From c6af159ec27cc1920d709cf5eba42f03f242f8f6 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Fri, 27 Jun 2025 00:10:04 +0200
Subject: [PATCH 1/2] fix: ensure unowned deriveds can add themselves as
 reactions while connected

Fixes #15829 and potentially #15853
---
 .changeset/little-kings-smoke.md              |  5 +++
 .../svelte/src/internal/client/runtime.js     |  7 ++-
 packages/svelte/tests/signals/test.ts         | 44 +++++++++++++++++++
 3 files changed, 55 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/little-kings-smoke.md

diff --git a/.changeset/little-kings-smoke.md b/.changeset/little-kings-smoke.md
new file mode 100644
index 000000000000..1e7f82352a7e
--- /dev/null
+++ b/.changeset/little-kings-smoke.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure unowned deriveds can add themselves as reactions while connected
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 56bc157f33cf..d057bfdf0d9d 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -294,7 +294,12 @@ export function update_reaction(reaction) {
 				reaction.deps = deps = new_deps;
 			}
 
-			if (!skip_reaction) {
+			if (
+				!skip_reaction ||
+				// Deriveds that already have reactions can cleanup, so we still add them as reactions
+				((flags & DERIVED) !== 0 &&
+					/** @type {import('#client').Derived} */ (reaction).reactions !== null)
+			) {
 				for (i = skipped_deps; i < deps.length; i++) {
 					(deps[i].reactions ??= []).push(reaction);
 				}
diff --git a/packages/svelte/tests/signals/test.ts b/packages/svelte/tests/signals/test.ts
index 78d7919e0fb1..6c75936e064c 100644
--- a/packages/svelte/tests/signals/test.ts
+++ b/packages/svelte/tests/signals/test.ts
@@ -112,6 +112,50 @@ describe('signals', () => {
 		};
 	});
 
+	test('unowned deriveds are not added as reactions but trigger effects', () => {
+		var obj = state(undefined);
+
+		class C1 {
+			#v = state(0);
+			get v() {
+				return $.get(this.#v);
+			}
+			set v(v: number) {
+				set(this.#v, v);
+			}
+		}
+
+		return () => {
+			console.log('1');
+			let d = derived(() => $.get(obj)?.v || '-');
+
+			const log: number[] = [];
+			assert.equal($.get(d), '-');
+			console.log('2');
+
+			let destroy = effect_root(() => {
+				render_effect(() => {
+					log.push($.get(d));
+				});
+			});
+
+			console.log('3');
+			set(obj, new C1());
+			flushSync();
+			console.log('4');
+			assert.equal($.get(d), '-');
+			$.get(obj)!.v = 1;
+			flushSync();
+			console.log('5');
+			assert.equal($.get(d), 1);
+			assert.deepEqual(log, ['-', 1]);
+			destroy();
+			// ensure we're not leaking reactions
+			assert.equal(obj.reactions, null);
+			assert.equal(d.reactions, null);
+		};
+	});
+
 	test('derived from state', () => {
 		const log: number[] = [];
 

From 065690bf0fa464e21327a544b65cacd070726ade Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Fri, 27 Jun 2025 00:16:40 +0200
Subject: [PATCH 2/2] lint

---
 packages/svelte/tests/signals/test.ts | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/packages/svelte/tests/signals/test.ts b/packages/svelte/tests/signals/test.ts
index 6c75936e064c..719c936df002 100644
--- a/packages/svelte/tests/signals/test.ts
+++ b/packages/svelte/tests/signals/test.ts
@@ -113,7 +113,7 @@ describe('signals', () => {
 	});
 
 	test('unowned deriveds are not added as reactions but trigger effects', () => {
-		var obj = state(undefined);
+		var obj = state<any>(undefined);
 
 		class C1 {
 			#v = state(0);
@@ -126,12 +126,10 @@ describe('signals', () => {
 		}
 
 		return () => {
-			console.log('1');
 			let d = derived(() => $.get(obj)?.v || '-');
 
 			const log: number[] = [];
 			assert.equal($.get(d), '-');
-			console.log('2');
 
 			let destroy = effect_root(() => {
 				render_effect(() => {
@@ -139,14 +137,11 @@ describe('signals', () => {
 				});
 			});
 
-			console.log('3');
 			set(obj, new C1());
 			flushSync();
-			console.log('4');
 			assert.equal($.get(d), '-');
-			$.get(obj)!.v = 1;
+			$.get(obj).v = 1;
 			flushSync();
-			console.log('5');
 			assert.equal($.get(d), 1);
 			assert.deepEqual(log, ['-', 1]);
 			destroy();