diff --git a/src/stdmock.ts b/src/stdmock.ts
index 420ea5b..8ec95c2 100644
--- a/src/stdmock.ts
+++ b/src/stdmock.ts
@@ -1,13 +1,32 @@
 import * as mock from 'stdout-stderr'
 
+export interface Stdout {
+  before(): {stdout: string}
+  after(): void
+  catch(): void
+  finally(): void
+}
+
+export interface Stderr {
+  before(): {stderr: string}
+  after(): void
+  catch(): void
+  finally(): void
+}
+
 const create = <T extends 'stdout' | 'stderr'>(std: T) => () => {
   const _finally = () => mock[std].stop()
   return {
     before() {
       mock[std].start()
+      if (std === 'stdout') {
+        return {
+          get stdout() { return mock.stdout.output }
+        }
+      }
       return {
-        get [std]() { return mock[std].output }
-      } as {[P in T]: string}
+        get stderr() { return mock.stderr.output }
+      }
     },
     after: _finally,
     catch: _finally,
@@ -15,5 +34,5 @@ const create = <T extends 'stdout' | 'stderr'>(std: T) => () => {
   }
 }
 
-export const stdout = create('stdout')
-export const stderr = create('stderr')
+export const stdout = create('stdout') as () => Stdout
+export const stderr = create('stderr') as () => Stderr
diff --git a/src/test.ts b/src/test.ts
index 115b6b1..2be6845 100644
--- a/src/test.ts
+++ b/src/test.ts
@@ -4,7 +4,7 @@ import env from './env'
 import mock from './mock'
 import {stderr, stdout} from './stdmock'
 
-export type Extension<O extends object, A1, A2, A3, A4> = (a1: A1, a2: A2, a3: A3, a4: A4) => Filters<O>
+export type Extension<O extends object, A1 = void, A2 = void, A3 = void, A4 = void> = (a1: A1, a2: A2, a3: A3, a4: A4) => Filters<O>
 export interface Filters<C extends object> {
   before?(input: any): Promise<C> | C | void
   after?(context: C): Promise<any> | any
@@ -62,10 +62,10 @@ export interface TestBase<E extends Extensions, C = {}> {
    *  }
    * })
    */
-  extend<K extends string, O extends object>(key: K, filter: Extension<O, void, void, void, void>): Test<AddExtension0<E, K, O>, C>
-  extend<K extends string, O extends object, A1>(key: K, filter: Extension<O, A1, void, void, void>): Test<AddExtension1<E, K, O, A1>, C>
-  extend<K extends string, O extends object, A1, A2>(key: K, filter: Extension<O, A1, A2, void, void>): Test<AddExtension2<E, K, O, A1, A2>, C>
-  extend<K extends string, O extends object, A1, A2, A3>(key: K, filter: Extension<O, A1, A2, A3, void>): Test<AddExtension3<E, K, O, A1, A2, A3>, C>
+  extend<K extends string, O extends object>(key: K, filter: Extension<O>): Test<AddExtension0<E, K, O>, C>
+  extend<K extends string, O extends object, A1>(key: K, filter: Extension<O, A1>): Test<AddExtension1<E, K, O, A1>, C>
+  extend<K extends string, O extends object, A1, A2>(key: K, filter: Extension<O, A1, A2>): Test<AddExtension2<E, K, O, A1, A2>, C>
+  extend<K extends string, O extends object, A1, A2, A3>(key: K, filter: Extension<O, A1, A2, A3>): Test<AddExtension3<E, K, O, A1, A2, A3>, C>
   extend<K extends string, O extends object, A1, A2, A3, A4>(key: K, filter: Extension<O, A1, A2, A3, A4>): Test<AddExtension4<E, K, O, A1, A2, A3, A4>, C>
 }
 export type Test<E extends Extensions, C> = TestBase<E, C> &