From 9002ee6846efe04f22f4453b5dadda23a67d5f1d Mon Sep 17 00:00:00 2001
From: okxint <cashmein.eth@gmail.com>
Date: Sat, 22 Aug 2026 15:59:12 +0530
Subject: [PATCH] fix(svelte-query): useMutationState array never shrank when
 mutations stopped matching filter
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Object.assign copies enumerable own properties but never sets Array.length.
When nextResult was shorter than result (fewer mutations matched the filter),
trailing stale entries were never removed — the array only ever grew.

Fix: replace Object.assign with result.splice(0, result.length, ...nextResult).
splice keeps the reactive $state array reference intact (required by Svelte 5
runes) while correctly adjusting length, adding, and removing elements.

Adds a regression test: two pending mutations, both settle, assert the
filtered result shrinks to [].

Fixes #11152

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
---
 .../fix-svelte-use-mutation-state-shrink.md   |  5 ++
 .../src/useMutationState.svelte.ts            |  2 +-
 .../useMutationState.svelte.test.ts           | 51 +++++++++++++++++++
 3 files changed, 57 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/fix-svelte-use-mutation-state-shrink.md

diff --git a/.changeset/fix-svelte-use-mutation-state-shrink.md b/.changeset/fix-svelte-use-mutation-state-shrink.md
new file mode 100644
index 00000000000..17bcb2996fb
--- /dev/null
+++ b/.changeset/fix-svelte-use-mutation-state-shrink.md
@@ -0,0 +1,5 @@
+---
+"@tanstack/svelte-query": patch
+---
+
+fix(svelte-query): use splice in useMutationState so the result array shrinks when mutations no longer match the filter
diff --git a/packages/svelte-query/src/useMutationState.svelte.ts b/packages/svelte-query/src/useMutationState.svelte.ts
index b92c900e8cf..215956b7628 100644
--- a/packages/svelte-query/src/useMutationState.svelte.ts
+++ b/packages/svelte-query/src/useMutationState.svelte.ts
@@ -44,7 +44,7 @@ export function useMutationState<
         getResult(mutationCache, options),
       )
       if (result !== nextResult) {
-        Object.assign(result, nextResult)
+        result.splice(0, result.length, ...nextResult)
       }
     })
 
diff --git a/packages/svelte-query/tests/useMutationState/useMutationState.svelte.test.ts b/packages/svelte-query/tests/useMutationState/useMutationState.svelte.test.ts
index 703c05df631..77bb1134b5f 100644
--- a/packages/svelte-query/tests/useMutationState/useMutationState.svelte.test.ts
+++ b/packages/svelte-query/tests/useMutationState/useMutationState.svelte.test.ts
@@ -157,3 +157,54 @@ describe('useMutationState', () => {
     expect(rendered.getByText('Data: ["success"]')).toBeInTheDocument()
   })
 })
+
+describe('useMutationState - array shrinks when mutations no longer match filter', () => {
+  let queryClient: QueryClient
+
+  beforeEach(() => {
+    vi.useFakeTimers()
+    queryClient = new QueryClient()
+  })
+
+  afterEach(() => {
+    queryClient.clear()
+    vi.useRealTimers()
+  })
+
+  it('should remove mutations that no longer match the filter', async () => {
+    const firstKey = queryKey()
+    const secondKey = queryKey()
+    const mutationFn = vi.fn(() => sleep(10).then(() => 'data'))
+
+    const rendered = render(Base, {
+      props: {
+        queryClient,
+        successMutationOpts: () => ({
+          mutationKey: firstKey,
+          mutationFn,
+        }),
+        errorMutationOpts: () => ({
+          mutationKey: secondKey,
+          mutationFn,
+        }),
+        mutationStateOpts: {
+          filters: { status: 'pending' },
+        },
+      },
+    })
+
+    fireEvent.click(rendered.getByRole('button', { name: /Success/i }))
+    fireEvent.click(rendered.getByRole('button', { name: /Error/i }))
+
+    await vi.advanceTimersByTimeAsync(0)
+    expect(
+      rendered.getByText('Data: ["pending","pending"]'),
+    ).toBeInTheDocument()
+
+    await vi.advanceTimersByTimeAsync(10)
+
+    // Both mutations settled — zero now match status: 'pending'
+    // Previously Object.assign never shrank the array so this stayed ["pending","pending"]
+    expect(rendered.getByText('Data: []')).toBeInTheDocument()
+  })
+})