diff --git a/packages/client/src/types.ts b/packages/client/src/types.ts
index b5aef5ac9..6b946f8dc 100644
--- a/packages/client/src/types.ts
+++ b/packages/client/src/types.ts
@@ -1,3 +1,5 @@
+import type { PromiseWithError } from '@orpc/shared'
+
 export type HTTPPath = `/${string}`
 export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'DELETE' | 'PATCH'
 
@@ -13,7 +15,7 @@ export type ClientRest<TClientContext extends ClientContext, TInput> = Record<ne
     : [input: TInput, options?: FriendlyClientOptions<TClientContext>]
   : [input: TInput, options: FriendlyClientOptions<TClientContext>]
 
-export type ClientPromiseResult<TOutput, TError extends Error> = Promise<TOutput> & { __error?: { type: TError } }
+export type ClientPromiseResult<TOutput, TError extends Error> = PromiseWithError<TOutput, TError>
 
 export interface Client<TClientContext extends ClientContext, TInput, TOutput, TError extends Error> {
   (...rest: ClientRest<TClientContext, TInput>): ClientPromiseResult<TOutput, TError>
diff --git a/packages/shared/src/interceptor.test-d.ts b/packages/shared/src/interceptor.test-d.ts
index 301f54c45..73afd6dbd 100644
--- a/packages/shared/src/interceptor.test-d.ts
+++ b/packages/shared/src/interceptor.test-d.ts
@@ -1,11 +1,12 @@
 import type { Interceptor } from './interceptor'
+import type { PromiseWithError } from './types'
 import { onError, onFinish, onStart, onSuccess } from './interceptor'
 
 it('onStart', () => {
   const interceptor: Interceptor<{ foo: string }, 'success', 'error'> = onStart((options) => {
     expectTypeOf(options.foo).toEqualTypeOf<string>()
     expectTypeOf(options.next).toBeCallableWith<[options?: { foo: string }]>()
-    expectTypeOf(options.next()).toEqualTypeOf<Promise<'success'> & { __error?: { type: 'error' } }>()
+    expectTypeOf(options.next()).toEqualTypeOf<PromiseWithError<'success', 'error'>>()
   })
 })
 
@@ -15,7 +16,7 @@ it('onSuccess', () => {
 
     expectTypeOf(options.foo).toEqualTypeOf<string>()
     expectTypeOf(options.next).toBeCallableWith<[options?: { foo: string }]>()
-    expectTypeOf(options.next()).toEqualTypeOf<Promise<'success'> & { __error?: { type: 'error' } }>()
+    expectTypeOf(options.next()).toEqualTypeOf<PromiseWithError<'success', 'error'>>()
   })
 })
 
@@ -25,7 +26,7 @@ it('onError', () => {
 
     expectTypeOf(options.foo).toEqualTypeOf<string>()
     expectTypeOf(options.next).toBeCallableWith<[options?: { foo: string }]>()
-    expectTypeOf(options.next()).toEqualTypeOf<Promise<'success'> & { __error?: { type: 'error' } }>()
+    expectTypeOf(options.next()).toEqualTypeOf<PromiseWithError<'success', 'error'>>()
   })
 })
 
@@ -35,6 +36,6 @@ it('onFinish', () => {
 
     expectTypeOf(options.foo).toEqualTypeOf<string>()
     expectTypeOf(options.next).toBeCallableWith<[options?: { foo: string }]>()
-    expectTypeOf(options.next()).toEqualTypeOf<Promise<'success'> & { __error?: { type: 'error' } }>()
+    expectTypeOf(options.next()).toEqualTypeOf<PromiseWithError<'success', 'error'>>()
   })
 })
diff --git a/packages/shared/src/interceptor.ts b/packages/shared/src/interceptor.ts
index 92fd86ef7..067a9b199 100644
--- a/packages/shared/src/interceptor.ts
+++ b/packages/shared/src/interceptor.ts
@@ -1,4 +1,5 @@
 import type { Promisable } from 'type-fest'
+import type { PromiseWithError } from './types'
 
 export type InterceptableOptions = Record<string, any>
 
@@ -7,14 +8,14 @@ export type InterceptorOptions<
   TResult,
   TError,
 > = Omit<TOptions, 'next'> & {
-  next(options?: TOptions): Promise<TResult> & { __error?: { type: TError } }
+  next(options?: TOptions): PromiseWithError<TResult, TError>
 }
 
 export type Interceptor<
   TOptions extends InterceptableOptions,
   TResult,
   TError,
-> = (options: InterceptorOptions<TOptions, TResult, TError>) => Promise<TResult> & { __error?: { type: TError } }
+> = (options: InterceptorOptions<TOptions, TResult, TError>) => PromiseWithError<TResult, TError>
 
 /**
  * Can used for interceptors or middlewares
@@ -44,15 +45,19 @@ export function onSuccess<TOptions extends { next(): any }, TRest extends any[]>
 /**
  * Can used for interceptors or middlewares
  */
-export function onError<TError, TOptions extends { next(): any }, TRest extends any[]>(
-  callback: NoInfer<(error: TError, options: TOptions, ...rest: TRest) => Promisable<void>>,
-): (options: TOptions, ...rest: TRest) => Promise<Awaited<ReturnType<TOptions['next']>>> & { __error?: { type: TError } } {
+export function onError<TOptions extends { next(): any }, TRest extends any[]>(
+  callback: NoInfer<(
+    error: ReturnType<TOptions['next']> extends PromiseWithError<any, infer E> ? E : unknown,
+    options: TOptions,
+    ...rest: TRest
+  ) => Promisable<void>>,
+): (options: TOptions, ...rest: TRest) => Promise<Awaited<ReturnType<TOptions['next']>>> {
   return async (options, ...rest) => {
     try {
       return await options.next()
     }
     catch (error) {
-      await callback(error as TError, options, ...rest)
+      await callback(error as any, options, ...rest)
       throw error
     }
   }
@@ -63,10 +68,18 @@ export type OnFinishState<TResult, TError> = [TResult, null, 'success'] | [undef
 /**
  * Can used for interceptors or middlewares
  */
-export function onFinish<TError, TOptions extends { next(): any }, TRest extends any[]>(
-  callback: NoInfer<(state: OnFinishState<Awaited<ReturnType<TOptions['next']>>, TError>, options: TOptions, ...rest: TRest) => Promisable<void>>,
-): (options: TOptions, ...rest: TRest) => Promise<Awaited<ReturnType<TOptions['next']>>> & { __error?: { type: TError } } {
-  let state: OnFinishState<Awaited<ReturnType<TOptions['next']>>, TError> | undefined
+export function onFinish<TOptions extends { next(): any }, TRest extends any[]>(
+  callback: NoInfer<(
+    state: OnFinishState<
+      Awaited<ReturnType<TOptions['next']>>,
+      ReturnType<TOptions['next']> extends PromiseWithError<any, infer E> ? E : unknown
+    >,
+    options: TOptions,
+    ...rest: TRest
+  ) => Promisable<void>>,
+): (options: TOptions, ...rest: TRest) => Promise<Awaited<ReturnType<TOptions['next']>>> {
+  let state: any
+
   return async (options, ...rest) => {
     try {
       const result = await options.next()
@@ -74,11 +87,11 @@ export function onFinish<TError, TOptions extends { next(): any }, TRest extends
       return result
     }
     catch (error) {
-      state = [undefined, error as TError, 'error']
+      state = [undefined, error, 'error']
       throw error
     }
     finally {
-      await callback(state as Exclude<typeof state, undefined>, options, ...rest)
+      await callback(state, options, ...rest)
     }
   }
 }
diff --git a/packages/shared/src/types.test-d.ts b/packages/shared/src/types.test-d.ts
index 976581bea..29cb44794 100644
--- a/packages/shared/src/types.test-d.ts
+++ b/packages/shared/src/types.test-d.ts
@@ -1,4 +1,4 @@
-import type { IntersectPick, SetOptional } from './types'
+import type { IntersectPick, PromiseWithError, SetOptional } from './types'
 
 it('SetOptional', () => {
   expectTypeOf<SetOptional<{ a: number }, 'a'>>().toMatchTypeOf<{ a?: number }>()
@@ -12,3 +12,10 @@ it('IntersectPick', () => {
   expectTypeOf<IntersectPick<{ a: number, b: number }, { b: number }>>().toEqualTypeOf<{ b: number }>()
   expectTypeOf<IntersectPick<{ a: number }, { b: number }>>().toEqualTypeOf<Empty>()
 })
+
+it('PromiseWithError', () => {
+  type C = PromiseWithError<number | undefined | null, Error | undefined | null>
+
+  expectTypeOf<C extends Promise<infer T> ? T : never>().toEqualTypeOf<number | undefined | null>()
+  expectTypeOf<C extends PromiseWithError<infer T, infer E> ? [T, E] : never>().toEqualTypeOf<[number | undefined | null, Error | undefined | null]>()
+})
diff --git a/packages/shared/src/types.ts b/packages/shared/src/types.ts
index 502967837..8e9d78950 100644
--- a/packages/shared/src/types.ts
+++ b/packages/shared/src/types.ts
@@ -1,3 +1,5 @@
 export type SetOptional<T, K extends keyof T> = Omit<T, K> & Partial<Pick<T, K>>
 
 export type IntersectPick<T, U> = Pick<T, keyof T & keyof U>
+
+export type PromiseWithError<T, TError> = Promise<T> & { __error?: { type: TError } }