From 794b8f3dfd14fad5d2d67dea7607bae8185cf800 Mon Sep 17 00:00:00 2001
From: Simon H <5968653+dummdidumm@users.noreply.github.com>
Date: Tue, 2 Dec 2025 23:27:04 +0100
Subject: [PATCH] fix: keep reactions up to date even when read outside of
 effect (#17295)

In #17105 one line in `update_reaction` was changed that can cause reactivity loss. It checks if the reaction is updated inside of an effect and only then will push to the reactions. The prior version had an additional check to still add to the reactions if there is already at least one reaction on the derived, indicating it is connected. Removing this check fixes #17263 while keeping correctness: a connected derived by definition at least has one reaction and therefore can properly cleanup.
---
 .changeset/small-llamas-drum.md               |  5 ++++
 .../svelte/src/internal/client/runtime.js     |  2 +-
 packages/svelte/tests/signals/test.ts         | 25 +++++++++++++++++++
 3 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/small-llamas-drum.md

diff --git a/.changeset/small-llamas-drum.md b/.changeset/small-llamas-drum.md
new file mode 100644
index 0000000000..3848f83920
--- /dev/null
+++ b/.changeset/small-llamas-drum.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: keep reactions up to date even when read outside of effect
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 100804a974..64c8409b8f 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -278,7 +278,7 @@ export function update_reaction(reaction) {
 				reaction.deps = deps = new_deps;
 			}
 
-			if (is_updating_effect && effect_tracking() && (reaction.f & CONNECTED) !== 0) {
+			if (effect_tracking() && (reaction.f & CONNECTED) !== 0) {
 				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 13430609a8..23c4bb42f9 100644
--- a/packages/svelte/tests/signals/test.ts
+++ b/packages/svelte/tests/signals/test.ts
@@ -1418,6 +1418,31 @@ describe('signals', () => {
 		};
 	});
 
+	test('derived when connected should add new dependency to its reaction even when read outside effect', () => {
+		let count_a = state(0);
+		let count_b = state(0);
+		let which = state(true);
+		let double = derived(() => ($.get(which) ? $.get(count_a) * 2 : $.get(count_b) * 2));
+
+		render_effect(() => {
+			$.get(double);
+		});
+
+		return () => {
+			flushSync();
+			assert.equal($.get(double!), 0);
+
+			set(which, false);
+			$.get(double); // read before render effect has a chance to rerun
+			flushSync();
+			assert.equal($.get(double!), 0);
+
+			set(count_b, 1);
+			flushSync();
+			assert.equal($.get(double!), 2);
+		};
+	});
+
 	test('$effect.root inside deriveds stay alive independently', () => {
 		const log: any[] = [];
 		const c = state(0);