From a224dd58671eb11f9b21e2a1c9f8fc48c0ece63a Mon Sep 17 00:00:00 2001
From: Yuuki Oohara <gmcthmy@gmail.com>
Date: Sat, 15 Aug 2026 01:07:16 +0900
Subject: [PATCH] fix: give effect teardowns the value from before the first
 write in a flush (#18620)

Fixes #18619

Make sure to not override old values with newer values that are part of the same flush

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
---
 .changeset/lucky-pears-smoke.md               |  5 +++++
 .../src/internal/client/reactivity/batch.js   |  1 +
 .../src/internal/client/reactivity/sources.js |  8 ++++++-
 .../_config.js                                | 17 +++++++++++++++
 .../main.svelte                               | 21 +++++++++++++++++++
 5 files changed, 51 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/lucky-pears-smoke.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/effect-teardown-multiple-writes/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/effect-teardown-multiple-writes/main.svelte

diff --git a/.changeset/lucky-pears-smoke.md b/.changeset/lucky-pears-smoke.md
new file mode 100644
index 0000000000..816a37dca5
--- /dev/null
+++ b/.changeset/lucky-pears-smoke.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: give effect teardowns the value from before the first write in a flush
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 5becae2dc9..8251f79298 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -420,6 +420,7 @@ export class Batch {
 		}
 
 		if (next_batch !== null) {
+			old_values.clear();
 			next_batch.#process();
 		}
 	}
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 218941ab67..70be6489a4 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -180,7 +180,13 @@ export function set(source, value, should_proxy = false) {
  */
 export function internal_set(source, value, updated_during_traversal = null) {
 	if (!source.equals(value)) {
-		old_values.set(source, is_destroying_effect ? value : source.v);
+		if (is_destroying_effect) {
+			old_values.set(source, value);
+		} else if (!old_values.has(source)) {
+			// only record the value from before the first write in this flush, otherwise a
+			// teardown would see the value from before whichever write happened to be last
+			old_values.set(source, source.v);
+		}
 
 		var batch = Batch.ensure();
 		batch.capture(source, value);
diff --git a/packages/svelte/tests/runtime-runes/samples/effect-teardown-multiple-writes/_config.js b/packages/svelte/tests/runtime-runes/samples/effect-teardown-multiple-writes/_config.js
new file mode 100644
index 0000000000..7db4966106
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/effect-teardown-multiple-writes/_config.js
@@ -0,0 +1,17 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		const button = target.querySelector('button');
+
+		flushSync(() => button?.click());
+
+		assert.deepEqual(logs, [
+			'register: one',
+			'unregister: one',
+			'leftover: none',
+			'register: three'
+		]);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/effect-teardown-multiple-writes/main.svelte b/packages/svelte/tests/runtime-runes/samples/effect-teardown-multiple-writes/main.svelte
new file mode 100644
index 0000000000..a3932aaa22
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/effect-teardown-multiple-writes/main.svelte
@@ -0,0 +1,21 @@
+<script>
+	let value = $state('one');
+
+	const registry = new Set();
+
+	$effect(() => {
+		registry.add(value);
+		console.log(`register: ${value}`);
+
+		return () => {
+			registry.delete(value);
+			console.log(`unregister: ${value}`);
+			console.log(`leftover: ${[...registry].join(', ') || 'none'}`);
+		};
+	});
+</script>
+
+<button onclick={() => {
+	value = 'two';
+	value = 'three';
+}}>go</button>