Extending `Request` in `firebase-admin`

How can I extend the Request interface in firebase-admin. I see that it is exported as-is from express

I used to be able to do it with,

// extensions.d.ts
/// <reference types="express" />

declare module Express {
    export interface Request {
        authenticated: boolean

But it doesn't work this time around. I've tested this with import { Request as R } from 'express' also.

Is there anything I'm missing? I'm quite sure that tsc saw the file by enabling listFiles in tsconfig.json