
+
+	// ...before `d` is computed for the first time in that batch
+	$effect(() => {
+		console.log(y, x, d);
+	});
+</script>
+
+<button onclick={() => y++}>y</button>
+<button onclick={() => x++}>x</button>
+<button onclick={() => queue.shift()()}>shift</button>
+
+<p>{y ? await push(y) : y}</p>
+
+{#if x}
+	<svelte:boundary>
+		<p>{await push(x)}</p>
+		{#snippet pending()}
+			<p>loading</p>
+		{/snippet}
+	</svelte:boundary>
+{/if}