How to access the router instance inside createMiddleware({ type: "function" }).client()?

const withRouterContext = createMiddleware({ type: "function" })
  .client(async (ctx) => {
    const router = ???; // How do I get this?  
    return await ctx.next({
      sendContext: {
        // values derived from router.state
      },
    });
  });

const myServerFn = createServerFn({ method: "GET" })
  .middleware([withRouterContext])
  .handler(({ context }) => {
    // router context is available here automatically
  });

// router-ref.ts
let _router: AnyRouter | null = null;
export const setRouterRef = (r: AnyRouter) => { _router = r; };
export const getRouterRef = () => _router;

// root layout component
const router = useRouter();
useEffect(() => { setRouterRef(router); }, [router]);