From 737de2e4f3020493474d509e4cda799cd4fd7a16 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 removal of batch_values.delete(derived) in
 mark_reactions

A derived's batch-local value can be a later batch's previous value; deleting it makes the next
read fall back to the real (later batch's) value.
---
 .../src/internal/client/reactivity/sources.js |  1 -
 .../_config.js                                | 33 +++++++++++++++++++
 .../main.svelte                               | 32 ++++++++++++++++++
 3 files changed, 65 insertions(+), 1 deletion(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/main.svelte

diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 09b6d3bb97..f2ed02a23e 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -411,7 +411,6 @@ function mark_reactions(signal, status, updated_during_traversal) {
 		} else if ((flags & DERIVED) !== 0) {
 			var derived = /** @type {Derived} */ (reaction);
 
-			batch_values?.delete(derived);
 			mark_reactions(derived, MAYBE_DIRTY, updated_during_traversal);
 		} else if (not_dirty) {
 			var effect = /** @type {Effect} */ (reaction);
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/_config.js b/packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/_config.js
new file mode 100644
index 0000000000..fb0b6e8a4f
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/_config.js
@@ -0,0 +1,33 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons = '<button>x</button><button>t</button><button>shift</button>';
+
+// While batch A (x) commits, a pre effect writes to `s`. That marks `label` (via `big`) as
+// maybe dirty, but it doesn't actually change. `label` must still show the value from A's
+// point of view, not the one from the later, still-pending batch B (t).
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [x, t, shift] = target.querySelectorAll('button');
+
+		// A
+		x.click();
+		await tick();
+
+		// B
+		t.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>0</p><p>0</p><p>0 false-0</p>`);
+
+		// A commits, B is still pending
+		shift.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>1</p><p>0</p><p>1 false-0</p>`);
+
+		// B commits
+		shift.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>1</p><p>1</p><p>1 false-1</p>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/main.svelte
new file mode 100644
index 0000000000..1791827eae
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/main.svelte
@@ -0,0 +1,32 @@
+<script>
+	import { untrack } from 'svelte';
+
+	let x = $state(0);
+	let t = $state(0);
+	let s = $state(0);
+
+	// `s` changes, but `big` doesn't
+	let big = $derived(s > 100);
+	let label = $derived(`${big}-${t}`);
+
+	const queue = [];
+	function push(v) {
+		if (!v) return v;
+		return new Promise((r) => queue.push(() => r(v)));
+	}
+
+	$effect.pre(() => {
+		if (x) untrack(() => s++);
+	});
+</script>
+
+<button onclick={() => x++}>x</button>
+<button onclick={() => t++}>t</button>
+<button onclick={() => queue.shift()()}>shift</button>
+
+<p>{await push(x)}</p>
+<p>{await push(t)}</p>
+<!-- evaluates `label` during traversal of B -->
+{#key label}{/key}
+
+<p>{x} {label}</p>