From 7e6a3bfea8f7023d725288c9ef82cb67d810ffe0 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 9 Sep 2025 22:05:25 -0400
Subject: [PATCH 01/14] fix: flush effects scheduled during boundary's pending
 phase

---
 .changeset/ninety-pandas-move.md               |  5 +++++
 .../src/internal/client/dom/blocks/boundary.js | 10 +++++++++-
 .../samples/async-attachment/Inner.svelte      | 10 ++++++++++
 .../samples/async-attachment/_config.js        | 18 ++++++++++++++++++
 .../samples/async-attachment/main.svelte       | 16 ++++++++++++++++
 5 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/ninety-pandas-move.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-attachment/Inner.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-attachment/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-attachment/main.svelte

diff --git a/.changeset/ninety-pandas-move.md b/.changeset/ninety-pandas-move.md
new file mode 100644
index 0000000000..65f57ddbbf
--- /dev/null
+++ b/.changeset/ninety-pandas-move.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: flush effects scheduled during boundary's pending phase
diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index b7f1803782..659f6014b8 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -23,7 +23,7 @@ import { queue_micro_task } from '../task.js';
 import * as e from '../../errors.js';
 import * as w from '../../warnings.js';
 import { DEV } from 'esm-env';
-import { Batch, effect_pending_updates } from '../../reactivity/batch.js';
+import { Batch, current_batch, effect_pending_updates } from '../../reactivity/batch.js';
 import { internal_set, source } from '../../reactivity/sources.js';
 import { tag } from '../../dev/tracing.js';
 import { createSubscriber } from '../../../../reactivity/create-subscriber.js';
@@ -259,6 +259,14 @@ export class Boundary {
 				this.#anchor.before(this.#offscreen_fragment);
 				this.#offscreen_fragment = null;
 			}
+
+			const batch = current_batch;
+
+			if (batch) {
+				queue_micro_task(() => {
+					batch.flush();
+				});
+			}
 		}
 	}
 
diff --git a/packages/svelte/tests/runtime-runes/samples/async-attachment/Inner.svelte b/packages/svelte/tests/runtime-runes/samples/async-attachment/Inner.svelte
new file mode 100644
index 0000000000..b9b9d7a3d0
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-attachment/Inner.svelte
@@ -0,0 +1,10 @@
+<script>
+	function renderContent(node) {
+		node.textContent = 'foo';
+	}
+ 
+	const test = await Promise.resolve('foo');
+</script>
+
+<p>{test}</p>
+<div {@attach renderContent}></div>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-attachment/_config.js b/packages/svelte/tests/runtime-runes/samples/async-attachment/_config.js
new file mode 100644
index 0000000000..f6b48b38b1
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-attachment/_config.js
@@ -0,0 +1,18 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<button>toggle</button> <p>foo</p><div>foo</div>');
+
+		const [toggle] = target.querySelectorAll('button');
+		toggle.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<button>toggle</button>');
+
+		toggle.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<button>toggle</button> <p>foo</p><div>foo</div>');
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-attachment/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-attachment/main.svelte
new file mode 100644
index 0000000000..6cef6e8f5c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-attachment/main.svelte
@@ -0,0 +1,16 @@
+<script>
+	import Inner from './Inner.svelte';
+
+	let show = $state(true);
+</script>
+
+<svelte:boundary>
+	<button onclick={() => show = !show}>toggle</button>
+	{#if show}
+		<Inner />
+	{/if}
+
+	{#snippet pending()}
+		<p>pending</p>
+	{/snippet}
+</svelte:boundary>

From bcd3acad210c22b8c39d6408e94207ee1754cf21 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 9 Sep 2025 22:22:22 -0400
Subject: [PATCH 02/14] simpler fix

---
 .../svelte/src/internal/client/dom/blocks/boundary.js     | 8 --------
 packages/svelte/src/internal/client/reactivity/batch.js   | 2 +-
 2 files changed, 1 insertion(+), 9 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index 659f6014b8..28f35b0b66 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -259,14 +259,6 @@ export class Boundary {
 				this.#anchor.before(this.#offscreen_fragment);
 				this.#offscreen_fragment = null;
 			}
-
-			const batch = current_batch;
-
-			if (batch) {
-				queue_micro_task(() => {
-					batch.flush();
-				});
-			}
 		}
 	}
 
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 5176a4f74b..555286e53a 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -685,7 +685,7 @@ export function suspend() {
 			batch.activate();
 			batch.decrement();
 		} else {
-			batch.deactivate();
+			batch.flush();
 		}
 
 		unset_context();

From e36137d161e88013f116e5b9e2b141d7d86ca1a7 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Wed, 10 Sep 2025 08:18:54 -0400
Subject: [PATCH 03/14] add async-effect-after-await test

---
 .../samples/async-effect-after-await/Child.svelte        | 7 +++++++
 .../samples/async-effect-after-await/_config.js          | 9 +++++++++
 .../samples/async-effect-after-await/main.svelte         | 9 +++++++++
 3 files changed, 25 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-effect-after-await/main.svelte

diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte
new file mode 100644
index 0000000000..682f7a0631
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte
@@ -0,0 +1,7 @@
+<script>
+	await 1;
+
+	$effect(() => {
+		console.log('hello');
+	});
+</script>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js
new file mode 100644
index 0000000000..81548a25ea
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js
@@ -0,0 +1,9 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, logs }) {
+		await tick();
+		assert.deepEqual(logs, ['hello']);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/main.svelte
new file mode 100644
index 0000000000..d4b67f8803
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/main.svelte
@@ -0,0 +1,9 @@
+<script>
+	import Child from './Child.svelte';
+</script>
+
+<svelte:boundary>
+	<Child />
+
+	{#snippet pending()}{/snippet}
+</svelte:boundary>

From f41dad72226104a417149442323df39126925dfd Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Wed, 10 Sep 2025 12:33:08 -0400
Subject: [PATCH 04/14] failing test

---
 .../samples/async-effect-after-await/Child.svelte           | 6 +++++-
 .../samples/async-effect-after-await/_config.js             | 3 ++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte
index 682f7a0631..758ebc0004 100644
--- a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte
@@ -1,7 +1,11 @@
 <script>
+	$effect(() => {
+		console.log('before');
+	});
+
 	await 1;
 
 	$effect(() => {
-		console.log('hello');
+		console.log('after');
 	});
 </script>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js
index 81548a25ea..0908b6a9fe 100644
--- a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js
@@ -3,7 +3,8 @@ import { test } from '../../test';
 
 export default test({
 	async test({ assert, logs }) {
+		assert.deepEqual(logs, []);
 		await tick();
-		assert.deepEqual(logs, ['hello']);
+		assert.deepEqual(logs, ['before', 'after']);
 	}
 });

From d50b4d955a8633aceeddad5711f65434e02c3a39 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Mon, 22 Sep 2025 14:58:02 -0400
Subject: [PATCH 05/14] add another test

---
 .../async-boundary-coordination/_config.js    | 46 +++++++++++++++++++
 .../async-boundary-coordination/main.svelte   | 35 ++++++++++++++
 2 files changed, 81 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte

diff --git a/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/_config.js b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/_config.js
new file mode 100644
index 0000000000..f3e60e7b32
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/_config.js
@@ -0,0 +1,46 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<p>hello from server</p>
+				<p>hello from server</p>
+				<p>hello from server</p>
+				<p>hello from server</p>
+			`
+		);
+
+		const [button1, button2] = target.querySelectorAll('button');
+
+		button1.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<p>hello from browser</p>
+				<p>hello from browser</p>
+				<p>hello from server</p>
+				<p>hello from server</p>
+			`
+		);
+
+		button2.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<p>hello from browser</p>
+				<p>hello from browser</p>
+				<p>hello from browser</p>
+				<p>hello from browser</p>
+			`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte
new file mode 100644
index 0000000000..3986c45046
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte
@@ -0,0 +1,35 @@
+<script>
+	let browser = typeof window !== 'undefined';
+
+	let a = Promise.withResolvers();
+	let b = Promise.withResolvers();
+
+	if (!browser) {
+		a.resolve(true);
+		b.resolve(true);
+	}
+</script>
+
+<button onclick={() => a.resolve(true)}>
+	resolve a
+</button>
+
+<button onclick={() => b.resolve(true)}>
+	resolve b
+</button>
+
+<svelte:boundary>
+	{#if await a.promise}
+		<p>hello from {browser ? 'browser' : 'server'}</p>
+	{/if}
+
+	<p>hello from {browser ? 'browser' : 'server'}</p>
+</svelte:boundary>
+
+<svelte:boundary>
+	{#if await b.promise}
+		<p>hello from {browser ? 'browser' : 'server'}</p>
+	{/if}
+
+	<p>hello from {browser ? 'browser' : 'server'}</p>
+</svelte:boundary>

From ea29258bb0ad6c492a1c75785b08515155b1ee29 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 13 Feb 2026 17:14:32 -0500
Subject: [PATCH 06/14] fix test

---
 .../async-boundary-coordination/_config.js       | 16 ++++++++++++++++
 .../async-boundary-coordination/main.svelte      |  2 +-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/_config.js b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/_config.js
index f3e60e7b32..15e4ab45ef 100644
--- a/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/_config.js
@@ -2,12 +2,24 @@ import { tick } from 'svelte';
 import { test } from '../../test';
 
 export default test({
+	mode: ['hydrate'],
+
+	props: {
+		browser: true
+	},
+
+	server_props: {
+		browser: false
+	},
+
 	async test({ assert, target }) {
 		await tick();
 
 		assert.htmlEqual(
 			target.innerHTML,
 			`
+				<button>resolve a</button>
+				<button>resolve b</button>
 				<p>hello from server</p>
 				<p>hello from server</p>
 				<p>hello from server</p>
@@ -23,6 +35,8 @@ export default test({
 		assert.htmlEqual(
 			target.innerHTML,
 			`
+				<button>resolve a</button>
+				<button>resolve b</button>
 				<p>hello from browser</p>
 				<p>hello from browser</p>
 				<p>hello from server</p>
@@ -36,6 +50,8 @@ export default test({
 		assert.htmlEqual(
 			target.innerHTML,
 			`
+				<button>resolve a</button>
+				<button>resolve b</button>
 				<p>hello from browser</p>
 				<p>hello from browser</p>
 				<p>hello from browser</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte
index 3986c45046..e837aac030 100644
--- a/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte
@@ -1,5 +1,5 @@
 <script>
-	let browser = typeof window !== 'undefined';
+	let { browser } = $props();
 
 	let a = Promise.withResolvers();
 	let b = Promise.withResolvers();

From 82d9880f6132543a8fb3e02a10ed57dfe4f371ea Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 13 Feb 2026 18:08:50 -0500
Subject: [PATCH 07/14] WIP

---
 .../internal/client/dom/blocks/boundary.js    | 42 ++++++++++++++++++-
 .../src/internal/client/reactivity/effects.js | 22 +++++++++-
 2 files changed, 61 insertions(+), 3 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index ef5f0e116d..ff125aa93a 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -37,7 +37,7 @@ import { queue_micro_task } from '../task.js';
 import * as e from '../../errors.js';
 import * as w from '../../warnings.js';
 import { DEV } from 'esm-env';
-import { Batch, schedule_effect } from '../../reactivity/batch.js';
+import { Batch, flushSync, schedule_effect } from '../../reactivity/batch.js';
 import { internal_set, source } from '../../reactivity/sources.js';
 import { tag } from '../../dev/tracing.js';
 import { createSubscriber } from '../../../../reactivity/create-subscriber.js';
@@ -164,7 +164,9 @@ export class Boundary {
 				} else {
 					this.#hydrate_resolved_content();
 
-					if (this.#pending_count === 0) {
+					// Match the non-hydrating logic: only stay pending if there's
+					// actual pending async work
+					if (this.#local_pending_count === 0) {
 						this.is_pending = false;
 					}
 				}
@@ -261,6 +263,15 @@ export class Boundary {
 		return !!this.#props.pending;
 	}
 
+	/**
+	 * Returns true if there's pending async work in this boundary or any ancestor boundary
+	 * @returns {boolean}
+	 */
+	has_pending_async() {
+		if (this.#local_pending_count > 0) return true;
+		return this.parent ? this.parent.has_pending_async() : false;
+	}
+
 	/**
 	 * @param {() => Effect | null} fn
 	 */
@@ -360,6 +371,33 @@ export class Boundary {
 
 		this.#local_pending_count += d;
 
+		if (d === -1 && this.#local_pending_count === 0) {
+			// async work completed — if we don't have a pending snippet,
+			// we need to reschedule deferred effects here
+			if (!this.has_pending_snippet()) {
+				this.is_pending = false;
+
+				// Ensure there's a batch to process the rescheduled effects
+				Batch.ensure();
+
+				for (const e of this.#dirty_effects) {
+					set_signal_status(e, DIRTY);
+					schedule_effect(e);
+				}
+
+				for (const e of this.#maybe_dirty_effects) {
+					set_signal_status(e, MAYBE_DIRTY);
+					schedule_effect(e);
+				}
+
+				this.#dirty_effects.clear();
+				this.#maybe_dirty_effects.clear();
+
+				// Force flush the scheduled effects
+				flushSync();
+			}
+		}
+
 		if (!this.#effect_pending || this.#pending_count_update_queued) return;
 		this.#pending_count_update_queued = true;
 
diff --git a/packages/svelte/src/internal/client/reactivity/effects.js b/packages/svelte/src/internal/client/reactivity/effects.js
index 512c435a27..e22f80878a 100644
--- a/packages/svelte/src/internal/client/reactivity/effects.js
+++ b/packages/svelte/src/internal/client/reactivity/effects.js
@@ -41,6 +41,8 @@ import { define_property } from '../../shared/utils.js';
 import { get_next_sibling } from '../dom/operations.js';
 import { component_context, dev_current_component_function, dev_stack } from '../context.js';
 import { Batch, current_batch, schedule_effect } from './batch.js';
+import { hydrating } from '../dom/hydration.js';
+import { async_mode_flag } from '../../flags/index.js';
 import { flatten } from './async.js';
 import { without_reactive_context } from '../dom/elements/bindings/shared.js';
 import { set_signal_status } from './status.js';
@@ -119,7 +121,23 @@ function create_effect(type, fn, sync) {
 		effect.component_function = dev_current_component_function;
 	}
 
-	if (sync) {
+	// During hydration, if we're inside a pending boundary with actual async work,
+	// defer render effects instead of running them immediately to avoid hydration mismatches.
+	// We only defer RENDER_EFFECT (and similar), not ASYNC effects which need to run
+	// to set up the async machinery. Also, only defer effects with an actual fn.
+	// Note: We use has_pending_async() instead of is_pending because is_pending can be
+	// true just because a pending snippet exists, even without actual async work.
+	var should_defer =
+		async_mode_flag &&
+		hydrating &&
+		fn !== null &&
+		(type & RENDER_EFFECT) !== 0 &&
+		effect.b?.has_pending_async();
+
+	if (should_defer) {
+		// Store the effect in the boundary so it can be rescheduled when async work completes
+		/** @type {import('../dom/blocks/boundary.js').Boundary} */ (effect.b).defer_effect(effect);
+	} else if (sync) {
 		try {
 			update_effect(effect);
 		} catch (e) {
@@ -136,8 +154,10 @@ function create_effect(type, fn, sync) {
 	// if an effect has already ran and doesn't need to be kept in the tree
 	// (because it won't re-run, has no DOM, and has no teardown etc)
 	// then we skip it and go to its child (if any)
+	// NOTE: We only do this pruning if the effect actually ran (!should_defer)
 	if (
 		sync &&
+		!should_defer &&
 		e.deps === null &&
 		e.teardown === null &&
 		e.nodes === null &&

From a51b6639d2f1286412169fc9f27fe8fdac5154ed Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 13 Feb 2026 18:15:39 -0500
Subject: [PATCH 08/14] WIP

---
 packages/svelte/src/internal/client/dom/blocks/boundary.js | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index ff125aa93a..f5acd3efe3 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -264,12 +264,11 @@ export class Boundary {
 	}
 
 	/**
-	 * Returns true if there's pending async work in this boundary or any ancestor boundary
+	 * Returns true if there's pending async work in this boundary
 	 * @returns {boolean}
 	 */
 	has_pending_async() {
-		if (this.#local_pending_count > 0) return true;
-		return this.parent ? this.parent.has_pending_async() : false;
+		return this.#local_pending_count > 0;
 	}
 
 	/**

From 706ce6d2dcb73f9b7c32e2a8f0f21eee817e2b8c Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 13 Feb 2026 18:29:46 -0500
Subject: [PATCH 09/14] DRY out

---
 .../internal/client/dom/blocks/boundary.js    | 57 ++++++++-----------
 1 file changed, 24 insertions(+), 33 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index f5acd3efe3..82ff3daefe 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -309,6 +309,27 @@ export class Boundary {
 		}
 	}
 
+	#reschedule_deferred_effects() {
+		this.is_pending = false;
+
+		// any effects that were encountered and deferred during traversal
+		// should be rescheduled — after the next traversal (which will happen
+		// immediately, due to the same update that brought us here)
+		// the effects will be flushed
+		for (const e of this.#dirty_effects) {
+			set_signal_status(e, DIRTY);
+			schedule_effect(e);
+		}
+
+		for (const e of this.#maybe_dirty_effects) {
+			set_signal_status(e, MAYBE_DIRTY);
+			schedule_effect(e);
+		}
+
+		this.#dirty_effects.clear();
+		this.#maybe_dirty_effects.clear();
+	}
+
 	/**
 	 * Updates the pending count associated with the currently visible pending snippet,
 	 * if any, such that we can replace the snippet with content once work is done
@@ -327,24 +348,7 @@ export class Boundary {
 		this.#pending_count += d;
 
 		if (this.#pending_count === 0) {
-			this.is_pending = false;
-
-			// any effects that were encountered and deferred during traversal
-			// should be rescheduled — after the next traversal (which will happen
-			// immediately, due to the same update that brought us here)
-			// the effects will be flushed
-			for (const e of this.#dirty_effects) {
-				set_signal_status(e, DIRTY);
-				schedule_effect(e);
-			}
-
-			for (const e of this.#maybe_dirty_effects) {
-				set_signal_status(e, MAYBE_DIRTY);
-				schedule_effect(e);
-			}
-
-			this.#dirty_effects.clear();
-			this.#maybe_dirty_effects.clear();
+			this.#reschedule_deferred_effects();
 
 			if (this.#pending_effect) {
 				pause_effect(this.#pending_effect, () => {
@@ -370,27 +374,14 @@ export class Boundary {
 
 		this.#local_pending_count += d;
 
-		if (d === -1 && this.#local_pending_count === 0) {
+		if (this.#local_pending_count === 0) {
 			// async work completed — if we don't have a pending snippet,
 			// we need to reschedule deferred effects here
 			if (!this.has_pending_snippet()) {
-				this.is_pending = false;
-
 				// Ensure there's a batch to process the rescheduled effects
 				Batch.ensure();
 
-				for (const e of this.#dirty_effects) {
-					set_signal_status(e, DIRTY);
-					schedule_effect(e);
-				}
-
-				for (const e of this.#maybe_dirty_effects) {
-					set_signal_status(e, MAYBE_DIRTY);
-					schedule_effect(e);
-				}
-
-				this.#dirty_effects.clear();
-				this.#maybe_dirty_effects.clear();
+				this.#reschedule_deferred_effects();
 
 				// Force flush the scheduled effects
 				flushSync();

From b4823b0e63b31ab4c86ffe0b6c27ba0b8ccabbaa Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 13 Feb 2026 18:31:58 -0500
Subject: [PATCH 10/14] simplify

---
 .../src/internal/client/dom/blocks/boundary.js   | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index 82ff3daefe..0f3c0057f9 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -374,18 +374,10 @@ export class Boundary {
 
 		this.#local_pending_count += d;
 
-		if (this.#local_pending_count === 0) {
-			// async work completed — if we don't have a pending snippet,
-			// we need to reschedule deferred effects here
-			if (!this.has_pending_snippet()) {
-				// Ensure there's a batch to process the rescheduled effects
-				Batch.ensure();
-
-				this.#reschedule_deferred_effects();
-
-				// Force flush the scheduled effects
-				flushSync();
-			}
+		// async work completed — if we don't have a pending snippet,
+		// we need to reschedule deferred effects here
+		if (this.#local_pending_count === 0 && !this.has_pending_snippet()) {
+			this.#reschedule_deferred_effects();
 		}
 
 		if (!this.#effect_pending || this.#pending_count_update_queued) return;

From c86398e627bc304616f2cef893f99d6a741a70e4 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Mon, 16 Feb 2026 08:31:44 -0500
Subject: [PATCH 11/14] tidy up

---
 .../svelte/src/internal/client/reactivity/effects.js   | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/effects.js b/packages/svelte/src/internal/client/reactivity/effects.js
index e22f80878a..3893c8391e 100644
--- a/packages/svelte/src/internal/client/reactivity/effects.js
+++ b/packages/svelte/src/internal/client/reactivity/effects.js
@@ -1,3 +1,4 @@
+/** @import { Boundary } from '../dom/blocks/boundary' */
 /** @import { Blocker, ComponentContext, ComponentContextLegacy, Derived, Effect, TemplateNode, TransitionManager } from '#client' */
 import {
 	is_dirty,
@@ -121,12 +122,7 @@ function create_effect(type, fn, sync) {
 		effect.component_function = dev_current_component_function;
 	}
 
-	// During hydration, if we're inside a pending boundary with actual async work,
-	// defer render effects instead of running them immediately to avoid hydration mismatches.
-	// We only defer RENDER_EFFECT (and similar), not ASYNC effects which need to run
-	// to set up the async machinery. Also, only defer effects with an actual fn.
-	// Note: We use has_pending_async() instead of is_pending because is_pending can be
-	// true just because a pending snippet exists, even without actual async work.
+	// During hydration, defer template effects until local promises have resolved
 	var should_defer =
 		async_mode_flag &&
 		hydrating &&
@@ -136,7 +132,7 @@ function create_effect(type, fn, sync) {
 
 	if (should_defer) {
 		// Store the effect in the boundary so it can be rescheduled when async work completes
-		/** @type {import('../dom/blocks/boundary.js').Boundary} */ (effect.b).defer_effect(effect);
+		/** @type {Boundary} */ (effect.b).defer_effect(effect);
 	} else if (sync) {
 		try {
 			update_effect(effect);

From 94a230e4c49015523fa9622bda1c342c075c3a39 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Mon, 16 Feb 2026 10:39:27 -0500
Subject: [PATCH 12/14] beef up test. current implementation does not pass

---
 .../samples/async-boundary-coordination/main.svelte  | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte
index e837aac030..4dc515ca7a 100644
--- a/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte
@@ -19,17 +19,21 @@
 </button>
 
 <svelte:boundary>
-	{#if await a.promise}
+	{#if true}
 		<p>hello from {browser ? 'browser' : 'server'}</p>
 	{/if}
 
-	<p>hello from {browser ? 'browser' : 'server'}</p>
+	{#if await a.promise}
+		<p>hello from {browser ? 'browser' : 'server'}</p>
+	{/if}
 </svelte:boundary>
 
 <svelte:boundary>
-	{#if await b.promise}
+	{#if true}
 		<p>hello from {browser ? 'browser' : 'server'}</p>
 	{/if}
 
-	<p>hello from {browser ? 'browser' : 'server'}</p>
+	{#if await b.promise}
+		<p>hello from {browser ? 'browser' : 'server'}</p>
+	{/if}
 </svelte:boundary>

From 439ef8bb5754e09dd8181c35e0f8b974a0435b71 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Mon, 16 Feb 2026 11:00:08 -0500
Subject: [PATCH 13/14] another failing test

---
 .../async-boundary-pending-live/_config.js    | 38 +++++++++++++++++++
 .../async-boundary-pending-live/main.svelte   | 31 +++++++++++++++
 2 files changed, 69 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-boundary-pending-live/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-boundary-pending-live/main.svelte

diff --git a/packages/svelte/tests/runtime-runes/samples/async-boundary-pending-live/_config.js b/packages/svelte/tests/runtime-runes/samples/async-boundary-pending-live/_config.js
new file mode 100644
index 0000000000..1d860cded5
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-boundary-pending-live/_config.js
@@ -0,0 +1,38 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `
+		<button>increment</button>
+		<button>shift</button>
+		<p>0</p>
+	`,
+
+	async test({ assert, target }) {
+		const [increment, shift] = target.querySelectorAll('button');
+
+		increment.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>increment</button>
+				<button>shift</button>
+				<p>1</p>
+			`
+		);
+
+		shift.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>increment</button>
+				<button>shift</button>
+				<p>resolved</p>
+			`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-boundary-pending-live/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-boundary-pending-live/main.svelte
new file mode 100644
index 0000000000..764fd20a51
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-boundary-pending-live/main.svelte
@@ -0,0 +1,31 @@
+<script>
+	let resolvers = [];
+
+	function push(value) {
+		const deferred = Promise.withResolvers();
+		resolvers.push(() => deferred.resolve(value));
+		return deferred.promise;
+	}
+
+	function shift() {
+		resolvers.shift()?.();
+	}
+
+	let count = $state(0);
+</script>
+
+<button onclick={() => count += 1}>
+	increment
+</button>
+
+<button onclick={shift}>
+	shift
+</button>
+
+<svelte:boundary>
+	<p>{await push('resolved')}</p>
+
+	{#snippet pending()}
+		<p>{count}</p>
+	{/snippet}
+</svelte:boundary>

From a3d46fc3627f9d5caa96e21df3751adc533c0bf1 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Mon, 16 Feb 2026 11:16:28 -0500
Subject: [PATCH 14/14] i take it back, opencode didn't get it to work at all.
 revert revert revert

---
 .../internal/client/dom/blocks/boundary.js    | 20 ++-----------------
 .../src/internal/client/reactivity/effects.js | 18 +----------------
 2 files changed, 3 insertions(+), 35 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index 0f3c0057f9..44bd0f22f4 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -37,7 +37,7 @@ import { queue_micro_task } from '../task.js';
 import * as e from '../../errors.js';
 import * as w from '../../warnings.js';
 import { DEV } from 'esm-env';
-import { Batch, flushSync, schedule_effect } from '../../reactivity/batch.js';
+import { Batch, schedule_effect } from '../../reactivity/batch.js';
 import { internal_set, source } from '../../reactivity/sources.js';
 import { tag } from '../../dev/tracing.js';
 import { createSubscriber } from '../../../../reactivity/create-subscriber.js';
@@ -164,9 +164,7 @@ export class Boundary {
 				} else {
 					this.#hydrate_resolved_content();
 
-					// Match the non-hydrating logic: only stay pending if there's
-					// actual pending async work
-					if (this.#local_pending_count === 0) {
+					if (this.#pending_count === 0) {
 						this.is_pending = false;
 					}
 				}
@@ -263,14 +261,6 @@ export class Boundary {
 		return !!this.#props.pending;
 	}
 
-	/**
-	 * Returns true if there's pending async work in this boundary
-	 * @returns {boolean}
-	 */
-	has_pending_async() {
-		return this.#local_pending_count > 0;
-	}
-
 	/**
 	 * @param {() => Effect | null} fn
 	 */
@@ -374,12 +364,6 @@ export class Boundary {
 
 		this.#local_pending_count += d;
 
-		// async work completed — if we don't have a pending snippet,
-		// we need to reschedule deferred effects here
-		if (this.#local_pending_count === 0 && !this.has_pending_snippet()) {
-			this.#reschedule_deferred_effects();
-		}
-
 		if (!this.#effect_pending || this.#pending_count_update_queued) return;
 		this.#pending_count_update_queued = true;
 
diff --git a/packages/svelte/src/internal/client/reactivity/effects.js b/packages/svelte/src/internal/client/reactivity/effects.js
index 3893c8391e..512c435a27 100644
--- a/packages/svelte/src/internal/client/reactivity/effects.js
+++ b/packages/svelte/src/internal/client/reactivity/effects.js
@@ -1,4 +1,3 @@
-/** @import { Boundary } from '../dom/blocks/boundary' */
 /** @import { Blocker, ComponentContext, ComponentContextLegacy, Derived, Effect, TemplateNode, TransitionManager } from '#client' */
 import {
 	is_dirty,
@@ -42,8 +41,6 @@ import { define_property } from '../../shared/utils.js';
 import { get_next_sibling } from '../dom/operations.js';
 import { component_context, dev_current_component_function, dev_stack } from '../context.js';
 import { Batch, current_batch, schedule_effect } from './batch.js';
-import { hydrating } from '../dom/hydration.js';
-import { async_mode_flag } from '../../flags/index.js';
 import { flatten } from './async.js';
 import { without_reactive_context } from '../dom/elements/bindings/shared.js';
 import { set_signal_status } from './status.js';
@@ -122,18 +119,7 @@ function create_effect(type, fn, sync) {
 		effect.component_function = dev_current_component_function;
 	}
 
-	// During hydration, defer template effects until local promises have resolved
-	var should_defer =
-		async_mode_flag &&
-		hydrating &&
-		fn !== null &&
-		(type & RENDER_EFFECT) !== 0 &&
-		effect.b?.has_pending_async();
-
-	if (should_defer) {
-		// Store the effect in the boundary so it can be rescheduled when async work completes
-		/** @type {Boundary} */ (effect.b).defer_effect(effect);
-	} else if (sync) {
+	if (sync) {
 		try {
 			update_effect(effect);
 		} catch (e) {
@@ -150,10 +136,8 @@ function create_effect(type, fn, sync) {
 	// if an effect has already ran and doesn't need to be kept in the tree
 	// (because it won't re-run, has no DOM, and has no teardown etc)
 	// then we skip it and go to its child (if any)
-	// NOTE: We only do this pruning if the effect actually ran (!should_defer)
 	if (
 		sync &&
-		!should_defer &&
 		e.deps === null &&
 		e.teardown === null &&
 		e.nodes === null &&