From bc6921107e2fad08e19c90032dc29fb91c8412ca Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Tue, 11 Nov 2025 16:04:29 +0100
Subject: [PATCH] fix: ensure eager effects don't break reactions chain

Execution of eager effects didn't set `is_updating_effect`, which meant the logic in `get` would wrongfully prevent dependencies being added to `reactions` of sources/deriveds.

Fixes #17133
---
 .changeset/silent-teeth-invent.md             |  5 +++
 .../src/internal/client/reactivity/sources.js | 26 +++++++++-----
 .../samples/inspect-derived-4/_config.js      | 35 +++++++++++++++++++
 .../samples/inspect-derived-4/main.svelte     | 14 ++++++++
 4 files changed, 71 insertions(+), 9 deletions(-)
 create mode 100644 .changeset/silent-teeth-invent.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/inspect-derived-4/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/inspect-derived-4/main.svelte

diff --git a/.changeset/silent-teeth-invent.md b/.changeset/silent-teeth-invent.md
new file mode 100644
index 000000000000..54603cc27e35
--- /dev/null
+++ b/.changeset/silent-teeth-invent.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure eager effects don't break reactions chain
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 8ae406b57b30..052ca97dc071 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -14,7 +14,9 @@ import {
 	is_dirty,
 	untracking,
 	is_destroying_effect,
-	push_reaction_value
+	push_reaction_value,
+	set_is_updating_effect,
+	is_updating_effect
 } from '../runtime.js';
 import { equals, safe_equals } from './equality.js';
 import {
@@ -246,19 +248,25 @@ export function internal_set(source, value) {
 
 export function flush_eager_effects() {
 	eager_effects_deferred = false;
+	var prev_is_updating_effect = is_updating_effect;
+	set_is_updating_effect(true);
 
 	const inspects = Array.from(eager_effects);
 
-	for (const effect of inspects) {
-		// Mark clean inspect-effects as maybe dirty and then check their dirtiness
-		// instead of just updating the effects - this way we avoid overfiring.
-		if ((effect.f & CLEAN) !== 0) {
-			set_signal_status(effect, MAYBE_DIRTY);
-		}
+	try {
+		for (const effect of inspects) {
+			// Mark clean inspect-effects as maybe dirty and then check their dirtiness
+			// instead of just updating the effects - this way we avoid overfiring.
+			if ((effect.f & CLEAN) !== 0) {
+				set_signal_status(effect, MAYBE_DIRTY);
+			}
 
-		if (is_dirty(effect)) {
-			update_effect(effect);
+			if (is_dirty(effect)) {
+				update_effect(effect);
+			}
 		}
+	} finally {
+		set_is_updating_effect(prev_is_updating_effect);
 	}
 
 	eager_effects.clear();
diff --git a/packages/svelte/tests/runtime-runes/samples/inspect-derived-4/_config.js b/packages/svelte/tests/runtime-runes/samples/inspect-derived-4/_config.js
new file mode 100644
index 000000000000..3a3bca7221a3
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/inspect-derived-4/_config.js
@@ -0,0 +1,35 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+import { normalise_inspect_logs } from '../../../helpers';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+
+	async test({ assert, target, logs }) {
+		const [b] = target.querySelectorAll('button');
+
+		b.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<button>first unseen: 1</button>`);
+
+		b.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<button>first unseen: 2</button>`);
+
+		b.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<button>first unseen:</button>`);
+
+		assert.deepEqual(normalise_inspect_logs(logs), [
+			[0, 1, 2],
+			[1, 2],
+			'at SvelteSet.add',
+			[2],
+			'at SvelteSet.add',
+			[],
+			'at SvelteSet.add'
+		]);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/inspect-derived-4/main.svelte b/packages/svelte/tests/runtime-runes/samples/inspect-derived-4/main.svelte
new file mode 100644
index 000000000000..eb4ea891dbeb
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/inspect-derived-4/main.svelte
@@ -0,0 +1,14 @@
+<script>
+	import {SvelteSet} from "svelte/reactivity";
+	const ids = [0,1,2];
+	const seenIds = new SvelteSet();
+
+	const unseenIds = $derived(ids.filter((id) => !seenIds.has(id)));
+	
+	const currentId = $derived(unseenIds.at(0));
+	$inspect(unseenIds)
+</script>
+
+<button onclick={() => seenIds.add(currentId)}>
+	first unseen: {currentId}
+</button>