From 2cd6ddf9eba47b1dd78cf6088a30ca2f4e058663 Mon Sep 17 00:00:00 2001
From: David Roizenman <david@hmnd.io>
Date: Mon, 15 Dec 2025 00:06:08 -0800
Subject: [PATCH 1/3] add test sample

---
 .../fork-derived-class-instance/Child.svelte  | 10 ++++++++
 .../fork-derived-class-instance/_config.js    | 24 +++++++++++++++++++
 .../fork-derived-class-instance/main.svelte   | 17 +++++++++++++
 3 files changed, 51 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/Child.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/main.svelte

diff --git a/packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/Child.svelte b/packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/Child.svelte
new file mode 100644
index 000000000000..fdb0e5fc7b5e
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/Child.svelte
@@ -0,0 +1,10 @@
+<script>
+	class Y {
+		foo = $state(0);
+	}
+
+	let y = $derived(new Y());
+</script>
+
+<button onclick={() => y.foo++}>click</button>
+<p>{y.foo}</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/_config.js b/packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/_config.js
new file mode 100644
index 000000000000..6da74a4e99dd
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/_config.js
@@ -0,0 +1,24 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const forkButton = target.querySelector('button');
+
+		flushSync(() => {
+			forkButton?.click();
+		});
+
+		const [, clickButton] = target.querySelectorAll('button');
+		const p = target.querySelector('p');
+
+		assert.equal(p?.textContent, '0');
+
+		flushSync(() => {
+			clickButton?.click();
+		});
+
+		assert.equal(p?.textContent, '1');
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/main.svelte b/packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/main.svelte
new file mode 100644
index 000000000000..2b94475d5008
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/main.svelte
@@ -0,0 +1,17 @@
+<script>
+	import { fork } from 'svelte';
+	import Child from './Child.svelte';
+
+	let x = $state(false);
+</script>
+
+<button onclick={() => {
+	const f = fork(() => {
+		x = true;
+	});
+	f.commit();
+}}>fork</button>
+
+{#if x}
+	<Child />
+{/if}

From 95efd0e9d940df87ecdad604981e12221bdc628a Mon Sep 17 00:00:00 2001
From: David Roizenman <david@hmnd.io>
Date: Sun, 14 Dec 2025 23:59:33 -0800
Subject: [PATCH 2/3] fix: maintain reactivity for properties of deriveds with
 no deps

fixes #17111
---
 .../svelte/src/internal/client/reactivity/deriveds.js  | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index b2d5d59ae198..017dad01f694 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -360,7 +360,10 @@ export function update_derived(derived) {
 		// the underlying value will be updated when the fork is committed.
 		// otherwise, the next time we get here after a 'real world' state
 		// change, `derived.equals` may incorrectly return `true`
-		if (!current_batch?.is_fork) {
+		//
+		// deriveds with no deps should always update `derived.v`
+		// since they will never change and need the value after fork commits
+		if (!current_batch?.is_fork || derived.deps === null) {
 			derived.v = value;
 		}
 
@@ -381,6 +384,11 @@ export function update_derived(derived) {
 		if (effect_tracking() || current_batch?.is_fork) {
 			batch_values.set(derived, value);
 		}
+		// For deriveds with no deps, set CLEAN to prevent re-evaluation
+		// since they can never become dirty from dependency changes
+		if (derived.deps === null) {
+			set_signal_status(derived, CLEAN);
+		}
 	} else {
 		var status = (derived.f & CONNECTED) === 0 ? MAYBE_DIRTY : CLEAN;
 		set_signal_status(derived, status);

From e66b494cc4e0cdd483b1618105e1c9ae90c627be Mon Sep 17 00:00:00 2001
From: David Roizenman <david@hmnd.io>
Date: Mon, 15 Dec 2025 00:11:36 -0800
Subject: [PATCH 3/3] add changeset

---
 .changeset/old-pens-help.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/old-pens-help.md

diff --git a/.changeset/old-pens-help.md b/.changeset/old-pens-help.md
new file mode 100644
index 000000000000..b0374b96792e
--- /dev/null
+++ b/.changeset/old-pens-help.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: maintain reactivity for properties of deriveds with no deps