From f7ac5f54d5f82ad425dfb6755c02d3e662e469e7 Mon Sep 17 00:00:00 2001
From: S-jooyoung <jooyoung.dev@gmail.com>
Date: Tue, 13 Oct 2026 15:45:17 +0900
Subject: [PATCH] fix(vue-query): throw falsy errors from useMutation to the
 error boundary

The error watcher in `useMutation` only evaluated `throwOnError` when
the error value was truthy, so a mutation that rejected with a falsy
value (e.g. `undefined` or `null`) never reached `shouldThrowError` and
was not propagated. Watch `state.isError` alongside `state.error` and
check `isError` instead, the same approach proposed for react-query in
#11311 and preact-query in #11312, and already used by vue-query's
`useBaseQuery`.
---
 .changeset/vue-mutation-falsy-error.md        |  5 ++++
 .../src/__tests__/useMutation.test.ts         | 29 +++++++++++++++++++
 packages/vue-query/src/useMutation.ts         | 19 +++++-------
 3 files changed, 42 insertions(+), 11 deletions(-)
 create mode 100644 .changeset/vue-mutation-falsy-error.md

diff --git a/.changeset/vue-mutation-falsy-error.md b/.changeset/vue-mutation-falsy-error.md
new file mode 100644
index 00000000000..6bbed215273
--- /dev/null
+++ b/.changeset/vue-mutation-falsy-error.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/vue-query': patch
+---
+
+fix(vue-query): throw falsy errors from `useMutation` to the error boundary
diff --git a/packages/vue-query/src/__tests__/useMutation.test.ts b/packages/vue-query/src/__tests__/useMutation.test.ts
index 2ac8aad0b86..b1d414ab152 100644
--- a/packages/vue-query/src/__tests__/useMutation.test.ts
+++ b/packages/vue-query/src/__tests__/useMutation.test.ts
@@ -619,6 +619,35 @@ describe('useMutation', () => {
         )
       },
     )
+
+    // These tests let throwOnError return false on purpose: under vitest,
+    // throwing a falsy value from the watcher breaks every watcher created
+    // later in this file, while the throw path is already covered above.
+    it('should evaluate throwOnError when the mutation rejects with undefined', async () => {
+      const throwOnError = vi.fn().mockReturnValue(false)
+      const { mutate } = useMutation<string, undefined>({
+        mutationFn: () => sleep(10).then(() => Promise.reject(undefined)),
+        throwOnError,
+      })
+
+      mutate()
+      await vi.advanceTimersByTimeAsync(10)
+      expect(throwOnError).toHaveBeenCalledTimes(1)
+      expect(throwOnError).toHaveBeenCalledWith(undefined)
+    })
+
+    it('should evaluate throwOnError when the mutation rejects with null', async () => {
+      const throwOnError = vi.fn().mockReturnValue(false)
+      const { mutate } = useMutation<string, null>({
+        mutationFn: () => sleep(10).then(() => Promise.reject(null)),
+        throwOnError,
+      })
+
+      mutate()
+      await vi.advanceTimersByTimeAsync(10)
+      expect(throwOnError).toHaveBeenCalledTimes(1)
+      expect(throwOnError).toHaveBeenCalledWith(null)
+    })
   })
 
   describe('optimistic updates', () => {
diff --git a/packages/vue-query/src/useMutation.ts b/packages/vue-query/src/useMutation.ts
index 799b321ba4d..4ecdd325470 100644
--- a/packages/vue-query/src/useMutation.ts
+++ b/packages/vue-query/src/useMutation.ts
@@ -291,17 +291,14 @@ export function useMutation<
     Readonly<MutationResult<TData, TError, TVariables, TOnMutateResult>>
   >
 
-  watch(
-    () => state.error,
-    (error) => {
-      if (
-        error &&
-        shouldThrowError(defaultedOptions.value.throwOnError, [error as TError])
-      ) {
-        throw error
-      }
-    },
-  )
+  watch([() => state.isError, () => state.error], ([isError, error]) => {
+    if (
+      isError &&
+      shouldThrowError(defaultedOptions.value.throwOnError, [error as TError])
+    ) {
+      throw error
+    }
+  })
 
   return {
     ...resultRefs,