From 499973454af1081e19d7eaae87658f3b76d3d091 Mon Sep 17 00:00:00 2001
From: ComputerGuy <63362464+Ocean-OS@users.noreply.github.com>
Date: Wed, 1 Oct 2025 09:03:50 -0700
Subject: [PATCH] fix: check boundary `pending` attribute at runtime on server
 (#16855)

* fix: check boundary `pending` attribute at runtime on server

* fix

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>
---
 .changeset/lazy-cooks-return.md               |  5 ++
 .../server/visitors/SvelteBoundary.js         | 50 ++++++++++++++-----
 packages/svelte/src/compiler/phases/scope.js  |  6 +++
 .../_expected.html                            |  1 +
 .../async-nullish-pending-snippet/main.svelte |  6 +++
 5 files changed, 55 insertions(+), 13 deletions(-)
 create mode 100644 .changeset/lazy-cooks-return.md
 create mode 100644 packages/svelte/tests/server-side-rendering/samples/async-nullish-pending-snippet/_expected.html
 create mode 100644 packages/svelte/tests/server-side-rendering/samples/async-nullish-pending-snippet/main.svelte

diff --git a/.changeset/lazy-cooks-return.md b/.changeset/lazy-cooks-return.md
new file mode 100644
index 0000000000..d2f346f76d
--- /dev/null
+++ b/.changeset/lazy-cooks-return.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: check boundary `pending` attribute at runtime on server
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 39e6912be0..b272870ac3 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
@@ -7,6 +7,7 @@ import {
 	block_open,
 	block_open_else,
 	build_attribute_value,
+	build_template,
 	create_async_block
 } from './shared/utils.js';
 
@@ -19,6 +20,11 @@ export function SvelteBoundary(node, context) {
 	const pending_attribute = /** @type {AST.Attribute} */ (
 		node.attributes.find((node) => node.type === 'Attribute' && node.name === 'pending')
 	);
+	const is_pending_attr_nullish =
+		pending_attribute &&
+		typeof pending_attribute.value === 'object' &&
+		!Array.isArray(pending_attribute.value) &&
+		!context.state.scope.evaluate(pending_attribute.value.expression).is_defined;
 
 	const pending_snippet = /** @type {AST.SnippetBlock} */ (
 		node.fragment.nodes.find(
@@ -27,20 +33,38 @@ export function SvelteBoundary(node, context) {
 	);
 
 	if (pending_attribute || pending_snippet) {
-		const pending = pending_attribute
-			? b.call(
-					build_attribute_value(
-						pending_attribute.value,
-						context,
-						(expression) => expression,
-						false,
-						true
-					),
-					b.id('$$renderer')
+		if (pending_attribute && is_pending_attr_nullish && !pending_snippet) {
+			const callee = build_attribute_value(
+				pending_attribute.value,
+				context,
+				(expression) => expression,
+				false,
+				true
+			);
+			const pending = b.call(callee, b.id('$$renderer'));
+			const block = /** @type {BlockStatement} */ (context.visit(node.fragment));
+			context.state.template.push(
+				b.if(
+					callee,
+					b.block(build_template([block_open_else, pending, block_close])),
+					b.block(build_template([block_open, block, block_close]))
 				)
-			: /** @type {BlockStatement} */ (context.visit(pending_snippet.body));
-
-		context.state.template.push(block_open_else, pending, block_close);
+			);
+		} else {
+			const pending = pending_attribute
+				? b.call(
+						build_attribute_value(
+							pending_attribute.value,
+							context,
+							(expression) => expression,
+							false,
+							true
+						),
+						b.id('$$renderer')
+					)
+				: /** @type {BlockStatement} */ (context.visit(pending_snippet.body));
+			context.state.template.push(block_open_else, pending, block_close);
+		}
 	} else {
 		const block = /** @type {BlockStatement} */ (context.visit(node.fragment));
 		const statement = node.fragment.metadata.has_await
diff --git a/packages/svelte/src/compiler/phases/scope.js b/packages/svelte/src/compiler/phases/scope.js
index 887bc47c56..0c6b64dd04 100644
--- a/packages/svelte/src/compiler/phases/scope.js
+++ b/packages/svelte/src/compiler/phases/scope.js
@@ -260,6 +260,12 @@ class Evaluation {
 						break;
 					}
 
+					if (binding.initial?.type === 'SnippetBlock') {
+						this.is_defined = true;
+						this.is_known = false;
+						break;
+					}
+
 					if (!binding.updated && binding.initial !== null && !is_prop) {
 						binding.scope.evaluate(/** @type {Expression} */ (binding.initial), this.values);
 						break;
diff --git a/packages/svelte/tests/server-side-rendering/samples/async-nullish-pending-snippet/_expected.html b/packages/svelte/tests/server-side-rendering/samples/async-nullish-pending-snippet/_expected.html
new file mode 100644
index 0000000000..1cc0b4b00f
--- /dev/null
+++ b/packages/svelte/tests/server-side-rendering/samples/async-nullish-pending-snippet/_expected.html
@@ -0,0 +1 @@
+<!--[--><!--[--><!---->awaited<!--]--><!--]-->
\ No newline at end of file
diff --git a/packages/svelte/tests/server-side-rendering/samples/async-nullish-pending-snippet/main.svelte b/packages/svelte/tests/server-side-rendering/samples/async-nullish-pending-snippet/main.svelte
new file mode 100644
index 0000000000..b4e8dbcb75
--- /dev/null
+++ b/packages/svelte/tests/server-side-rendering/samples/async-nullish-pending-snippet/main.svelte
@@ -0,0 +1,6 @@
+<script>
+    let pending = null;
+</script>
+<svelte:boundary {pending}>
+    {await 'awaited'}
+</svelte:boundary>
\ No newline at end of file