From 90a70cb012733be22858dc4db9a18868159bd9fa Mon Sep 17 00:00:00 2001
From: Simon H <5968653+dummdidumm@users.noreply.github.com>
Date: Thu, 30 Apr 2026 00:18:09 +0200
Subject: [PATCH] fix: flush eager effects in production (#18107)

While working on #18106 I noticed that we're not adding eager effects
inside `mark_reactions` when `DEV` is `false`. As a result production
build could have `$state.eager` or `$state.pending` not working
correctly.

~No test because I can't get Vitest to not run with `DEV` being `true`.~
added a test
---
 .changeset/flat-shrimps-worry.md              |  5 ++++
 .../src/internal/client/reactivity/sources.js | 21 ++++++-----------
 .../samples/async-eager-derived/_config.js    | 23 +++++++++++++++++++
 .../samples/async-eager-derived/main.svelte   | 22 ++++++++++++++++++
 4 files changed, 57 insertions(+), 14 deletions(-)
 create mode 100644 .changeset/flat-shrimps-worry.md
 create mode 100644 packages/svelte/tests/runtime-production/samples/async-eager-derived/_config.js
 create mode 100644 packages/svelte/tests/runtime-production/samples/async-eager-derived/main.svelte

diff --git a/.changeset/flat-shrimps-worry.md b/.changeset/flat-shrimps-worry.md
new file mode 100644
index 0000000000..a5f76a0f9d
--- /dev/null
+++ b/.changeset/flat-shrimps-worry.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: flush eager effects in production
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 1831183e6f..a5dd94badf 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -47,7 +47,7 @@ import { proxy } from '../proxy.js';
 import { execute_derived } from './deriveds.js';
 import { set_signal_status, update_derived_status } from './status.js';
 
-/** @type {Set<any>} */
+/** @type {Set<Effect>} */
 export let eager_effects = new Set();
 
 /** @type {Map<Source, any>} */
@@ -266,12 +266,6 @@ export function flush_eager_effects() {
 	eager_effects_deferred = false;
 
 	for (const effect of eager_effects) {
-		// 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);
 		}
@@ -338,12 +332,6 @@ function mark_reactions(signal, status, updated_during_traversal) {
 		// In legacy mode, skip the current effect to prevent infinite loops
 		if (!runes && reaction === active_effect) continue;
 
-		// Inspect effects need to run immediately, so that the stack trace makes sense
-		if (DEV && (flags & EAGER_EFFECT) !== 0) {
-			eager_effects.add(reaction);
-			continue;
-		}
-
 		var not_dirty = (flags & DIRTY) === 0;
 
 		// don't set a DIRTY reaction to MAYBE_DIRTY
@@ -351,7 +339,12 @@ function mark_reactions(signal, status, updated_during_traversal) {
 			set_signal_status(reaction, status);
 		}
 
-		if ((flags & DERIVED) !== 0) {
+		if ((flags & EAGER_EFFECT) !== 0) {
+			// Eager effects need to run immediately:
+			// - for $inspect so that the stack trace makes sense
+			// - for $state.eager because they might be without an effect parent
+			eager_effects.add(/** @type {Effect} */ (reaction));
+		} else if ((flags & DERIVED) !== 0) {
 			var derived = /** @type {Derived} */ (reaction);
 
 			batch_values?.delete(derived);
diff --git a/packages/svelte/tests/runtime-production/samples/async-eager-derived/_config.js b/packages/svelte/tests/runtime-production/samples/async-eager-derived/_config.js
new file mode 100644
index 0000000000..043f1610fb
--- /dev/null
+++ b/packages/svelte/tests/runtime-production/samples/async-eager-derived/_config.js
@@ -0,0 +1,23 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [increment, shift] = target.querySelectorAll('button');
+
+		increment.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>clicks: 0 - 0 - 0</button> <button>shift</button> <p>true - true</p>`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>clicks: 1 - 1 - 1</button> <button>shift</button> <p>false - false</p>`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-production/samples/async-eager-derived/main.svelte b/packages/svelte/tests/runtime-production/samples/async-eager-derived/main.svelte
new file mode 100644
index 0000000000..d1d979126d
--- /dev/null
+++ b/packages/svelte/tests/runtime-production/samples/async-eager-derived/main.svelte
@@ -0,0 +1,22 @@
+<script>
+	
+	let count = $state(0);
+	const delayedCount = $derived(await push(count));
+	const derivedCount = $derived(count);
+
+	let resolvers = [];
+
+	function push(value) {
+        if (!value) return value;
+		const { promise, resolve } = Promise.withResolvers();
+		resolvers.push(() => resolve(value));
+		return promise;
+	}
+</script>
+
+<button onclick={() => count += 1}>
+	clicks: {count} - {delayedCount} - {derivedCount}
+</button> 
+<button onclick={() => resolvers.shift()?.()}>shift</button>
+
+<p>{$state.eager(count) !== count} - {$state.eager(derivedCount) !== derivedCount}</p>