Firebase functions parsing request body before I can handle it in Express

I'm trying to handle invalid requests in Firebase functions, so making a post request with invalid JSON, with the intent of handling it in express. but I get 400 error 'SyntaxError: Unexpected token a in JSON at position 20' before it even reaches express layer, and the worst thing is the function runs for 60 seconds untill it hits timeout error.

my function

import * as functions from 'firebase-functions';
import * as express from 'express';
import * as admin from 'firebase-admin';

admin.initializeApp();


const app = express();

app.use((err: any, req: any, res: any, next: any) => {
  res.json({ error: 'invalid request' });
  next(err);
});

app.post('/test', (req: any, res: any) => {
  res.json({ error: 'invalid request' });
  res.end();
  return;
});



const server = functions.runWith({ maxInstances: 100 }).https.onRequest(app);

export { server as api };

the invalid json,

{
    "es":"adfasdf"asdf
}

I suspect this has something to do with the https://firebase.google.com/docs/functions/http-events#read_values_from_the_request at "This parsing is done by the following body parsers:"