From ba66d6796f6ac81e4e46f14377aa4858a1f74eda Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Sat, 25 Oct 2025 23:33:38 +0200
Subject: [PATCH 1/3] fix: support async Svelte

Svelte 5 now supports async await in components. One behavior there is that when a component is created as part of a boundary that has pending async work, its `$effect`s will not run until that async work is done.

The way the code is written right now means you can end up in an infinite pending state: If you do `await someQuery.promise`, the `$effect` for the subscription will never run, and therefore the `await` will never resolve. This PR fixes it.
---
 .changeset/thin-bugs-change.md                |  5 ++++
 .../src/createBaseQuery.svelte.ts             | 26 ++++++++++++++++---
 2 files changed, 27 insertions(+), 4 deletions(-)
 create mode 100644 .changeset/thin-bugs-change.md

diff --git a/.changeset/thin-bugs-change.md b/.changeset/thin-bugs-change.md
new file mode 100644
index 0000000000..efaca4fccb
--- /dev/null
+++ b/.changeset/thin-bugs-change.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/svelte-query': patch
+---
+
+fix: support async Svelte
diff --git a/packages/svelte-query/src/createBaseQuery.svelte.ts b/packages/svelte-query/src/createBaseQuery.svelte.ts
index 03fc6b28db..be829cd597 100644
--- a/packages/svelte-query/src/createBaseQuery.svelte.ts
+++ b/packages/svelte-query/src/createBaseQuery.svelte.ts
@@ -8,6 +8,7 @@ import type {
   CreateBaseQueryOptions,
   CreateBaseQueryResult,
 } from './types.js'
+import { onDestroy } from 'svelte'
 
 /**
  * Base implementation for `createQuery` and `createInfiniteQuery`
@@ -71,12 +72,29 @@ export function createBaseQuery<
     createResult(),
   )
 
-  $effect(() => {
-    const unsubscribe = isRestoring.current
+  // The following is convoluted but necessary:
+  // Call eagerly so subscription happens on the server and on suspended branches in the client...
+  let unsubscribe =
+    isRestoring.current && typeof window !== 'undefined'
       ? () => undefined
       : observer.subscribe(() => update(createResult()))
-    observer.updateResult()
-    return unsubscribe
+  // ...but also watch for state changes to resubscribe, and because Svelte right now doesn't
+  // run onDestroy on components with pending work that are destroyed again before they are resolved...
+  watchChanges(
+    () => [isRestoring.current, observer] as const,
+    'pre',
+    () => {
+      unsubscribe()
+      unsubscribe = isRestoring.current
+        ? () => undefined
+        : observer.subscribe(() => update(createResult()))
+      observer.updateResult()
+      return unsubscribe
+    },
+  )
+  // ...and finally also cleanup via onDestroy because that one runs on the server whereas $effect.pre does not.
+  onDestroy(() => {
+    unsubscribe()
   })
 
   watchChanges(

From 1cacfcb666474e89696e5d6413916a4d30c0bea3 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Sat, 25 Oct 2025 23:40:39 +0200
Subject: [PATCH 2/3] lint

---
 packages/svelte-query/src/createBaseQuery.svelte.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/svelte-query/src/createBaseQuery.svelte.ts b/packages/svelte-query/src/createBaseQuery.svelte.ts
index be829cd597..eae6aaebf7 100644
--- a/packages/svelte-query/src/createBaseQuery.svelte.ts
+++ b/packages/svelte-query/src/createBaseQuery.svelte.ts
@@ -1,3 +1,4 @@
+import { onDestroy } from 'svelte'
 import { useIsRestoring } from './useIsRestoring.js'
 import { useQueryClient } from './useQueryClient.js'
 import { createRawRef } from './containers.svelte.js'
@@ -8,7 +9,6 @@ import type {
   CreateBaseQueryOptions,
   CreateBaseQueryResult,
 } from './types.js'
-import { onDestroy } from 'svelte'
 
 /**
  * Base implementation for `createQuery` and `createInfiniteQuery`

From 59a1acc5697b56e3c73296dd245b3b36b9fff949 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Sat, 25 Oct 2025 23:56:27 +0200
Subject: [PATCH 3/3] fix

---
 packages/svelte-query/src/createBaseQuery.svelte.ts | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/packages/svelte-query/src/createBaseQuery.svelte.ts b/packages/svelte-query/src/createBaseQuery.svelte.ts
index eae6aaebf7..401a471814 100644
--- a/packages/svelte-query/src/createBaseQuery.svelte.ts
+++ b/packages/svelte-query/src/createBaseQuery.svelte.ts
@@ -93,9 +93,13 @@ export function createBaseQuery<
     },
   )
   // ...and finally also cleanup via onDestroy because that one runs on the server whereas $effect.pre does not.
-  onDestroy(() => {
-    unsubscribe()
-  })
+  // (in a try-catch because it theoretically can be called in a non-component context - that should not happen
+  // but it would be a breaking change technically to error out here. SSR-safe because this wouldn't be called during SSR if it was not in a component)
+  try {
+    onDestroy(() => {
+      unsubscribe()
+    })
+  } catch (e) {}
 
   watchChanges(
     () => resolvedOptions,