From 1445341ba83b238708ea2f5ecd88910c829c34c9 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Mon, 1 Dec 2025 13:14:12 -0500
Subject: [PATCH] fix: handle cross-realm Promises in `hydratable`

---
 .changeset/olive-mangos-march.md                  |  5 +++++
 packages/svelte/src/internal/server/hydratable.js | 12 +++++++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/olive-mangos-march.md

diff --git a/.changeset/olive-mangos-march.md b/.changeset/olive-mangos-march.md
new file mode 100644
index 000000000000..28253eb49aa7
--- /dev/null
+++ b/.changeset/olive-mangos-march.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle cross-realm Promises in `hydratable`
diff --git a/packages/svelte/src/internal/server/hydratable.js b/packages/svelte/src/internal/server/hydratable.js
index 59fa97da4c6d..5711885ed41c 100644
--- a/packages/svelte/src/internal/server/hydratable.js
+++ b/packages/svelte/src/internal/server/hydratable.js
@@ -56,7 +56,7 @@ function encode(key, value, unresolved) {
 	let uid = 1;
 
 	entry.serialized = devalue.uneval(entry.value, (value, uneval) => {
-		if (value instanceof Promise) {
+		if (is_promise(value)) {
 			const p = value
 				.then((v) => `r(${uneval(v)})`)
 				.catch((devalue_error) =>
@@ -90,6 +90,16 @@ function encode(key, value, unresolved) {
 	return entry;
 }
 
+/**
+ * @param {any} value
+ * @returns {value is Promise<any>}
+ */
+function is_promise(value) {
+	// we use this check rather than `instanceof Promise`
+	// because it works cross-realm
+	return Object.prototype.toString.call(value) === '[object Promise]';
+}
+
 /**
  * @param {string} key
  * @param {HydratableLookupEntry} a