diff --git a/CHANGELOG.md b/CHANGELOG.md
index bbf2084..58ec6f9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,7 @@
 ## Changelog
 
+- **v5.0.3**
+  - fixed: (TypeScript) middleware corrupting downstream request types and args
 - **v5.0.2**
   - fixed: AutoRouter was missing the router-level generics support of the other 2 routers.
   - fixed: All 3 routers had their 3rd generic argument, ResponseType added per the spec.
diff --git a/examples/types/additional-arguments.ts b/examples/types/additional-arguments.ts
new file mode 100644
index 0000000..6ad8933
--- /dev/null
+++ b/examples/types/additional-arguments.ts
@@ -0,0 +1,56 @@
+import { RequestHandler } from '../../src/types'
+import { IRequestStrict } from '../../src/types'
+import { Router } from '../../src/Router'
+import { IttyRouter } from '../../src/IttyRouter'
+import { AutoRouter } from '../../src/AutoRouter'
+import { IRequest } from 'itty-router'
+
+// we define our environment
+type Environment = { age: number }
+type Pet = { name: string }
+
+// and now both args combined (that Workers send to the .fetch())
+type Args = [Environment]
+type AlternativeArgs = [Pet]
+
+// creating some middleware that needs access to CF variables
+export const withUser: RequestHandler<IRequest, Args> =
+  (request, env) => {
+    request.user = 'Kevin'
+    env.age = 123
+    env.name = 'Kevin' // invalid
+  }
+
+const router = Router<IRequestStrict, Args>()
+
+router
+  // before middleware
+  .get('/', (request, env) => {
+    request.user = 'kevin' // invalid (strict)
+    env.whatever = 123 // valid (any)
+    env.age = 123 // valid (any)
+  })
+
+  // route-level overrides
+  .get<IRequest, Args>('/', (request, env) => {
+    request.foo = 'bar' // invalid
+    env.whatever = 123 // invalid
+    env.age = 123 // valid
+  })
+  // route-level overrides
+  .get<IRequest, AlternativeArgs>('/', (request, env) => {
+    request.foo = 'bar' // invalid
+    env.age = 123 // invalid
+    env.name = 'Mittens' // valid
+  })
+
+  // after middleware
+  .get('/', withUser, (request, env) => {
+    request.user = 'Kevin'
+    env.age = 123 // valid
+    env.whatever = 123 // invalid
+  })
+
+  .get('/', (request, env) => {
+    env.age = 'foo' // valid (any)
+  })
diff --git a/examples/types/global-and-route-level-request.ts b/examples/types/global-and-route-level-request.ts
index 5bbc7e5..b4cbd96 100644
--- a/examples/types/global-and-route-level-request.ts
+++ b/examples/types/global-and-route-level-request.ts
@@ -1,7 +1,10 @@
-import { IRequestStrict } from 'IttyRouter'
-import { Router } from 'Router'
+import { IRequestStrict } from '../../src/types'
+import { Router } from '../../src/Router'
 
-const router = Router<IRequestStrict>()
+type Pet = { name: string }
+type List = number[]
+
+const router = Router<IRequestStrict, [Pet]>()
 
 type FooRequest = {
   foo: string
@@ -10,6 +13,11 @@ type FooRequest = {
 router
   .get('/basic', () => new Response('Success!'))
   .get('/text', () => 'Success!')
+  .get('/text', (request, env) => {
+    env.name = 'foo'
+    env.bar = 'baz' // invalid
+  })
+  .get('/', (r, env) => env.foo )
   // .get('/params/:foo', ({ foo }) => foo)              // should NOT work
   .get<FooRequest>('/params/:foo', ({ foo }) => foo)  // should work
   .get('/json', () => ({ foo: 'bar' }))
diff --git a/examples/types/middleware.ts b/examples/types/middleware.ts
index 7642bbf..2ad26b4 100644
--- a/examples/types/middleware.ts
+++ b/examples/types/middleware.ts
@@ -1,41 +1,37 @@
 import { IRequestStrict, IRequest, RequestHandler } from '../../src/types'
-import { IttyRouter } from 'IttyRouter'
+import { IttyRouter } from '../../src/IttyRouter'
 
 type UserRequest = {
   user: string
 } & IRequestStrict
 
+// we define a *strict* router for this demo
 const router = IttyRouter<IRequestStrict>()
 
-const withUser: RequestHandler = (request) => {
+// middleware with explicit request-type (generic)
+const withUser: RequestHandler<UserRequest> = (request) => {
   request.user = 'Kevin'
 }
 
 router
-  // upstream request sees the request as IRequest (default), so anything goes
+  // request will be IRequestStrict here, thus no user property
   .get('/', (request) => {
-    request.user = 123 // not OK
+    request.user = 'Kevin' // invalid
   })
 
-  // then we add the middleware defined above as <UserRequest>
+  // then we add the middleware defined above, allowing the handler chain to inherit the request type
   .all('*', withUser, (request) => {
     request.user = 'Kevin'
-    request.user = 123  // NOT ok
-  })
-
-  // and if we ever need to restore control, add the generic back in
-  .get<UserRequest, []>('/', (request) => {
-    request.user = 'Kevin'
-    request.user = 123  // NOT ok
   })
 
+  // request will be back to IRequestStrict here, thus no user property
   .get('/', (request) => {
-    request.user = 'Kevin' // still ok
-    request.user = 123  // NOT ok
+    request.user = 'Kevin' // invalid
   })
 
-  router.get('/', (request) => {
-    request.user = 123 // NOT ok
+  // and if we ever need to restore control, add the generic back in
+  .get<UserRequest>('/', (request) => {
+    request.user = 'Kevin'
   })
 
 export default router
diff --git a/src/IttyRouter.ts b/src/IttyRouter.ts
index c6305ea..bbed2a3 100644
--- a/src/IttyRouter.ts
+++ b/src/IttyRouter.ts
@@ -9,13 +9,15 @@ import {
 export const IttyRouter = <
   RequestType extends IRequest = IRequest,
   Args extends any[] = any[],
-  ResponseType = any
->({ base = '', routes = [], ...other }: IttyRouterOptions = {}): IttyRouterType<RequestType, Args, ResponseType> =>
+  ResponseType = any,
+  GlobalRequestType = RequestType,
+>({ base = '', routes = [], ...other }: IttyRouterOptions = {}): IttyRouterType<RequestType, Args, ResponseType, GlobalRequestType> =>
+// @ts-ignore
   ({
     __proto__: new Proxy({}, {
       // @ts-expect-error (we're adding an expected prop "path" to the get)
       get: (target: any, prop: string, receiver: object, path: string) =>
-        (route: string, ...handlers: RequestHandler<RequestType, Args>[]) =>
+        (route: string, ...handlers: RequestHandler<GlobalRequestType, Args>[]) =>
           routes.push(
             [
               prop.toUpperCase(),
@@ -53,4 +55,4 @@ export const IttyRouter = <
             if ((response = await handler(request.proxy ?? request, ...args)) != null) return response
         }
     },
-  } as IttyRouterType<RequestType, Args>)
+  })
diff --git a/src/types/IttyRouterType.ts b/src/types/IttyRouterType.ts
index e94ce9d..4279e64 100644
--- a/src/types/IttyRouterType.ts
+++ b/src/types/IttyRouterType.ts
@@ -8,17 +8,18 @@ import { CustomRoutes } from './CustomRoutes'
 export type IttyRouterType<
   R = IRequest,
   A extends any[] = any[],
-  ResponseType = any
+  ResponseType = any,
+  GlobalRequestType = R,
 > = {
   __proto__: IttyRouterType<R>
   routes: RouteEntry[]
   fetch: <Args extends any[] = A>(request: RequestLike, ...extra: Args) => Promise<ResponseType>
-  all: Route<R, A>
-  delete: Route<R, A>
-  get: Route<R, A>
-  head: Route<R, A>
-  options: Route<R, A>
-  patch: Route<R, A>
-  post: Route<R, A>
-  put: Route<R, A>
-} & CustomRoutes<Route<R, A>> & GenericTraps
+  all: Route<GlobalRequestType, A>
+  delete: Route<GlobalRequestType, A>
+  get: Route<GlobalRequestType, A>
+  head: Route<GlobalRequestType, A>
+  options: Route<GlobalRequestType, A>
+  patch: Route<GlobalRequestType, A>
+  post: Route<GlobalRequestType, A>
+  put: Route<GlobalRequestType, A>
+} & CustomRoutes<Route<GlobalRequestType, A>> & GenericTraps
diff --git a/src/types/Route.ts b/src/types/Route.ts
index b4c10d6..fac6254 100644
--- a/src/types/Route.ts
+++ b/src/types/Route.ts
@@ -11,4 +11,4 @@ export type Route<
 >(
   path: string,
   ...handlers: RequestHandler<RequestType, Args>[]
-) => IttyRouterType<RequestType, Args>
+) => IttyRouterType<R, A>