From aadc97ce1be06e3dad4b98469ac385a07f34fd06 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 25 Aug 2026 12:06:14 -0400
Subject: [PATCH] fix: simplify boundary error handling and handle hydration
 errors correctly (#18539)

remove the `catch` clause in `this.#run` - in two locations there's already a catch, and in the hydration-location we have to add special handling for hydration errors.

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>
---
 .../internal/client/dom/blocks/boundary.js    | 33 ++++++++++++-------
 .../async-error-boundary-5/Child.svelte       |  7 ++++
 .../samples/async-error-boundary-5/_config.js | 14 ++++++++
 .../async-error-boundary-5/main.svelte        | 24 ++++++++++++++
 4 files changed, 66 insertions(+), 12 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-error-boundary-5/Child.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-error-boundary-5/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-error-boundary-5/main.svelte

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index 4f655a3695..fd6bbdf5b4 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -1,14 +1,8 @@
 /** @import { Effect, Source, TemplateNode, } from '#client' */
-import {
-	BOUNDARY_EFFECT,
-	DIRTY,
-	EFFECT_PRESERVED,
-	EFFECT_TRANSPARENT,
-	MAYBE_DIRTY
-} from '#client/constants';
+import { BOUNDARY_EFFECT, EFFECT_PRESERVED, EFFECT_TRANSPARENT } from '#client/constants';
 import { HYDRATION_START_ELSE, HYDRATION_START_FAILED } from '../../../../constants.js';
 import { component_context, set_component_context } from '../../context.js';
-import { handle_error, invoke_error_boundary } from '../../error-handling.js';
+import { invoke_error_boundary } from '../../error-handling.js';
 import {
 	block,
 	branch,
@@ -271,13 +265,31 @@ export class Boundary {
 		queue_micro_task(() => {
 			var fragment = (this.#offscreen_fragment = document.createDocumentFragment());
 			var anchor = create_text();
+			var handled = false;
 
 			fragment.append(anchor);
 
 			this.#main_effect = this.#run(() => {
-				return branch(() => this.#children(anchor));
+				try {
+					return branch(() => this.#children(anchor));
+				} catch (error) {
+					try {
+						this.error(error);
+						handled = true;
+					} catch (error) {
+						invoke_error_boundary(error, this.#effect.parent);
+					}
+
+					return null;
+				}
 			});
 
+			if (this.#main_effect === null) {
+				this.#offscreen_fragment = null;
+				if (handled) this.#resolve(/** @type {Batch} */ (current_batch));
+				return;
+			}
+
 			if (this.#pending_count === 0) {
 				this.#anchor.before(fragment);
 				this.#offscreen_fragment = null;
@@ -362,9 +374,6 @@ export class Boundary {
 		try {
 			Batch.ensure();
 			return fn();
-		} catch (e) {
-			handle_error(e);
-			return null;
 		} finally {
 			set_active_effect(previous_effect);
 			set_active_reaction(previous_reaction);
diff --git a/packages/svelte/tests/runtime-runes/samples/async-error-boundary-5/Child.svelte b/packages/svelte/tests/runtime-runes/samples/async-error-boundary-5/Child.svelte
new file mode 100644
index 0000000000..a49033a8cc
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-error-boundary-5/Child.svelte
@@ -0,0 +1,7 @@
+<script>
+	const { environment } = $props();
+
+	if (environment === 'client') {
+		throw new Error('oops');
+	}
+</script>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-error-boundary-5/_config.js b/packages/svelte/tests/runtime-runes/samples/async-error-boundary-5/_config.js
new file mode 100644
index 0000000000..29e470e46f
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-error-boundary-5/_config.js
@@ -0,0 +1,14 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['hydrate'],
+	server_props: { environment: 'server' },
+	props: { environment: 'client' },
+	ssrHtml: 'loading inner loading nested',
+
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, 'inner failed: oops outer failed: oops');
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-error-boundary-5/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-error-boundary-5/main.svelte
new file mode 100644
index 0000000000..16a54630d0
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-error-boundary-5/main.svelte
@@ -0,0 +1,24 @@
+<script>
+	import Child from './Child.svelte';
+
+	const { environment } = $props();
+</script>
+
+<svelte:boundary>
+	<Child {environment} />
+	{await new Promise(() => {})}
+
+	{#snippet pending()}loading inner{/snippet}
+	{#snippet failed(error)}inner failed: {error.message}{/snippet}
+</svelte:boundary>
+
+<svelte:boundary>
+	<svelte:boundary>
+		<Child {environment} />
+		{await new Promise(() => {})}
+
+		{#snippet pending()}loading nested{/snippet}
+	</svelte:boundary>
+
+	{#snippet failed(error)}outer failed: {error.message}{/snippet}
+</svelte:boundary>