From 1e899cba354d2b25593afc1bbf04bf3555b5d26a Mon Sep 17 00:00:00 2001
From: Simon H <5968653+dummdidumm@users.noreply.github.com>
Date: Mon, 18 May 2026 17:36:41 +0200
Subject: [PATCH] fix: execute uninitialized derived even if it's destroyed
 (#18228)

Turns out there are a few unavoidable cases where we have to execute the
derived even if we otherwise wouldn't, because of its lazy nature.
Fixes #18139
---
 .changeset/hungry-ads-beam.md                 |  5 +++
 .../internal/client/reactivity/deriveds.js    |  7 +++-
 .../Child.svelte                              | 10 ++++++
 .../_config.js                                | 32 +++++++++++++++++++
 .../main.svelte                               | 11 +++++++
 5 files changed, 64 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/hungry-ads-beam.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/transition-derived-uninitialized/Child.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/transition-derived-uninitialized/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/transition-derived-uninitialized/main.svelte

diff --git a/.changeset/hungry-ads-beam.md b/.changeset/hungry-ads-beam.md
new file mode 100644
index 0000000000..260d177119
--- /dev/null
+++ b/.changeset/hungry-ads-beam.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: execute uninitialized derived even if it's destroyed
diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index 83e23dfab6..75be018552 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -338,7 +338,12 @@ export function execute_derived(derived) {
 	var prev_active_effect = active_effect;
 	var parent = derived.parent;
 
-	if (!is_destroying_effect && parent !== null && (parent.f & (DESTROYED | INERT)) !== 0) {
+	if (
+		!is_destroying_effect &&
+		parent !== null &&
+		derived.v !== UNINITIALIZED && // if it was never evaluated before, it's guaranteed to fail downstream, so we try to execute instead
+		(parent.f & (DESTROYED | INERT)) !== 0
+	) {
 		w.derived_inert();
 
 		return derived.v;
diff --git a/packages/svelte/tests/runtime-runes/samples/transition-derived-uninitialized/Child.svelte b/packages/svelte/tests/runtime-runes/samples/transition-derived-uninitialized/Child.svelte
new file mode 100644
index 0000000000..20bd606d9b
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/transition-derived-uninitialized/Child.svelte
@@ -0,0 +1,10 @@
+<script>
+	import { fly } from 'svelte/transition';
+
+	// Not read before the outro, by which time the derived is destroyed
+	let duration = $derived(10);
+</script>
+
+<div in:fly={{ duration: 10 }} out:fly={{ duration }}>
+	hello
+</div>
diff --git a/packages/svelte/tests/runtime-runes/samples/transition-derived-uninitialized/_config.js b/packages/svelte/tests/runtime-runes/samples/transition-derived-uninitialized/_config.js
new file mode 100644
index 0000000000..734c84a0c4
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/transition-derived-uninitialized/_config.js
@@ -0,0 +1,32 @@
+import { flushSync } from 'svelte';
+import { raf } from '../../../animation-helpers';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [fly_in, fly_out] = target.querySelectorAll('button');
+
+		fly_in.click();
+		flushSync();
+		raf.tick(25);
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>fly in</button>
+				<button>fly out</button>
+				<div style="">hello</div>
+			`
+		);
+
+		fly_out.click();
+		flushSync();
+		raf.tick(50);
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>fly in</button>
+				<button>fly out</button>
+			`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/transition-derived-uninitialized/main.svelte b/packages/svelte/tests/runtime-runes/samples/transition-derived-uninitialized/main.svelte
new file mode 100644
index 0000000000..4fc3395af8
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/transition-derived-uninitialized/main.svelte
@@ -0,0 +1,11 @@
+<script>
+	import Child from './Child.svelte';
+	let show = $state(false);
+</script>
+
+<button onclick={() => (show = true)}>fly in</button>
+<button onclick={() => (show = false)}>fly out</button>
+
+{#if show}
+	<Child />
+{/if}