From 9e695442380a7c3bec5bf90d6c7341c3e1ae9bac Mon Sep 17 00:00:00 2001
From: JY Wey <34165386+JaiWey@users.noreply.github.com>
Date: Fri, 4 Sep 2026 02:06:28 +1200
Subject: [PATCH] fix: in non-async mode, only push variable to current_sources
 when active_reaction is updating (#18550)

fix: #16814

in non-async mode, when the derived inside `SvelteDate` is created and
evaluated, the `#reaction` is from the `active_reaction` when the
`SvelteDate` instance is created, which could possible in NON_UPDATING
state, so the derived should not be added to `current_sources` to
prevent re-run.

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>
---
 .changeset/tricky-cooks-help.md               |  5 +++
 .../svelte/src/internal/client/runtime.js     |  6 ++-
 packages/svelte/tests/signals/test.ts         | 39 +++++++++++++++++++
 3 files changed, 49 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/tricky-cooks-help.md

diff --git a/.changeset/tricky-cooks-help.md b/.changeset/tricky-cooks-help.md
new file mode 100644
index 0000000000..f61c7944a1
--- /dev/null
+++ b/.changeset/tricky-cooks-help.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: in non-async mode, only push variable to current_sources when active_reaction is updating
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index a881403f58..d86c349445 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -100,7 +100,11 @@ export let current_sources = null;
 
 /** @param {Value} value */
 export function push_reaction_value(value) {
-	if (active_reaction !== null && (!async_mode_flag || (active_reaction.f & DERIVED) !== 0)) {
+	if (
+		active_reaction !== null &&
+		((!async_mode_flag && (active_reaction.f & REACTION_IS_UPDATING) !== 0) ||
+			(active_reaction.f & DERIVED) !== 0)
+	) {
 		(current_sources ??= new Set()).add(value);
 	}
 }
diff --git a/packages/svelte/tests/signals/test.ts b/packages/svelte/tests/signals/test.ts
index c1ebc3ec75..285dc1b3ce 100644
--- a/packages/svelte/tests/signals/test.ts
+++ b/packages/svelte/tests/signals/test.ts
@@ -1591,4 +1591,43 @@ describe('signals', () => {
 			assert.equal(s.reactions, null);
 		};
 	});
+
+	// https://github.com/sveltejs/svelte/issues/16814
+	it('does not treat values created for inactive reactions as current', () => {
+		push({}, true);
+
+		const trigger = state(false);
+		const value = state(0);
+		const log: number[] = [];
+		let inactive_reaction: Effect;
+
+		const destroy = effect_root(() => {
+			render_effect(() => {
+				$.get(trigger);
+				inactive_reaction = $.active_reaction as Effect;
+			});
+
+			render_effect(() => {
+				$.get(trigger);
+
+				const reaction = $.active_reaction;
+				$.set_active_reaction(inactive_reaction);
+				$.push_reaction_value(value);
+				$.set_active_reaction(reaction);
+
+				log.push($.get(value));
+			});
+		});
+
+		try {
+			flushSync();
+			flushSync(() => set(trigger, true));
+			flushSync(() => set(value, 1));
+
+			assert.deepEqual(log, [0, 0, 1]);
+		} finally {
+			destroy();
+			pop();
+		}
+	});
 });