diff --git a/src/lib/queries/mutationOptions.ts b/src/lib/queries/mutationOptions.ts
new file mode 100644
index 0000000..327bd2e
--- /dev/null
+++ b/src/lib/queries/mutationOptions.ts
@@ -0,0 +1,71 @@
+import type { UseMutationOptions } from "@tanstack/react-query"
+import type { Observable } from "rxjs"
+
+/**
+ * Resolves the public `mutationFn` option — an Observable or a function
+ * returning one — to its source Observable for the given variables.
+ */
+export function resolveMutationFnSource<TData, TVariables>(
+  mutationFn:
+    | ((variables: TVariables) => Observable<TData>)
+    | Observable<TData>,
+  variables: TVariables,
+) {
+  return typeof mutationFn === "function" ? mutationFn(variables) : mutationFn
+}
+
+type MutationCallbacks<TData, TError, TVariables, TOnMutateResult> = Pick<
+  UseMutationOptions<TData, TError, TVariables, TOnMutateResult>,
+  "onMutate" | "onSuccess" | "onError" | "onSettled"
+>
+
+/**
+ * Same callbacks, but keyed as required so that spreading the adapted
+ * callbacks after `...options` overrides (rather than unions with) the
+ * raw-variables callbacks still present in `options`.
+ */
+type AdaptedMutationCallbacks<TData, TError, TVariables, TOnMutateResult> = {
+  [K in "onMutate" | "onSuccess" | "onError" | "onSettled"]:
+    | MutationCallbacks<
+        TData,
+        TError,
+        { variables: TVariables },
+        TOnMutateResult
+      >[K]
+    | undefined
+}
+
+/**
+ * Hooks built on top of `useMutation$` (`useSwitchMutation$`,
+ * `useConcatMutation$`) run their inner mutation with the user variables
+ * wrapped in an envelope (`{ variables, ... }`). This adapts the user-facing
+ * callbacks, which expect the raw variables, to that envelope.
+ */
+export function adaptCallbacksToWrappedVariables<
+  TData,
+  TError,
+  TVariables,
+  TOnMutateResult,
+>({
+  onMutate,
+  onSuccess,
+  onError,
+  onSettled,
+}: MutationCallbacks<
+  TData,
+  TError,
+  TVariables,
+  TOnMutateResult
+>): AdaptedMutationCallbacks<TData, TError, TVariables, TOnMutateResult> {
+  return {
+    onMutate: onMutate
+      ? ({ variables }, ...rest) => onMutate(variables, ...rest)
+      : undefined,
+    onSuccess: (data, { variables }, ...rest) =>
+      onSuccess?.(data, variables, ...rest),
+    onError: (error, { variables }, ...rest) =>
+      onError?.(error, variables, ...rest),
+    onSettled: (data, error, { variables }, ...rest) =>
+      onSettled?.(data, error, variables, ...rest),
+  }
+}
diff --git a/src/lib/queries/useConcatMutation$.ts b/src/lib/queries/useConcatMutation$.ts
index 6bc337e..2a34ba3 100644
--- a/src/lib/queries/useConcatMutation$.ts
+++ b/src/lib/queries/useConcatMutation$.ts
@@ -13,6 +13,10 @@ import {
   type Subject,
   switchMap,
 } from "rxjs"
+import {
+  adaptCallbacksToWrappedVariables,
+  resolveMutationFnSource,
+} from "./mutationOptions"
 import { type UseMutation$Options, useMutation$ } from "./useMutation$"
 
 export function useConcatMutation$<
@@ -21,10 +25,7 @@ export function useConcatMutation$<
   TVariables = void,
   TContext = unknown,
 >(
-  {
-    onMutate,
-    ...options
-  }: UseMutation$Options<TData | null, TError, TVariables, TContext> & {
+  options: UseMutation$Options<TData | null, TError, TVariables, TContext> & {
     mutationKey: MutationKey
   },
   queryClient?: QueryClient,
@@ -41,23 +42,9 @@ export function useConcatMutation$<
   >(
     {
       ...options,
-      onMutate: onMutate
-        ? ({ variables }, ...rest) => onMutate(variables, ...rest)
-        : undefined,
-      onSuccess(data, { variables }, ...rest) {
-        return options.onSuccess?.(data, variables, ...rest)
-      },
-      onError(error, { variables }, ...rest) {
-        return options.onError?.(error, variables, ...rest)
-      },
-      onSettled(data, error, { variables }, ...rest) {
-        return options.onSettled?.(data, error, variables, ...rest)
-      },
+      ...adaptCallbacksToWrappedVariables(options),
       mutationFn: ({ ready$, variables }) => {
-        const source =
-          typeof options.mutationFn === "function"
-            ? options.mutationFn(variables)
-            : options.mutationFn
+        const source = resolveMutationFnSource(options.mutationFn, variables)
 
         return ready$.pipe(
           filter((isReady) => isReady),
diff --git a/src/lib/queries/useMutation$.ts b/src/lib/queries/useMutation$.ts
index 334200c..1c98c17 100644
--- a/src/lib/queries/useMutation$.ts
+++ b/src/lib/queries/useMutation$.ts
@@ -8,6 +8,7 @@ import {
 import { useEffect } from "react"
 import { BehaviorSubject, type Observable, take } from "rxjs"
 import { useConstant } from "../utils/react/useConstant"
+import { resolveMutationFnSource } from "./mutationOptions"
 
 export type UseMutation$Options<
   TData = unknown,
@@ -53,10 +54,7 @@ export function useMutation$<
         let lastData: { value: TData } | undefined
 
         return new Promise<TData>((resolve, reject) => {
-          const source =
-            typeof options.mutationFn === "function"
-              ? options.mutationFn(variables)
-              : options.mutationFn
+          const source = resolveMutationFnSource(options.mutationFn, variables)
 
           source.pipe(take(1)).subscribe({
             next: (data) => {
diff --git a/src/lib/queries/useSwitchMutation$.ts b/src/lib/queries/useSwitchMutation$.ts
index e67eb95..1397d97 100644
--- a/src/lib/queries/useSwitchMutation$.ts
+++ b/src/lib/queries/useSwitchMutation$.ts
@@ -9,6 +9,10 @@ import {
   tap,
 } from "rxjs"
 import { useRefOnce } from "../utils"
+import {
+  adaptCallbacksToWrappedVariables,
+  resolveMutationFnSource,
+} from "./mutationOptions"
 import { type UseMutation$Options, useMutation$ } from "./useMutation$"
 
 export class SwitchMutationCancelError extends Error {
@@ -26,9 +30,6 @@ export function useSwitchMutation$<
 >(
   {
     mutationFn,
-    onMutate,
-    onError,
-    onSettled,
     ...options
   }: UseMutation$Options<TData | null, TError, TVariables, TOnMutateResult>,
   queryClient?: QueryClient,
@@ -56,10 +57,7 @@ export function useSwitchMutation$<
             throw new SwitchMutationCancelError()
           }
 
-          const source =
-            typeof mutationFn === "function"
-              ? mutationFn(variables)
-              : mutationFn
+          const source = resolveMutationFnSource(mutationFn, variables)
 
           /**
            * `defaultIfEmpty` must sit on the source itself: the abort stream
@@ -79,20 +77,7 @@ export function useSwitchMutation$<
         },
         [mutationFn],
       ),
-      onMutate: onMutate
-        ? ({ variables }, ...rest) => {
-            return onMutate(variables, ...rest)
-          }
-        : undefined,
-      onSuccess: (data, { variables }, ...rest) => {
-        return options.onSuccess?.(data, variables, ...rest)
-      },
-      onError: (error, { variables }, ...rest) => {
-        return onError?.(error, variables, ...rest)
-      },
-      onSettled: (data, error, { variables }, ...rest) => {
-        return onSettled?.(data, error, variables, ...rest)
-      },
+      ...adaptCallbacksToWrappedVariables(options),
     },
     queryClient,
   )