From 585293fc45edb829d1d14986d40f31b5a8e17d4c Mon Sep 17 00:00:00 2001
From: paoloricciuti <ricciutipaolo@gmail.com>
Date: Thu, 16 Oct 2025 12:46:05 +0200
Subject: [PATCH] fix: add hydration markers in `pending` branch of SSR
 boundary

---
 .changeset/happy-numbers-stick.md                      |  5 +++++
 .../3-transform/server/visitors/SvelteBoundary.js      |  2 +-
 .../samples/boundary-pending-attribute/_config.js      | 10 ++++++++++
 .../samples/boundary-pending-attribute/_expected.html  |  1 +
 .../samples/boundary-pending-attribute/main.svelte     |  8 ++++++++
 5 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/happy-numbers-stick.md
 create mode 100644 packages/svelte/tests/hydration/samples/boundary-pending-attribute/_config.js
 create mode 100644 packages/svelte/tests/hydration/samples/boundary-pending-attribute/_expected.html
 create mode 100644 packages/svelte/tests/hydration/samples/boundary-pending-attribute/main.svelte

diff --git a/.changeset/happy-numbers-stick.md b/.changeset/happy-numbers-stick.md
new file mode 100644
index 000000000000..12fbd68ad85d
--- /dev/null
+++ b/.changeset/happy-numbers-stick.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: add hydration markers in `pending` branch of SSR boundary
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/SvelteBoundary.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/SvelteBoundary.js
index 41a18cf52da6..45f1b5aad2b3 100644
--- a/packages/svelte/src/compiler/phases/3-transform/server/visitors/SvelteBoundary.js
+++ b/packages/svelte/src/compiler/phases/3-transform/server/visitors/SvelteBoundary.js
@@ -49,7 +49,7 @@ export function SvelteBoundary(node, context) {
 			context.state.template.push(
 				b.if(
 					callee,
-					b.block([b.stmt(pending)]),
+					b.block(build_template([block_open_else, b.stmt(pending), block_close])),
 					b.block(build_template([block_open, statement, block_close]))
 				)
 			);
diff --git a/packages/svelte/tests/hydration/samples/boundary-pending-attribute/_config.js b/packages/svelte/tests/hydration/samples/boundary-pending-attribute/_config.js
new file mode 100644
index 000000000000..eec39eb84cef
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/boundary-pending-attribute/_config.js
@@ -0,0 +1,10 @@
+import { flushSync } from 'svelte';
+import { assert_ok, test } from '../../test';
+
+export default test({
+	compileOptions: {
+		experimental: {
+			async: true
+		}
+	}
+});
diff --git a/packages/svelte/tests/hydration/samples/boundary-pending-attribute/_expected.html b/packages/svelte/tests/hydration/samples/boundary-pending-attribute/_expected.html
new file mode 100644
index 000000000000..8b44c4cc91b9
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/boundary-pending-attribute/_expected.html
@@ -0,0 +1 @@
+<!--[--><!--[!--><!---->loading...<!--]--><!--]-->
\ No newline at end of file
diff --git a/packages/svelte/tests/hydration/samples/boundary-pending-attribute/main.svelte b/packages/svelte/tests/hydration/samples/boundary-pending-attribute/main.svelte
new file mode 100644
index 000000000000..15612dd27eac
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/boundary-pending-attribute/main.svelte
@@ -0,0 +1,8 @@
+{#snippet pending()}
+	loading...
+{/snippet}
+
+<svelte:boundary {pending}>
+	{@const data = await Promise.resolve("data")}
+	{data}
+</svelte:boundary>
\ No newline at end of file