This was a minimal example of Mixpanel not working properly with Vercel and Koa, but this repo has led us to a solution! :)

There are three situations you can test:

1. Koa + Vercel
   1. npm start
   2. curl localhost:3000/api/koa/
   3. Check Mixpanel events
2. Vercel and no Koa
   1. npm start
   2. curl localhost:3000/api/noKoa/
   3. Check Mixpanel events
3. Koa and no Vercel
   1. node index.js
   2. curl localhost:3001/api/koa/
   3. Check Mixpanel events

All three now work with the provided configuration.