Every short link is a redirect handler, and a redirect handler sits in an unusually good spot: it sees a full HTTP request that the destination page will never show you. It is also the only place where a few well-known measurement bugs can be fixed.

I build HopQ, a link shortener with analytics, so I have spent more time in this request than is reasonable. Here is what is actually in it, and the four things that surprise people.

The request itself

A click arrives as an ordinary GET:

GET /a1b2c3 HTTP/1.1