From 6b913f2c274d276589508f65b60bb83df0499638 Mon Sep 17 00:00:00 2001
From: Yogesh Kumar <webdev1@futeservices.com>
Date: Thu, 27 Aug 2026 11:42:55 +0530
Subject: [PATCH] fix(preact-query): propagate falsy errors from useMutation to
 the error boundary

useMutation gated the error boundary throw on `result.error` being truthy, so a
mutationFn that rejects with a falsy value such as `Promise.reject('')` leaves
the mutation in the error state without the boundary ever seeing it.

Gate on `result.isError` instead, matching useBaseQuery's getHasError and the
same fix applied to useQueries in #11309.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019MfJ49iHsRNpy4oibuXzrz
---
 .changeset/spotty-hounds-clap.md              |  5 +++
 .../src/__tests__/useMutation.test.tsx        | 40 +++++++++++++++++++
 packages/preact-query/src/useMutation.ts      |  2 +-
 3 files changed, 46 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/spotty-hounds-clap.md

diff --git a/.changeset/spotty-hounds-clap.md b/.changeset/spotty-hounds-clap.md
new file mode 100644
index 00000000000..2ed4f6bcee3
--- /dev/null
+++ b/.changeset/spotty-hounds-clap.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/preact-query': patch
+---
+
+fix(preact-query): throw falsy errors from `useMutation` to the error boundary
diff --git a/packages/preact-query/src/__tests__/useMutation.test.tsx b/packages/preact-query/src/__tests__/useMutation.test.tsx
index e24f79ddfba..8e8144f81f4 100644
--- a/packages/preact-query/src/__tests__/useMutation.test.tsx
+++ b/packages/preact-query/src/__tests__/useMutation.test.tsx
@@ -1364,6 +1364,46 @@ describe('useMutation', () => {
     consoleMock.mockRestore()
   })
 
+  it('should be able to throw a falsy error when throwOnError is set to true', async () => {
+    const consoleMock = vi
+      .spyOn(console, 'error')
+      .mockImplementation(() => undefined)
+    function Page() {
+      const { mutate } = useMutation<string, string>({
+        mutationFn: () => {
+          return Promise.reject('')
+        },
+        throwOnError: true,
+      })
+
+      return (
+        <div>
+          <button onClick={() => mutate()}>mutate</button>
+        </div>
+      )
+    }
+
+    const { getByText, queryByText } = renderWithClient(
+      queryClient,
+      <ErrorBoundary
+        fallbackRender={() => (
+          <div>
+            <span>error boundary</span>
+          </div>
+        )}
+      >
+        <Page />
+      </ErrorBoundary>,
+    )
+
+    fireEvent.click(getByText('mutate'))
+
+    await vi.advanceTimersByTimeAsync(0)
+    expect(queryByText('error boundary')).not.toBeNull()
+
+    consoleMock.mockRestore()
+  })
+
   it('should be able to throw an error when throwOnError is a function that returns true', async () => {
     const consoleMock = vi
       .spyOn(console, 'error')
diff --git a/packages/preact-query/src/useMutation.ts b/packages/preact-query/src/useMutation.ts
index 68a3759d93f..da5d0b79392 100644
--- a/packages/preact-query/src/useMutation.ts
+++ b/packages/preact-query/src/useMutation.ts
@@ -233,7 +233,7 @@ export function useMutation<
   )
 
   if (
-    result.error &&
+    result.isError &&
     shouldThrowError(observer.options.throwOnError, [result.error])
   ) {
     throw result.error