From a7c16935a511ea701cf571a312cdb21900ad14c0 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Fri, 18 Sep 2026 13:31:42 +0200
Subject: [PATCH] equality check against what the batch sees to update its
 value

---
 .../internal/client/reactivity/deriveds.js    |  6 ++++++
 .../src/internal/client/reactivity/sources.js |  9 ++++++++-
 .../samples/async-fork-write-back/_config.js  | 17 ++++++++++++++++
 .../samples/async-fork-write-back/main.svelte | 20 +++++++++++++++++++
 4 files changed, 51 insertions(+), 1 deletion(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-fork-write-back/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-fork-write-back/main.svelte

diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index 8fe8951a98..56bc0895f9 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -443,6 +443,12 @@ export function update_derived(derived) {
 				return;
 			}
 		}
+	} else if (
+		batch_values?.has(derived) &&
+		!derived.equals(/** @type {any[]} */ (batch_values?.get(derived))[0])
+	) {
+		current_batch?.capture(derived, derived.v);
+		// TODO also bump wv_values?
 	}
 
 	// don't mark derived clean if we're reading it inside a
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index ed8f1a146f..5c65f9ba0d 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -37,7 +37,8 @@ import {
 	batch_values,
 	eager_block_effects,
 	schedule_effect,
-	legacy_updates
+	legacy_updates,
+	current_batch
 } from './batch.js';
 import { proxy } from '../proxy.js';
 import { execute_derived } from './deriveds.js';
@@ -272,6 +273,12 @@ export function internal_set(source, value, updated_during_traversal = null) {
 		if (!batch.is_fork && eager_effects.size > 0 && !eager_effects_deferred) {
 			flush_eager_effects();
 		}
+	} else if (
+		batch_values?.has(source) &&
+		!source.equals(/** @type {any[]} */ (batch_values?.get(source))[0])
+	) {
+		current_batch?.capture(source, source.v);
+		// TODO also bump wv_values?
 	}
 
 	return value;
diff --git a/packages/svelte/tests/runtime-runes/samples/async-fork-write-back/_config.js b/packages/svelte/tests/runtime-runes/samples/async-fork-write-back/_config.js
new file mode 100644
index 0000000000..3d96cb58e6
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-fork-write-back/_config.js
@@ -0,0 +1,17 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [create, commit] = target.querySelectorAll('button');
+		const [p] = target.querySelectorAll('p');
+
+		create.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, '0:0');
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(p.innerHTML, '0:0');
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-fork-write-back/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-fork-write-back/main.svelte
new file mode 100644
index 0000000000..2d424c95aa
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-fork-write-back/main.svelte
@@ -0,0 +1,20 @@
+<script>
+	import { fork } from 'svelte';
+
+	let source = $state(0);
+	let writable = $derived(source);
+	let pending;
+</script>
+
+<button onclick={() => {
+	pending = fork(() => {
+		source = 1;
+		source = 0;
+		writable = 1;
+		writable = 0;
+	});
+}}>fork</button>
+
+<button onclick={() => pending.commit()}>commit</button>
+
+<p>{source}:{writable}</p>