From ece2e83eb9d0e2a97d7db50d41f68ce2edfb697d Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Wed, 28 Jan 2026 16:09:32 -0500
Subject: [PATCH] fix: increment signal versions when discarding forks (#17577)

* fix: always update an UNINITIALIZED derived on read

Co-authored-by: David Roizenman <hmnd@users.noreply.github.com>

* obsolete comments

* rename test

* add tests, fix

* revert

* Update .changeset/vast-hornets-draw.md

---------

Co-authored-by: David Roizenman <hmnd@users.noreply.github.com>
---
 .changeset/vast-hornets-draw.md               |  5 +++
 .../src/internal/client/reactivity/batch.js   |  7 ++++
 .../fork-derived-uncached-1/_config.js        | 40 +++++++++++++++++++
 .../fork-derived-uncached-1/main.svelte       | 23 +++++++++++
 .../fork-derived-uncached-2/_config.js        | 38 ++++++++++++++++++
 .../fork-derived-uncached-2/main.svelte       | 23 +++++++++++
 .../fork-derived-uncached-3/_config.js        | 32 +++++++++++++++
 .../fork-derived-uncached-3/main.svelte       | 23 +++++++++++
 8 files changed, 191 insertions(+)
 create mode 100644 .changeset/vast-hornets-draw.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-1/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-1/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-2/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-2/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-3/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-3/main.svelte

diff --git a/.changeset/vast-hornets-draw.md b/.changeset/vast-hornets-draw.md
new file mode 100644
index 0000000000..256439e9da
--- /dev/null
+++ b/.changeset/vast-hornets-draw.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: increment signal versions when discarding forks
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 134ebe53a2..d82f965541 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -972,6 +972,13 @@ export function fork(fn) {
 			await settled;
 		},
 		discard: () => {
+			// cause any MAYBE_DIRTY deriveds to update
+			// if they depend on things thath changed
+			// inside the discarded fork
+			for (var source of batch.current.keys()) {
+				source.wv = increment_write_version();
+			}
+
 			if (!committed && batches.has(batch)) {
 				batches.delete(batch);
 				batch.discard();
diff --git a/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-1/_config.js b/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-1/_config.js
new file mode 100644
index 0000000000..fb50ceb9b1
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-1/_config.js
@@ -0,0 +1,40 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const [fork, toggle, increment] = target.querySelectorAll('button');
+
+		// derived is first evaluated in block effect, then discarded
+		flushSync(() => fork.click());
+
+		// should not throw "Cannot convert a Symbol value to a string" due to cached UNINITIALIZED from first fork
+		flushSync(() => fork.click());
+
+		// should not reflect the temporary change to `clicks` inside the fork
+		flushSync(() => toggle.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>fork</button>
+				<button>toggle</button>
+				<button>clicks: 0</button>
+				<p>0</p>
+			`
+		);
+
+		flushSync(() => increment.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>fork</button>
+				<button>toggle</button>
+				<button>clicks: 1</button>
+				<p>2</p>
+			`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-1/main.svelte b/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-1/main.svelte
new file mode 100644
index 0000000000..08ebaba25a
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-1/main.svelte
@@ -0,0 +1,23 @@
+<script>
+	import { fork } from "svelte";
+
+	let clicks = $state(0);
+	let show = $state(false);
+	const derived = $derived(clicks * 2);
+</script>
+
+<button onclick={() => {
+	fork(() => {
+		clicks += 1;
+		show = true;
+	}).discard();
+}}>fork</button>
+
+<button onclick={() => show = !show}>toggle</button>
+
+<button onclick={() => clicks++}>clicks: {clicks}</button>
+
+{#if show}
+	<p>{derived}</p>
+{/if}
+
diff --git a/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-2/_config.js b/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-2/_config.js
new file mode 100644
index 0000000000..21ab140fc0
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-2/_config.js
@@ -0,0 +1,38 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const [fork, toggle, increment] = target.querySelectorAll('button');
+
+		// derived is first evaluated in block effect, then discarded
+		flushSync(() => fork.click());
+
+		// should not reflect the temporary change to `clicks` inside the fork
+		// or throw "Cannot convert a Symbol value to a string" due to cached UNINITIALIZED
+		flushSync(() => toggle.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>fork</button>
+				<button>toggle</button>
+				<button>clicks: 0</button>
+				<p>0</p>
+			`
+		);
+
+		flushSync(() => increment.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>fork</button>
+				<button>toggle</button>
+				<button>clicks: 1</button>
+				<p>2</p>
+			`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-2/main.svelte b/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-2/main.svelte
new file mode 100644
index 0000000000..08ebaba25a
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-2/main.svelte
@@ -0,0 +1,23 @@
+<script>
+	import { fork } from "svelte";
+
+	let clicks = $state(0);
+	let show = $state(false);
+	const derived = $derived(clicks * 2);
+</script>
+
+<button onclick={() => {
+	fork(() => {
+		clicks += 1;
+		show = true;
+	}).discard();
+}}>fork</button>
+
+<button onclick={() => show = !show}>toggle</button>
+
+<button onclick={() => clicks++}>clicks: {clicks}</button>
+
+{#if show}
+	<p>{derived}</p>
+{/if}
+
diff --git a/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-3/_config.js b/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-3/_config.js
new file mode 100644
index 0000000000..4827aa99f3
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-3/_config.js
@@ -0,0 +1,32 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const [fork, toggle, increment] = target.querySelectorAll('button');
+
+		// initialize derived by showing it
+		flushSync(() => toggle.click());
+		flushSync(() => toggle.click());
+
+		// increment clicks
+		flushSync(() => increment.click());
+
+		// update derived, but without writing to `derived.v`
+		flushSync(() => fork.click());
+
+		// show derived
+		flushSync(() => toggle.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>fork</button>
+				<button>toggle</button>
+				<button>clicks: 1</button>
+				<p>2</p>
+			`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-3/main.svelte b/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-3/main.svelte
new file mode 100644
index 0000000000..08ebaba25a
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/fork-derived-uncached-3/main.svelte
@@ -0,0 +1,23 @@
+<script>
+	import { fork } from "svelte";
+
+	let clicks = $state(0);
+	let show = $state(false);
+	const derived = $derived(clicks * 2);
+</script>
+
+<button onclick={() => {
+	fork(() => {
+		clicks += 1;
+		show = true;
+	}).discard();
+}}>fork</button>
+
+<button onclick={() => show = !show}>toggle</button>
+
+<button onclick={() => clicks++}>clicks: {clicks}</button>
+
+{#if show}
+	<p>{derived}</p>
+{/if}
+